U.S. patent application number 10/989992 was filed with the patent office on 2005-06-23 for multiple antibiotic resistance operon assays.
Invention is credited to Levy, Stuart B..
Application Number | 20050136460 10/989992 |
Document ID | / |
Family ID | 25470856 |
Filed Date | 2005-06-23 |
United States Patent
Application |
20050136460 |
Kind Code |
A1 |
Levy, Stuart B. |
June 23, 2005 |
Multiple antibiotic resistance operon assays
Abstract
An isolated and cloned region of a bacterial chromosome
containing a multiple antibiotic resistance operon is disclosed. A
description of the structure and function of the operon is provided
as are assorted recombinant DNA constructs involving the operon or
fragments thereof. The diagnostic, therapeutic and experimental
uses of these constructs are also disclosed. Methods of evaluating
the antibiotic effectiveness of compositions are disclosed and
methods of treatment employing effective compositions are
provided.
Inventors: |
Levy, Stuart B.; (Boston,
MA) |
Correspondence
Address: |
LAHIVE & COCKFIELD, LLP.
28 STATE STREET
BOSTON
MA
02109
US
|
Family ID: |
25470856 |
Appl. No.: |
10/989992 |
Filed: |
November 15, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10989992 |
Nov 15, 2004 |
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10131406 |
Apr 22, 2002 |
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10131406 |
Apr 22, 2002 |
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09118445 |
Jul 17, 1998 |
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6391545 |
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09118445 |
Jul 17, 1998 |
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08225480 |
Apr 8, 1994 |
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5817793 |
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08225480 |
Apr 8, 1994 |
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07938085 |
Aug 28, 1992 |
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Current U.S.
Class: |
435/6.15 ;
435/32 |
Current CPC
Class: |
C12N 15/52 20130101;
C12Q 1/6897 20130101 |
Class at
Publication: |
435/006 ;
435/032 |
International
Class: |
C12Q 001/68; C12Q
001/18 |
Goverment Interests
[0002] This work was funded, in part, by NIH grant AII6756. The
government may, therefore, have certain rights to this invention.
Claims
1. A method for predicting the antibiotic effectiveness of a
composition comprising exposing bacteria to said composition and
assaying the effect of said exposure on the expression of a genetic
locus wherein said expression is regulated at least in part and at
least indirectly by at least a functional fragment of a regulatory
region of a bacterial multiple antibiotic resistance operon within
said bacteria.
2-85. (canceled)
86. A method for screening the ability of a composition to inhibit
growth of a microbe in a host comprising contacting the microbe
with the composition and determining the effect of the composition
on the level of expression or the activity of a protein, wherein
the protein is structurally and functionally related to MarA.
87. A method for screening the ability of a composition to inhibit
growth of a microbe in a host comprising contacting the microbe
with the composition and determining the effect of the composition
on the level of expression or the activity of a protein that
promotes multiple antibiotic resistance in the microbe.
88. The method of claim 86, wherein the protein is selected from
the group consisting of: araC, rhaR, rhaS, melR, virF, xylS, rns,
soxS, and marA.
89. The method of claim 86, wherein the protein is a mar
enhancer.
90. The method of claim 86, wherein the protein is marA.
91. The method of claim 86, wherein the protein is soxS.
92. The method of claim 86, wherein the protein is araC.
93. The method of claim 86, wherein the protein is rhaR.
94. The method of claim 86, wherein the protein is rhaS.
95. The method of claim 86, wherein the protein is melR.
96. The method of claim 86, wherein the protein is virF.
97. The method of claim 86, wherein the protein is xylS.
98. The method of claim 86, wherein the protein is rns.
99. The method of claim 86, wherein the activity of the protein is
determined.
100. The method of claim 86, wherein the level of expression of the
protein is determined.
101. The method of claim 99, wherein the step of determining the
activity of the protein is performed by determining the level of
transcription of a regulatory region of an operon whose
transcription is regulated by the protein.
102. The method of claim 101, wherein the regulatory region of an
operon is micF or ompF.
103. The method of claim 101, wherein the step of determining the
level of transcription of gene whose transcription is regulated by
the protein comprises measuring the level of a product produced
from a marker locus operably linked to the regulatory region of an
operon whose transcription is regulated by the protein.
104. The method of claim 103, wherein the marker gene is lacZ.
105. The method of claim 103, wherein the marker gene is phoA.
106. The method of claim 86, wherein the protein is present in a
bacterium of a genus selected from the group consisting of:
Escherichia, Erwinia, Pseudomonas, Yersinia, Klebsiella, Serratia,
Citrobacter, Enterobacter, Hafnia, Salmonella, and Shigella.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
10/131,406 filed on Apr. 22, 2002, which is a divisional
application of Ser. No. 09/118,445 filed on Jul. 17, 1998 (now
issued as U.S. Pat. No. 6,391,545B1), which is a divisional
application of Ser. No. 08/225,480, filed on Apr. 8, 1994 (now
issued as U.S. Pat. No. 5,817,793), which is a continuation-in-part
of application Ser. No. 07/938,085 filed Aug. 28, 1992, and
entitled "Multiple Antibiotic Resistance Regulon Assays," now
abandoned. The entire contents of each of these applications are
incorporated herein by reference.
FIELD OF THE INVENTION
[0003] This invention relates generally to the field of
antimicrobial therapy. In particular, the present invention relates
to methods and products useful in inhibiting the growth of bacteria
or other microbes. In addition, this invention relates to
identifying loci in bacteria or other microbes which affect
antibiotic or antimicrobial susceptibility and to the production of
bacterial strains useful in the field of antimicrobial therapy.
BACKGROUND OF THE INVENTION
[0004] Antibiotic or antimicrobial substances have long been used
to inhibit the growth of bacteria or other microbes and to treat
bacterial or microbial infections in humans, other animals, and in
tissue culture. The use of antibiotics or antimicrobials in a
treatment regimen, however, has the undesirable effect of selecting
for bacteria or other microbes which are resistant to those
antibiotics or antimicrobials which are administered or applied. As
a result, treatment regimens can be adversely affected or, in some
cases, rendered ineffective. This necessitates a continual search
for new antibiotics and antimicrobials.
[0005] Of particular interest is the discovery of bacteria which
express a multiple antibiotic resistance phenotype (Mar). This
phenotype entails simultaneous resistance to a multiplicity of
antibiotics which are unrelated in chemical structure. The
appearance of such bacteria and infections by such bacteria greatly
increase the difficulty of identifying effective antibiotics and
treating infections in humans or other animals.
[0006] Multiple antibiotic resistance in bacteria is most commonly
associated with the presence of plasmids which contain one or more
resistance genes, each encoding a single antibiotic resistance
phenotype (Clewell 1981; Foster 1983). Multiple antibiotic
resistance associated with the chromosome, however, has been
reported in Klebsiella, Enterobacter, Serratia (Gutmann et al.
1985), Neisseria (Johnson and Morse 1988), and Escherichia (George
and Levy 1983a).
[0007] Bacteria expressing the multiple antibiotic resistance
phenotype can be isolated by selecting bacteria with a single
antibiotic and then screening for cross-resistance to structurally
unrelated antibiotics. For examples George and Levy initially
described a chromosomal multiple antibiotic resistance system which
exists in Escherichia coli and which can be selected by a single
drug, e.g., tetracycline or chloramphenicol (George and Levy
1983a). In addition to resistance to the selective agents, the Mar
phenotype includes resistance to structurally unrelated agents,
including nalidixic acid, rifampin, penicillins, and cephalosporins
(George and Levy 1983); more recently, resistance to the
fluoroquinolones has been described (Cohen et al. 1989).
[0008] The expression of a Mar phenotype, conferring substantially
increased, simultaneous and coordinated resistance to a
multiplicity of structurally unrelated compounds, appears to
involve coordinated changes in the expression of a multiplicity of
loci. This has been demonstrated in Mar phenotype bacteria of the
species E. coli (Cohen et al. 1989). Such coordinated control of
the expression of a multiplicity of loci implies the existence of
an operon which directly or indirectly regulates the expression of
the multiplicity of loci directly responsible for the Mar
phenotype. One locus in one such operon was identified in E. coli
and named marA by George and Levy (George and Levy 1983b).
[0009] Prior to the present invention, however, no multiple
antibiotic resistance (mar) operon had been isolated or cloned. In
addition, no mar operon had been characterized as to its structure
and operation so as to enable the use of such an operon or its
fragments for diagnostic, therapeutic or experimental purposes.
Finally, the several other contributions to the field of
antibacteriology in the claims were unavailable to those skilled in
the art prior to the present invention.
SUMMARY OF THE INVENTION
[0010] The present invention relates generally to developing and
evaluating antibiotic treatments effective against bacteria
possessing a multiple antibiotic resistance (mar) operon. Because
the expression of such an operon causes bacteria to become
simultaneously resistant to a multiplicity of structurally
unrelated antibiotics, it is a general object of the present
invention to provide methods and compositions useful in combating
bacteria possessing a mar operon or exhibiting a Mar phenotype. It
is one particular object of the present invention to provide tests
for compositions which are effective against bacteria expressing a
Mar phenotype but which do not induce the expression of a mar
operon, or which inhibit the expression of a mar operon. To this
end, it is also an object of the present invention to provide
cloned nucleotide sequences, as well as bacterial cells expressing
such sequences, which are useful in performing such tests and in
investigating bacterial multiple antibiotic resistance operons.
[0011] The present invention provides cloned bacterial mar operons
and cloned fragments thereof. In particular, a cloned repressor
locus and a cloned activator locus of a mar operon, as well as
cloned loci encoding anti-sense transcripts to the repressor and
activator loci, are provided. Using such clones, substantially pure
repressor protein and substantially pure activator protein are
provided. In addition, using such clones, isolated nucleotide
sequences, either sense or anti-sense to those loci, are provided.
These sequences are useful as probes for substantially homologous
loci in other species including bacteria, fungi, parasites, and
animal cells and are useful for altering the expression of a Mar
phenotype in bacteria, either by encoding repressor or activator
proteins or by encoding anti-sense transcripts which inhibit the
expression of either a mar repressor or mar activator locus.
[0012] The present invention also provides cloned nucleotide
sequences in which the regulatory region of a mar operon has been
operably joined to a marker locus. Such sequences are useful in
assaying the effect of compositions on the transcription of a mar
operon.
[0013] The present invention also provides methods for evaluating
the antibiotic effectiveness of compositions by assaying their
effects upon the transcription of a mar operon or upon the activity
of proteins encoded by a mar operon. In particular, the present
invention provides methods for assessing the ability or inability
of a composition to inhibit the activity of a mar repressor, to
enhance the activity of a mar repressor, or to inhibit the activity
of a mar activator. Compositions which enhance the activity of a
mar repressor or inhibit the activity of a mar activator will be
useful either alone or in combination with antibiotics to combat
bacteria. A method of treatment for bacterial infections using a
combination of such compositions along with antibiotics is thus
provided.
[0014] The present invention also provides methods for evaluating
the antibiotic effectiveness of compositions by assaying their
effects on bacteria in which the expression of a mar operon has
been substantially increased and on bacteria in which the
expression of a mar operon has been substantially decreased. To
this end, methods of producing such bacteria and such bacteria
themselves are provided.
[0015] The present invention also provides tests for identifying
loci in bacteria which are subject to regulation, directly or
indirectly, by a mar operon. Because such loci may be involved in
the expression of a Mar phenotype, their identification will be
useful in developing antibiotic compositions which affect the
products or expression of those loci.
[0016] The present invention also provides cloned bacterial loci
and fragments thereof which are subject to mar operon regulation
and which, therefore, form part of a mar regulon. Using such
clones, substantially pure protein encoded by these loci are
provided. In addition, using such clones, isolated nucleotide
sequences, either sense or anti-sense to these loci, are provided.
These sequences are useful as probes for substantially homologous
loci in other species including bacteria, fungi, parasites, and
animal cells and for altering the expression of a Mar phenotype in
bacteria.
BRIEF DESCRIPTION OF THE DRAWING
[0017] FIGURE 1 is a schematic diagram of the first 3.5 kb of SEQ
ID NO: 1.
DETAILED DESCRIPTION OF THE INVENTION
[0018] In the description that follows, a number of terms used in
biochemistry, molecular biology and recombinant DNA technology are
extensively utilized. In addition, certain new terms are introduced
for greater ease of exposition and to more clearly and distinctly
point out the subject matter of the invention. In order to provide
a clear and consistent understanding of the specification and
claims, including the scope to be given such terms, the following
definitions are provided.
[0019] By a "locus" is understood a nucleotide sequence encoding a
peptide. A locus consists of a start codon, a stop codon and at
least one codon encoding an amino acid residue in between.
Typically, a locus is transcribed to produce an mRNA transcript and
that transcript is translated to produce a peptide.
[0020] By "regulatory region" is understood a nucleotide sequence
involved in regulating the transcription of one or more loci.
Regulatory regions will include a promoter sequence at which an RNA
polymerase may bind and, typically, an operator sequence which may
be bound by a repressor protein. Additionally, regulatory regions
may include enhancers of transcription.
[0021] By "operon" is understood one or more loci operably joined
to a regulatory region such that, under appropriate conditions, an
RNA polymerase may bind to a promoter sequence in the regulatory
region and proceed to transcribe the loci. The loci within an
operon share a common regulatory region and, therefore, are
substantially regulated as a unit. Amongst the loci in an operon
may be a repressor locus which encodes a repressor protein which,
under appropriate conditions, binds to the operator of the operon
so as to substantially decrease expression of the loci in the
operon.
[0022] By "regulon" is understood two or more loci in two or more
different operons whose expression is regulated by a common
repressor or activator protein. A "first" operon may, for example,
encode a repressor protein which, under appropriate conditions,
binds to the operators of two or more different operons so as to
substantially inhibit transcription of the loci within those
operons. Or, a "first" operon may encode an activator protein which
interferes with the activity of one or more repressors of two or
more different operons so as to substantially increase the
transcription of the loci within those operons. Alternatively, a
"first" operon may encode a protein which affects the translation
or activity of proteins encoded by one or more loci in two or more
different operons. In each of these cases, the latter operons form
a regulon which is regulated by a common protein product of the
"first" operon.
[0023] By a "bacterial multiple antibiotic resistance regulon"
("mar regulon") is understood a regulon encoding a multiplicity of
protein products which are regulated in expression or activity by a
common protein product and which can cause a substantial increase
in resistance to a multiplicity of antibiotics, at least some of
which antibiotics are unrelated structurally.
[0024] By a "bacterial multiple antibiotic resistance operon" ("mar
operon") is understood a bacterial operon which, by its expression,
affects the expression of two or more different operons which form
a mar regulon. That is, by a "bacterial multiple antibiotic
resistance operon" is understood a bacterial operon which, by its
expression, affects the expression of two or more loci in two or
more different operons, or which affects the activity of two or
more protein products of such loci, so as to substantially increase
resistance to a multiplicity of antibiotics, at least some of which
are structurally unrelated. Amongst the loci in a bacterial
multiple antibiotic resistance operon, there is at least one locus
encoding an activator of a bacterial multiple antibiotic resistance
regulon. Amongst the loci in a bacterial multiple antibiotic
resistance operon, there may also be a locus encoding a repressor
of the bacterial multiple antibiotic resistance operon. The mar
operon of E. coli includes the marO region and marR, marA and marB
loci disclosed herein and is, therefore, also referred to as the
marRAB operon.
[0025] By a "repressor of a bacterial multiple antibiotic
resistance operon" ("mar repressor") is understood a protein which,
under appropriate conditions, binds to the operator of the operon
so as to substantially inhibit the transcription of the operon.
Such repressor proteins are encoded by repressor loci of bacterial
multiple antibiotic resistance operons.
[0026] By an "activator of a bacterial multiple antibiotic
resistance regulon" ("mar activator") is understood a protein
encoded by a locus within a mar operon which, under appropriate
conditions, affects the expression of two or more loci in a mar
regulon or the activity of two or more proteins from such a regulon
so as to cause expression of a bacterial multiple antibiotic
resistance phenotype.
[0027] By an "enhancer of a bacterial multiple antibiotic
resistance regulon" ("mar enhancer") is understood a protein
encoded by a locus within a mar operon which, under appropriate
conditions, enhances the expression or activity of a mar activator
so as to increase expression of a bacterial multiple antibiotic
resistance phenotype.
[0028] By a "bacterial multiple antibiotic resistance phenotype"
("Mar phenotype") is understood simultaneous and coordinated
resistance to a multiplicity of antibiotics, at least some of which
are structurally unrelated, which is substantially increased
relative to typical or wild-type bacteria. The antibiotic
resistance is simultaneous and coordinated in that the resistance
to the multiplicity of antibiotics increases or arises
simultaneously and may be decreased or lost simultaneously.
[0029] By an "inducer of a bacterial multiple antibiotic resistance
operon" ("mar inducer") is understood a chemical composition or
moiety which, under appropriate conditions, directly or indirectly
inhibits the binding of a repressor of a mar operon to the
regulatory region of that operon so as to substantially increase
the expression of that operon and, consequently, the expression of
a multiple antibiotic resistance phenotype.
[0030] By a "marker locus" is understood a locus whose expression
is easily assayed. A marker locus is typically a locus encoding an
enzyme and the assay may include a substance which changes color in
the presence of a product of the enzyme's activity. Alternatively,
a marker locus may encode a protein which directly or indirectly
affects a visually apparent phenotype of an organism such as color
or colony type in bacteria. Alternatively, a marker locus may
encode a protein which directly or indirectly confers substantial
resistance to, sensitivity to, or dependence upon a particular
composition.
[0031] By "expression" of a locus is understood the transcription
of the locus to produce mRNA and the translation of the mRNA
transcript to produce a peptide. By "substantially decreased
expression of a locus" is understood a decrease in detectable
expression of its mRNA transcript and/or protein product of at
least about 10% and preferably more than 25% of the previous level.
By "substantially increased expression of a locus" is understood an
increase in the level of its mRNA transcript and/or protein product
of at least about 10% and preferably about 25% of the previous
level.
[0032] By an "operable" locus is understood a locus capable of
being transcribed under appropriate conditions in vivo or in vitro.
A locus or nucleotide sequence is "operably joined" to a regulatory
region if, under appropriate conditions, an RNA polymerase may bind
to the promoter of the regulatory region and proceed to transcribe
the locus or nucleotide sequence in an appropriate reading frame. A
locus or nucleotide sequence operably joined to a regulatory region
is operable.
[0033] A coding sequence and a regulatory region are said to be
operably joined when they are covalently linked in such a way as to
place expression of the coding sequence under the influence or
control of the regulatory sequence. Two DNA sequences are said to
be operably joined if induction of the promoter function of one
results in the transcription of an mRNA sequence corresponding to
the coding sequences of the other. If it is desired that the RNA
transcript be translated into a protein or polypeptide, there are
further considerations. A coding sequencing which is to be
translated into a protein or polypeptide is said to be operably
joined to a regulatory region if induction of the promoter results
in the transcription of an mRNA transcript corresponding to the
coding sequences and if the nature of the linkage between the two
DNA sequences does not (1) result in the introduction of a
frame-shift mutation, (2) interfere with the ability of the
regulatory sequences to initiate and promote the transcription of
the coding sequences, or (3) interfere with the ability of the mRNA
template to be translated into a functional protein. Thus, a
regulatory region would be operably joined to a DNA sequence if the
promoter were capable of effecting transcription of that DNA
sequence such that the resulting transcript might be translated
into a functional protein or polypeptide.
[0034] If it is not desired that the coding sequence be eventually
expressed as a protein or polypeptide, as in the case of anti-sense
RNA expression, there is no need to ensure that the coding
sequences and regulatory region are joined without a frame-shift.
Thus, a coding sequence which need not be eventually expressed as a
protein or polypeptide is said to be operably joined to a
regulatory region if induction of promoter function results in the
transcription of an mRNA sequence corresponding to the coding
sequences.
[0035] The precise nature of the regulatory region needed for gene
expression may vary between species or cell types, but shall in
general include, as necessary, 5' non-transcribing and 5'
non-translating (non-coding) sequences involved with initiation of
transcription and translation respectively, such as a TATA box,
capping sequence, CAAT sequence, and the like, Especially, such 5'
non-transcribing regulatory sequences will include a region which
contains a promoter for transcriptional control of the operably
joined coding sequences. Such regulatory regions may also include
enhancer sequences or upstream activator sequences, as desired.
[0036] By "homology" of nucleotide sequences is understood a
correlation in the nucleotide composition and ordering of the
sequences. If the composition and ordering of the nucleotides are
the same or substantially the same, the sequences are characterized
by "sense" homology. If the composition and ordering of the
nucleotides of the sequences are substantially complementary such
that the sequences may, under appropriate conditions, hydrogen bond
in the manner of complementary strands of DNA, the sequences are
characterized by "anti-sense" homology. Sequences characterized by
sense homology to the mRNA transcript of a locus may, under
appropriate conditions, bind to the DNA of that locus so as to
inhibit further transcription. Sequences characterized by
anti-sense homology to the mRNA transcript of a locus may, under
appropriate conditions, bind to the DNA of that locus so as to
inhibit further transcription or bind to the mRNA transcript of
that locus so as to inhibit translation.
[0037] Two nucleotide sequences are substantially homologous if one
of them or its anti-sense complement can bind to the other under
strict hybridization conditions so as to distinguish that strand
from all or substantially all other sequences in a cDNA or genomic
library. Alternatively, one sequence is substantially homologous to
another if it or its anti-sense complement is useful as a probe in
screening for the presence of its homologous DNA or RNA sequence
under strict hybridization conditions. "Stringent hybridization"
conditions is a term of art understood by those of ordinary skill
in the art. For any given nucleotide sequence, stringent
hybridization conditions are those conditions of temperature and
buffer solution which will permit hybridization of that nucleotide
sequence to its complementary sequence and not to substantially
different sequences. The exact conditions which constitute
"stringent" conditions, depend upon the length of the nucleotide
sequence and the frequency of occurrence of subsets of that
sequence within other non-identical sequences. By varying
hybridization conditions from a level of stringency at which no
hybridization occurs to a level at which hybridization is first
observed, one of ordinary skill in the art can, without undue
experimentation, determine conditions which will allow a given
sequence to hybridize only with perfectly complementary sequences.
Hybridization conditions which permit hybridization to imperfectly
complementary sequences are employed to isolate nucleotide
sequences which are allelic to or evolutionary homologs of any
given sequence. Suitable ranges of such stringency conditions are
described in Krause, M. H. and S. A. Aaronson, Methods in
Enzymology, 200:546-556 (1991). By a sequence which is
"substantially homologous" to some specified sequence is understood
a sequence which will hybridize to the specified sequence, its
allelic variants and evolutionary homologs under stringent
hybridization conditions so as to distinguish those sequences from
non-allelic, non-homologous sequences.
[0038] By an "anti-sense locus" is understood a locus which encodes
an mRNA transcript characterized by substantial anti-sense homology
to the mRNA encoded by a specified locus. An anti-sense locus to an
activator locus of a bacterial multiple antibiotic resistance
operon, for example, will encode an mRNA transcript characterized
by substantial anti-sense homology to the mRNA transcript encoded
by the activator locus. The anti-sense mRNA may bind to the DNA of
the activator locus so as to inhibit further transcription or it
may bind to the mRNA transcript of the activator locus so as to
inhibit translation.
[0039] By "antibiotic" is understood a chemical composition or
moiety which decreases the viability or which inhibits the growth
or reproduction of microbes. As used in this disclosure, for
simplicity of exposition, antibiotics are intended to embrace
antibacterial, antiviral, antifungal and, generally, antimicrobial
compositions.
[0040] By an "isolated" nucleotide sequence is understood a
nucleotide sequence which has been: (1) amplified in vitro by, for
example, polymerase chain reaction (PCR); (2) recombinantly
produced by cloning; (3) purified, as by cleavage and gel
separation; or (4) synthesized by, for example, chemical synthesis.
An isolated nucleotide sequence is one which is readily manipulable
by recombinant DNA techniques well known in the art. Thus, a
nucleotide sequence contained in a vector in which 5' and 3'
restriction cytes are known or for which polymerase chain reaction
(PCR) primer sequences have been disclosed is considered isolated,
but a nucleotide sequence existing in its native state in its
natural host is not. An isolated nucleotide sequence may be
substantially purified, but need not be. For example, a nucleotide
sequence that is isolated within a cloning or expression vector is
not pure in that it may comprise only a tiny percentage of the
material in the cell in which it resides. Such a nucleotide
sequence is, however, isolated as the term is used herein because
it is readily manipulable by standard techniques of recombinant DNA
technology known to those of ordinary skill in the art.
[0041] By "fragment" is understood a unique fragment, a
substantially characteristic fragment, or a functional fragment as
defined below.
[0042] As used herein, a "unique fragment" of a protein or
nucleotide sequence is a substantially characteristic fragment not
currently known to occur elsewhere in nature (except in allelic or
allelomorphic variants). A unique fragment will generally exceed 15
nucleotides or 5 amino acid residues. One of ordinary skill in the
art can substantially identify unique fragments by searching
available computer databases of nucleotide and protein sequences
such as Genbank (Los Alamos National Laboratories, USA) or the
National Biomedical Research Foundation database. A unique fragment
is particularly useful, for example, in generating monoclonal
antibodies or in screening DNA or cDNA libraries.
[0043] A "substantially characteristic fragment" of a molecule,
such as a protein or nucleotide sequence, is meant to refer to any
portion of the molecule sufficiently rare or sufficiently
characteristic of that molecule so as to identify it as derived
from that molecule or to distinguish it from a class of related
molecules. A single amino acid or nucleotide cannot be a
substantially characteristic fragment. A substantially
characteristic fragment of a nucleotide sequence would have utility
as a probe in identifying the entire nucleotide sequence from which
it is derived within a sample of total genomic or DNA. A
substantially characteristic fragment of a protein would have
utility in generating antibodies which would distinguish the entire
protein from which it is derived from a mixture of many proteins.
It is within the knowledge and ability of one ordinarily skilled in
the art to recognize, produce and use substantially characteristic
fragments as, for example, probes for screening DNA libraries or
epitopes for generating antibodies.
[0044] By a "functional fragment" of a molecule is understood a
fragment retaining or possessing substantially the same biological
activity as the intact molecule. For example, a functional fragment
of a promoter sequence is a nucleotide sequence which retains or
possesses the ability to initiate and promote transcription of a
downstream nucleotide sequence by an RNA polymerase. Similarly, a
functional fragment of a repressor protein is a fragment which
retains or possesses the ability to bind to an operator sequence of
a regulatory region so as to substantially inhibit the ability of
an RNA polymerase to transcribe the downstream coding sequences. In
all instances, a functional fragment of a molecule retains at least
10% and at least about 25% of the biological activity of the intact
molecule.
[0045] The present invention in one aspect provides cloned
bacterial multiple antibiotic resistance operons. A bacterial mar
operon may be most easily isolated and cloned from any of a number
of species using the nucleotide sequences disclosed herein. Absent
the use of the nucleotide sequences disclosed herein, a mar operon
may still be isolated using the following procedures.
[0046] A strain of bacteria is first subjected to selection on
solid or in fluid medium containing an antibiotic. The selection
may be step-wise, with incrementally increasing concentrations of
the antibiotic. Amongst the surviving cells will be some
spontaneous mar operon mutants which express the Mar phenotype.
Such Mar mutants may be identified by their cross-resistance to a
multiplicity of antibiotics which are structurally unrelated to the
selective agent. The approximate map position of the operon may
then be determined by mating transfer experiments which are well
known in the art.
[0047] The Mar phenotype bacteria may then be mutagenized with a
transposon and cells reverting to the wild-type (non-Mar) phenotype
can be isolated by selecting for antibiotic susceptibility on
replicated plates. A high percentage of the revertants will be the
result of inactivation of the mar operon by insertion of the
transposon within the operon. Using a restriction enzyme known to
recognize a site within the transposon, fragments of the transposon
joined to segments of chromosomal DNA can be cloned into vectors.
These clones may, in turn, be used to probe a genomic library of
the cells to identify clones bearing at least a fragment of the mar
operon. Finally, these clones may be tested for their ability to
complement cells in which there is a large deletion around the
approximate mar map position. Clones capable of providing mar
operon activity to the deletants will contain an operable mar
operon and may be sequenced by standard techniques. This technique
was used to identify the mar operon of E. coli (See Example 1).
Given the nucleotide sequence of the mar operon of E. coli,
however, such a technique is not necessary for isolating
substantially homologous operons in other species.
[0048] In one particular aspect, therefore, the present invention
provides the cloned wild-type mar operon of E. coli. One sequence
containing a wild-type mar operon is presented as SEQ ID NO: 1.
This sequence has been entered into GenBank with Accession #M96235
and corresponds to the 7.8 kbp fragment isolated in Example 1. This
sequence or any fragment of it may, of course, be cloned into any
of a number of vectors which are known to those of ordinary skill
in the art.
[0049] Conservative variations on the DNA sequence SEQ ID NO: 1
exist which will have no substantial effect on the expression of
the operon. The substitution of synonymous codons is one example. A
small deletion or insertion which does not disrupt the reading
frame is another. Substantially homologous sequences from a mar
operon in a species other than E. coli are another example,
particularly when such operons are identified by the methods and
compositions disclosed herein. Such conservative variations would
be obvious to one of ordinary skill in the art and fall within the
spirit and scope of the claims.
[0050] The sequence of SEQ ID NO: 1 includes three regions. These
regions are depicted in FIGURE 1.
[0051] Analysis of this sequence reveals a regulatory region with
promoter-operator sequences, designated marO. The marO sequence
extends from approximately nucleotide positions 1234 to 1444 of SEQ
ID NO: 1 between Region I and Region II. Transcription of Region I
proceeds leftward from marO on the DNA strand complementary to SEQ
ID NO: 1 whereas transcription of Region II proceeds rightward from
marO on the DNA strand depicted as SEQ ID NO: 1. The marO sequence
includes two pairs of direct repeat (DR) elements. DR-1, 15 bp long
with one mismatch at position 9 of the DR (TACTTGCC[T/A]GGGCAA),
begins at position 1390 of SEQ ID NO: 1 and its partner, DR-1',
begins at position 1425. DR-1' is part of an imperfect palindrome
starting at position 1423 (ATTACTTGCCAGGGCAACTAAT) and DR-1 is part
of a similar shorter palindrome. A second DR (DR-2 and DR-2') of 9
bp (GCAACTAAT) flanks on both sides and partly overlaps the
downstream part of DR-1'. For Region marO includes a promoter with
-10 and -35 E. coli consensus sequences at about nucleotide
positions 1350 and 1370, respectively, and/or at about positions
1275 and 1301, respectively. For Region II, marO includes a
promoter with nearly perfect -10 and -35 E. coli consensus
sequences at about nucleotide positions 1408 and 1384,
respectively.
[0052] Region II includes four potential open reading frames (ORFs)
designated ORF 125, ORF 144, ORF 129 and ORF 72.
[0053] ORF 125 begins with an ATG start codon at nucleotide
positions 1502-1504 of SEQ ID NO: 1 and ends with a TAA stop codon
at positions 1877-1879, and encodes a protein of 125 amino acid
residues. Based upon sequencing of Mar mutants, ORF 125 was
originally considered the mar repressor locus but, based on
fusion-protein studies of binding to marO, described more fully
below (See Example 10), ORF 125 does not encode the full mar
repressor.
[0054] ORF 144 begins with a GTG start codon at nucleotide
positions 1445-1447 of SEQ ID NO: 1, ends with the same TAA stop
codon as ORF 125 at positions 1877-1879 (thereby encompassing all
of ORF 125). ORF 144 encodes the full mar repressor (144 amino acid
residues) and is, therefore, designated the marR locus. The mar
repressor regulates not only transcription of Region II (in which
marR is found) but also has a regulatory function for Region I. The
protein, MarR, encoded by ORF 144 is disclosed as SEQ ID NO: 4.
Note that the first GTG codon is translated as a Met residue.
[0055] The second locus of Region II, marA, corresponds to ORF 129
and encodes a protein of 129 amino acids, designated the mar
activator. The marA locus extends from nucleotide 1893 of SEQ ID
NO: 1 to nucleotide 2282. The mar activator (MarA) is a 13 kDa
polypeptide that shows strong similarity to the family of positive
regulators that includes regulators of carbohydrate metabolism in
Escherichia coli (AraC, RhaR, RhaS, and MelR), Erwinia corotovora
(AraC), and Pseudomonas putida (XylS); virulence in Yersinia
enterocolitica (VirF) and E. coli (Rns); and oxidative stress
response in E. coli (SoxS) (Cohen, Hachler and Levy, 1993). For
example, the MarA protein is strongly similar (42% identical, 65%
similar) to the SoxS protein which activates the soxRS regulon
genes (Amabile-Cuevas and Demple, 1991). The protein, MarA, encoded
by ORF 129 is disclosed as SEQ ID NO: 5.
[0056] The third locus of Region II, marB, corresponds to ORF 72
and encodes a protein of 72 amino acids, designated the mar
enhancer. The marB locus extends from nucleotide 2314 of SEQ ID NO:
1 to nucleotide 2531. The marB locus is necessary for expression of
the full Mar phenotype although its precise mode of action remains
unclear. The protein, MarB, encoded by ORF 72 is disclosed as SEQ
ID NO: 6.
[0057] The marO region and the marR, marA and marB loci form the
mar operon, or marRAB operon, of E. coli. The mar repressor acts to
repress expression of the operon by binding at marO. The mar
activator acts directly or indirectly to alter the expression of
other operons, loci or proteins which are part of the mar regulon
and which are directly involved with the expression of the Mar
phenotype. The mar enhancer augments the expression of the Mar
phenotype. In addition to these E. coli homologs of the bacterial
mar operon, hybridization studies, disclosed more fully below,
indicate that substantially homologous sequences are included in
the genomes of many other bacterial species and that bacterial mar
operons share a common ancestry and evolutionarily conserved
structure. In light of the present disclosure, one of ordinary
skill in the art may readily isolate the mar operons of other
species.
[0058] Transcription from marO through Region I (leftward with
respect to SEQ ID NO: 1 on the complementary DNA strand) proceeds
through two open reading frames designated ORF 64 and ORF 156/157.
ORF 64 begins with an ATG start codon complementary to the TAC
found (reading leftward) at positions 1233-31 of SEQ ID NO: 1 and
ends with a TGA stop codon complementary to the ACT at positions
1041-1039. ORF 156/157 begins either at the GTG complementary to
the CAC at positions 1042-1040 or at the ATG complementary to the
TAC at positions 1039-1037, and ends with the TAA complementary to
the ATT at positions 571-569 of SEQ ID NO: 1. The protein encoded
by ORF 157 is disclosed as SEQ ID NO: 2. Note that the first GTG is
translated as Met.
[0059] Although the functions of the Region I proteins are not yet
known, they are part of the mar regulon and function directly in
the phenotypic expression of Mar. Mar phenotype mutants produce
somewhat higher levels of Region I mRNA than wild-type cells and
clearly more in the presence of tetracycline. In addition,
deletants without both Region I and Region II show 2-3 fold lower
antibiotic resistance than deletants without only Region II (see
Example 11).
[0060] Region III contains a single significant open reading frame,
ORF 266, which encodes a protein of 266 residues. ORF 266 is
transcribed in the opposite (leftward) direction from the loci of
Region II and from the strand of DNA complementary to SEQ ID NO: 1.
ORF 266 begins with an ATG start complementary to the TAC at
positions 3363-3361 of SEQ ID NO: 1 and ends with the TAA stop
codon complementary to the ATT at positions 2565-2563. The function
of the ORF 266 protein is currently unknown and the level of Region
III mRNA transcripts are not detectably affected by the Mar status
of cells or the presence of tetracycline. The protein encoded by
ORF 266 is disclosed as SEQ ID NO: 7.
[0061] The present invention thus provides cloned fragments of
these various regions including regulatory regions, protein coding
sequences or both. Particular examples include cloned mar
regulatory sequences and cloned m repressor, mar activator, and mar
enhancer loci of bacterial mar operons. The particular examples
provided are the cloned E. coli marO sequences and marR, marA, and
marB loci. In addition, the cloned E. coli ORF 64 and ORF 157
sequences are provided. Such loci are cloned preferably so as to be
operably joined to a regulatory region so that they may be
expressed under conditions wherein the regulatory region is not
blocked by a repressor. The loci may be operably joined to a mar
regulatory region or to other regulatory regions depending upon the
desired manner of regulation and levels of expression. In addition,
the invention provides vectors containing an operably cloned mar
repressor locus without a mar activator locus and with or without a
mar enhancer locus (see Example 2), an operably cloned mar
activator locus without a mar repressor locus and with or without a
mar enhancer locus (see Example 3), and an operably cloned mar
enhancer locus without a mar repressor locus and with or without a
mar activator locus (see Example 3). Given the sequences disclosed
herein, as well as the methods provided for identifying
substantially homologous mar operons in other species, one of
ordinary skill in the art is enabled to produce such cloned loci.
In addition, anti-sense clones of these loci can be as easily
produced and are also an aspect of the present invention.
[0062] Conservative variations on SEQ ID NO: 1 exist which will
have no substantial effect on the expression of these loci. The
substitution of synonymous codons is one example. A small deletion
or insertion which does not disrupt the reading frame is another. A
sequence from a mar operon in a species other than E. coli is
another example, particularly when such a sequence is identified by
the methods and compositions disclosed herein. Such conservative
variations would be obvious to one of ordinary skill in the art and
fall within the spirit and scope of the claims.
[0063] The present invention also provides probes useful in
identifying mar operons in species other than E. coli. A cloned mar
operon or cloned fragment of a mar operon from one species can be
used to screen a DNA library of another species to identify
potential mar operons by methods which are known to those of
ordinary skill in the art. DNA homologous to marRAB has been found
among many members of the Enterobacteriaceae including Klebsiella
(Cohen, Yan and Levy, 1993). Similarly, induction of the Mar
phenotype by salicylate and acetyl salicylate has been commonly
observed among 58 clinical enteric isolates tested (Foulds and
Rosner, personal communication). In Klebsiella, Serratia and
Pseudomonas cepacia, the salicylate decreased the presence of
OmpF-like outer membrane porins (Burns and Clark, 1992; Sawai,
Hirano and Yamaguchi, 1987). Furthermore, in Klebsiella,
salicylates increased resistance to various antibiotics (including
.beta.-lactams and tetracycline), decreased resistance to
aminoglycosides and decreased the amounts of capsular
polysaccharide (Domenico, Hopkins and Cunha, 1990; Domenico,
Landolphi and Cunha, 1991). This indicates that mar operons are
involved in salicylate induction of Mar phenotypes in many
enterobacteria.
[0064] In one preferred embodiment, the cloned E. coli mar operon
disclosed as SEQ ID NO: 1 or a fragment thereof is used to produce
probes which are radioactively labeled with .sup.32P (see Example
4). In a particularly preferred embodiment, the probe is a fragment
of the E. coli marA or marR locus and, most preferably, a unique
fragment. Such probes have been found to hybridize with DNA
extracted from a wide variety of bacteria and may reveal an ancient
and evolutionarily highly conserved family of loci and operons in
species extending beyond bacteria.
[0065] Conservative variations on the disclosed DNA sequence exist
which will not substantially impair the effectiveness of such
probes. The substitution of a small percentage of the bases, small
insertions, and small deletions are examples. Sequences from a mar
operon in a species other than E. coli are another example,
particularly when such operons are identified by the methods and
compositions disclosed herein. Such conservative variations would
be obvious to one of ordinary skill in the art and fall within the
spirit and scope of the claims.
[0066] The present invention in another aspect provides
substantially pure mar repressor protein, substantially pure mar
activator protein and substantially pure mar enhancer protein. In
particular, the invention provides substantially pure E. coli mar
repressor protein, substantially pure E. coli mar activator protein
and substantially pure E. coli mar enhancer protein (see Example
5). Substantially pure proteins are suitable for protein sequencing
and are typically at least 90% pure by weight and preferably at
least 95% pure by weight. Given the sequences disclosed herein, as
well as the methods provided for identifying substantially
homologous mar operons in other species, such substantially pure
proteins can be produced and isolated by one of ordinary skill in
the art (Maniatis, et al. 1982).
[0067] The invention further provides a cloned fusion of a mar
regulatory region and a marker locus. In a preferred embodiment,
the mar regulatory region is the E. coli marO and the marker locus
is the .beta.-galactosidase gene, lacZ (see Example 6). Such a
fusion on a vector is useful for assaying the ability or inability
of compositions to increase or decrease the expression of a Mar
phenotype, as disclosed below.
[0068] The present invention also provides for the creation of
bacterial strains which exhibit the Mar phenotype. In particular,
the invention provides such strains by genetic manipulation and
provides such strains of E. coli.
[0069] In one embodiment, anti-sense to the mar repressor locus is
introduced within the cells. This may be accomplished by exposing
the cells to single-stranded nucleotides or nucleotide analogs
which enter the cell. The nucleotides are characterized by
substantial anti-sense homology to either the mar repressor locus
or its mRNA transcript and are of sufficient length such that they
either inhibit the transcription of the mar repressor locus by
binding to the mar repressor DNA or they inhibit translation of the
mar repressor by binding to the mRNA transcript of the mar
repressor locus. More preferably, an operable anti-sense locus is
introduced within the cells on a vector. Preferably, the anti-sense
locus is operably joined to a strong promoter and on a high
copy-number plasmid such that the anti-sense transcripts are
expressed at high levels. For some uses, as disclosed below, a
temperature sensitive plasmid may be preferred. Given the sequences
disclosed herein, as well as the methods provided for identifying
substantially homologous mar operons in other species, one of
ordinary skill in the art is enabled to produce such nucleotide
sequences and vectors operably expressing such sequences.
[0070] In a preferred embodiment, a strain is created which has a
deletion, insertion or substitution in the chromosomal mar
repressor locus such that functional mar repressor is not produced
but the mar activator locus is expressed. In a particularly
preferred embodiment, a deletion is introduced. This is achieved by
cloning the mar operon into a temperature sensitive plasmid which
replicates at lower temperatures but does not replicate at higher
temperatures. Using appropriate restriction enzymes, any one of
numerous possible deletions is introduced into the mar repressor
locus on the plasmid. The plasmid is then introduced into bacterial
cells and the cells are grown at the lower temperature to allow for
homologous recombination to introduce the partially deleted mar
repressor locus into the bacterial chromosome. The bacteria are
then grown at the higher temperature so that, at cell division, the
temperature sensitive plasmid is lost from the daughter cells.
Cells in which the deletion was introduced into the chromosome and
in which the activator is constitutively expressed may then be
selected with antibiotics.
[0071] In another preferred embodiment, a mar activator locus is
introduced within the cells on a vector. In one embodiment, the mar
activator locus is operably joined to the mar regulatory region on
a plasmid which does not include an operable mar repressor locus.
Homologous recombination is employed, as disclosed above, to
inactivate the mar repressor locus on the chromosome by partial
deletion, insertion, or substitution, so that functional mar
repressor is not produced and the plasmid copy of the mar activator
locus is expressed. In a most preferred embodiment, the mar
activator locus is operably joined to a regulatory region other
than the mar regulatory region such that it is expressed
irrespective of the presence of the mar repressor and the
chromosomal mar repressor locus need not be inactivated.
Preferably, the regulatory region contains a strong promoter. In
addition, it is preferred in both embodiments that the plasmids be
high copy-number plasmids. For some uses, as disclosed herein, it
may be preferable that the plasmid be temperature sensitive. Given
the sequences disclosed herein, as well as the methods provided for
identifying substantially homologous mar operons in other species,
one of ordinary skill in the art is enabled to produce such vectors
(see Examples 2 and 3).
[0072] The present invention further provides for the creation of
bacterial strains which exhibit increased sensitivity to
antibiotics, relative to wild-type cells, because they have at
least partially lost the ability to express a mar activator. In
particular, the invention provides such strains by genetic
manipulation and provides such strains of E. coli.
[0073] In one embodiment, anti-sense to the chromosomal mar
activator locus may be introduced within the cells. This may be
accomplished by exposing the cells to single-stranded nucleotides
or nucleotide analogs which enter the cell. The nucleotides are
characterized by substantial anti-sense homology to either the mar
activator locus or its mRNA transcript and are of sufficient length
such that they either inhibit the transcription of the chromosomal
mar activator locus by binding to the mar activator DNA or they
inhibit translation of the mar activator by binding to the mRNA
transcript of the mar activator locus. More preferably, an operable
anti-sense locus is introduced within the cells on a plasmid.
Preferably, the anti-sense locus is operably joined to a strong
promoter and on a high copy-number plasmid such that the anti-sense
transcripts are expressed at high levels. For some uses, as
disclosed below, a temperature sensitive plasmid may be preferred.
Given the sequences disclosed herein, as well as the methods
provided for identifying substantially homologous mar operons in
other species, one of ordinary skill in the art is enabled to
produce such nucleotide sequences and vectors operably expressing
such sequences.
[0074] In another preferred embodiment, a strain is created which
has a deletion, insertion or substitution in the chromosomal mar
activator locus such that functional mar activator cannot be
produced. In another embodiment, a chromosomal deletion, insertion
or substitution is introduced which is not in the mar activator
locus but which entails a frame-shift upstream of the locus such
that functional mar activator protein is not produced. In another
preferred embodiment, the entire mar operon or a substantial part
of it may be deleted (see Example 7). As disclosed above, such
deletions, insertions or substitutions may be achieved by
homologous recombination between the chromosomal mar operon and a
properly constructed plasmid. To further increase sensitivity to
antibiotics, the E. coli ORF 64 and/or ORF 156/157 loci, or their
homologs in other species, may be similarly inactivated. Thus, in
preferred embodiments, insertions, deletions, substitutions or
frame-shifts are introduced into a bacterial chromosome which
substantially decrease expression of ORF 64, ORF 156/157 or their
homologs.
[0075] In a most preferred embodiment, a mar repressor locus is
introduced within the cells on a vector. In this embodiment, the
mar repressor locus is operably joined to a regulatory region other
than a mar regulatory region such that it is expressed irrespective
of its own presence. Preferably, the regulatory region will contain
a strong promoter and the vectors are high copy-number plasmids.
For some uses, as disclosed below, it may be preferable that the
plasmid be temperature sensitive. Given the sequences disclosed
herein, as well as the methods provided for identifying
substantially homologous mar operons in other species, one of
ordinary skill in the art is enabled to produce such vectors (see
Examples 2 and 3).
[0076] The present invention also provides an assay for
compositions which induce a Mar phenotype by interfering with the
activity of a mar repressor or which increase the sensitivity of
cells to antibiotic compositions by enhancing the activity of a mar
repressor. In each embodiment, cells which are not characterized by
the Mar phenotype are exposed to a composition and then the level
of expression of a locus under the control of a mar regulatory
region is assayed. The locus may be contained within a mar operon
or may be operably joined to a mar regulatory region and introduced
into the cells on a vector.
[0077] In one embodiment, the level of a protein product of a locus
under the control of a mar regulatory region is directly measured.
This may be accomplished by, for example, polyacrylamide gel
electrophoresis or by any of a variety of other means which are
well known to those of ordinary skill in the art.
[0078] In another embodiment, the levels of the mRNA transcript of
a locus under the control of a mar regulatory region are directly
measured. This may be accomplished, as is well known in the art, by
performing a Northern hybridization with a probe which has been
radioactively labeled with 32P and measuring the level of
radioactive probe bound. In a preferred embodiment, the locus is a
mar operon activator locus (see Example 8). Probes for such a locus
are disclosed above. In addition, in another preferred embodiment,
the locus is at least one of the open reading frames of the E. coli
Region I (see FIGURE 1) disclosed in SEQ ID NO: 1.
[0079] In a preferred embodiment, a fusion of a marker locus to a
mar regulatory region is introduced within the cells. One such
fusion is disclosed in Example 6. In one embodiment, the marker
fusion of Example 6 is introduced within cells and the cells are
then grown in LB broth at 30.degree. C. for one hour in the
presence of the compound to be tested. The level of activity of the
.beta.-D-galactosidase marker can be determined by the
O-nitrophenyl-.beta.-D-galactoside assay described in Maniatis, et
al. (1982).
[0080] In each of these embodiments, it is further preferred that a
vector bearing an operable mar repressor locus be introduced within
the cells. This will cause increased repression of the mar operon
and improve the ability of the assay to detect inducers of the Mar
phenotype. The mar repressor locus is preferably introduced into
the chromosome by homologous recombination and is operably joined
to a regulatory region other than a mar regulatory region such that
it does not repress it own expression. Such vectors are disclosed
above.
[0081] The most preferred embodiment is an assay employing cells
into which have been introduced both a marker locus fused to a mar
regulatory region and an operable mar repressor locus which is not
controlled by a mar regulatory region.
[0082] The present invention also provides assays for compositions
which act to prevent the expression of a Mar phenotype or which
cause cells to become even more sensitive to certain compounds than
wild-type cells, by acting as inhibitors of mar operon expression.
In each embodiment, cells which possess an operable mar activator
locus are exposed to a composition and then the level of expression
of a locus, the expression of which is at least in part controlled
by mar operon expression, is assayed. The locus may be naturally
occurring in the cells or may be operably joined to a regulatory
region influenced by mar operon expression.
[0083] As disclosed above, the assay may be a direct assay for a
translation product of the locus by, for example, electrophoresis
of cellular proteins, for a transcription product of the locus by,
for example, a Northern blot of cellular mRNA or, in a preferred
embodiment, the locus is a marker locus, the activity of which is
easily assayed.
[0084] In a most preferred embodiment, a marker locus is operably
joined to the regulatory region of an operon which is affected by
mar operon expression. Means of identifying such loci are disclosed
below. The fusion of the regulatory region and marker is then
introduced into cells. After the cells have been exposed to a
composition, changes in the level of expression of the marker locus
can be assayed as an indication of the effect of the composition on
the expression of the mar operon. In a particularly preferred
embodiment, the regulatory region is from the micF or ompF loci of
E. coli. The expression of the micF locus is affected by mar operon
expression. The micF locus, in turn, affects the expression of the
ompF locus. These loci or their regulatory regions may be operably
fused to, for example, lacZ and the activity of the
.beta.-D-galactosidase determined as described above. In another
preferred embodiment, the regulatory region is marO and the marker
locus is one of the loci encoded by the E. coli Region I shown in
FIGURE 1 and disclosed in SEQ ID NO: 1. In this embodiment, the
assay is for the transcription or translation products of Region
I.
[0085] The present invention also provides assays for identifying
loci involved in the expression of a Mar phenotype other than mar
operons. That is, the invention provides assays for loci whose
expression is directly or indirectly regulated by a mar activator
protein.
[0086] In one embodiment, substantially purified mar activator
protein, disclosed above, is mixed with the fragmented genomic DNA
of a species under conditions which permit it to bind to
appropriate DNA sequences. DNA fragments to which the activator has
bound may then be isolated on filters, in polyacrylamide gels, or
by other methods well known to those of ordinary skill in the art.
Those fragments may then be cloned into vectors and used as probes
to locate and isolate their corresponding loci or may be sequenced
to identify gene products associated with them.
[0087] In another embodiment, a cell line is employed into which
has been introduced a vector bearing an operable mar activator
locus such that the cells express the Mar phenotype. Preferably,
the activator locus is joined to a regulatory region other than the
mar regulatory region such that its level of expression is high.
Alternatively, the mar repressor locus may be inactivated by
deletion, insertion or substitution, as disclosed above and a
plasmid bearing an operable activator locus but not an operable
repressor locus, as disclosed above, may be introduced within the
cells. The total mRNA from these cells may then be compared to the
total mRNA of cells which are not expressing the Mar phenotype. In
a most preferred embodiment, this is accomplished by creating a
cDNA library of the total mRNA from the mar strain and the non-mar
strain. This cDNA library is then used to generate probes to
screen, by standard Northern technique, the total mRNA from the mar
strain and the non-mar strain. Any cDNA probes that hybridize to
the mRNA of one strain but not to the mRNA of the other will
correspond to loci involved in the expression of the Mar phenotype.
Those probes may then be used to identify such loci by standard
techniques. An alternative approach would employ subtractive
screening. The cDNA from a strain expressing a mar activator locus
can be hybridized to excess mRNA from a strain deleted of that
locus. Subsequently, those cDNAs which do not hybridize can be
isolated by, for example, hydroxyapatite chromatography and used to
identify mar related loci.
[0088] In another embodiment, a promoterless and therefore
inoperable marker locus is introduced into the cell and allowed to
insert randomly into the chromosome. The cells are then manipulated
so as to change their phenotype either from non-mar to mar or from
mar to non-mar. Cells in which the expression of the marker changes
along with the change in Mar phenotype, contain markers which have
operably inserted into loci which are regulated directly or
indirectly by a mar operon (See Example 9). The two alternative
versions of this embodiment are described separately, below.
[0089] In one version of the above embodiment, the cells do not
initially express the Mar phenotype but contain an operable mar
operon which is capable of being induced. It is particularly
preferred that a vector bearing an operable mar repressor locus, as
disclosed above, be introduced within the cells such that the
expression of the mar activator locus is initially minimal. A
promoterless marker locus contained within a transposon and
inserted within a phage, for example .lambda.::TnphoA or
.lambda.::TnlacZ, is introduced into the cells and allowed to
randomly integrate into the genome. In addition, it is desirable
that the transposon also include a locus conferring resistance to
kanamycin or another appropriate antibiotic. A number of colonies,
preferably at least two thousand and, more preferably, at least ten
thousand, are then isolated on plates containing kanamycin or
another appropriate antibiotic. These colonies are then examined
for expression of the marker locus. If the marker is phoA or lacZ
and the cells are grown on plates with 5-bromo-4-chloro-3-indolyl
phosphate (XP plates) or 5-bromo-4-chloro-3-indolyl
.beta.-D-galactoside (XG plates), colonies in which the marker
operably inserted into an actively expressed locus will be blue
whereas colonies in which the marker failed to insert, inserted
inoperably, or inserted operably into a repressed locus will appear
white. The colonies in which the marker is not expressed are then
isolated and the cells are grown in the presence of a known inducer
of the mar operon (e.g. salicylate or tetracycline for E. coli).
Subsequent to such treatment, colonies which express the marker
(and, in this example, turn blue) are isolated. These colonies
contain the marker operably inserted in a locus that is subject to
regulation by a mar operon. The DNA of these colonies may then be
fragmented and cloned. Those clones which confer resistance to
kanamycin or another appropriate antibiotic will contain the marker
in the transposon as well as DNA adjacent to the insertion site.
The genomic DNA adjacent to the insertion site of the transposon
can then be isolated and the locus into which the transposon
inserted can be identified by techniques known to those of ordinary
skill in the art. That locus will, by this method, be identified as
one which is involved in the expression of the Mar phenotype.
[0090] In a most preferred version of the above embodiment, the
cells initially express the Mar phenotype but can easily be caused
to express the non-Mar phenotype. As above, a promoterless marker
in a transposon is introduced within the cells and allowed to
randomly integrate into the chromosome. And, as above, the
transposon also encodes a locus conferring resistance to kanamycin
or another appropriate antibiotic. A temperature sensitive plasmid,
such as pMAK705 (Hamilton, et al. 1989), bearing an operable mar
activator locus is introduced within the cells. The plasmid may
bear the activator locus operably joined to a mar regulatory region
but without the mar repressor locus or, preferably, may contain an
activator locus operably joined to a regulatory region other than a
mar regulatory region such that its level of expression is high. If
the mar activator locus is operably joined to a mar regulatory
region, the chromosomal mar repressor locus must be inactivated by
any of the means disclosed above. In addition, the chromosomal mar
activator locus is inactivated by any of the means disclosed above
so that the expression of the Mar phenotype is dependent upon the
plasmid copy of the activator locus and the cells are recombination
deficient (e.g. recA-) so that the activator locus on the plasmid
cannot be introduced into the chromosome. Initially, the cells are
grown at a temperature at which the temperature sensitive plasmid
replicates (e.g. 30.degree. C. for pMAK705) and in the presence of
kanamycin or another appropriate antibiotic. In this embodiment, a
number of colonies, preferably at least two thousand and, more
preferably, at least ten thousand, are then isolated and examined
for expression of the marker locus. If the marker is phoA or lacZ
and the cells are grown on X-P or X-G plates, for example, colonies
in which the marker operably inserted into an actively expressed
locus will be blue whereas colonies in which the marker failed to
insert, inserted inoperably, or inserted operably into a repressed
locus will appear white. The colonies in which the marker is
expressed are then isolated and the cells are grown at an elevated
temperature (e.g. 42.degree. C. for pMAK705) such that the
temperature sensitive plasmid and, consequently, the Mar phenotype
are lost. Then, colonies which no longer express the marker are
isolated. These colonies contain the marker in the transposon
operably inserted in a locus that is subject to regulation by a mar
operon. As described above, the kanamycin or other resistance locus
in the transposon can be used to isolate a fragment containing the
transposon and DNA adjacent to the insertion site of the
transposon. The locus into which the transposon inserted can then
be identified by techniques known to those of ordinary skill in the
art. That locus will, by this method, be identified as one which is
involved in the expression of the Mar phenotype.
[0091] The present invention further provides compositions and a
method for their use in treating bacterial infections. By employing
the assays disclosed above, one of ordinary skill in the art is
enabled to identify compositions which inhibit the expression of a
bacterial mar operon. These compositions may be administered to a
human or other animal along with known antibiotics. By inhibiting
the expression of the mar operon, these compositions will either
enhance the effectiveness of the known antibiotic or will render an
otherwise ineffective antibiotic effective. Such compositions, once
identified by the means disclosed herein, can be combined in
pharmaceutically effective amounts with known antibiotics by one
ordinarily skilled in the art.
EXAMPLE 1
[0092] A mar mutant of E. coli K12 designated AG102 was derived
from a wild-type strain designated AG100 by selection with
antibiotics (see George and Levy, 1983b). AG102 was then subjected
to .lambda. b221 c1857 rex:::Tn5 mutagenesis. A revertant from the
Mar phenotype, resulting from Tn5 insertion within the mar operon,
was isolated and designated AG1025. Exploiting the single BamHI
site in Tn5, the AG1025 chromosomal DNA was digested with BamHI or
partially digested with Sau3A. The resulting fragments were ligated
into the single BamHI site of the high copy-number plasmid vector
pUC18. Two clones, designated pKan1 and pKan2 (see Hachler, Cohen
and Levy, 1991), were isolated which contained the 3.2 kbp of Tn5
upstream of its internal BamHI site. A 2 kbp HpaI fragment of
pKan1, containing only 187 bp from the IS50L of Tn5 and 1.85 kbp of
chromosomal DNA was used as a probe to screen a .lambda. phasmid
library derived from partial Sau3A digests of the E. coli K12
derivative W3110 (see Elledge and Walker, 1985). Isolation was
performed in host strain PLK1738 in which the marA region has been
deleted. Two phasmids identified by the probe were introduced by
transduction into a deletion strain HH84 in which the region
including the mar operon had been deleted from the chromosome.
These phasmids were capable of restoring mar operon activity. One
of the fragments, 13.1 kbp in length, was used for subcloning into
the low copy-number vector pHSG415. mar activity was tested in
CH164, a .DELTA.marA strain genetically related to the original
AG100 mar mutants. Subclones containing either the 9 kbp PstI or
7.8 kbp HpaI-PstI fragment, but none containing any smaller
fragments, produced Mar mutants. (For detail on the genotypes of
the strains and vectors, see Hachler, Cohen, and Levy, 1991,
incorporated herein by reference.)
EXAMPLE 2
[0093] The mar repressor locus, marR, was cloned from the wild-type
plasmid pHHM183 (see Hachler, Cohen and Levy, 1991) as an 818 bp
DraI fragment into the SmaI site of the high copy-number cloning
vector pUC18. The plasmid was designated p125WT. A mutant mar
repressor locus causing expression of the Mar phenotype was cloned
on a 850 bp DraI-HpaI fragment from plasmid pKan1 (see Hachler,
Cohen and Levy, 1991) into pUC18. This plasmid was designated
p125mar. These plasmids have been introduced into a number of E.
coli K12 strains such as the wild-type AG100 and mar mutant AG102.
The full genotypes of these strains may be found in Cohen et al.,
1988, incorporated herein by reference.
EXAMPLE 3
[0094] The entire E. coli mar operon, marA, and marB have been
cloned into various vectors and introduced within various hosts
using the disclosed sequences and the polymerase chain reaction to
generate the fragments disclosed below. In particular, for cloning
the entire operon, the PCR primers at the 5' end were nucleotides
1311-1328 of SEQ ID NO: 1 and nucleotides 2575-2592 at the 3' end.
In addition PstI linkers were included at both ends. For cloning
the marA locus, the PCR primers were nucleotides 1893-1908 at the
5' end and nucleotides 2265-2282 at the 3' end with EcoRI linkers
at both ends. For cloning the marB locus, the PCR primers were
nucleotides 2314-2331 at the 5' end and nucleotides 2515-2532 at
the 3' end with EcoRI linkers at both ends. The PCR synthesized
genes have been cloned into several plasmid vectors at appropriate
single restriction enzyme sites: pUC18, a multicopy ColE1
derivative; pMAK705, a temperature-sensitive plasmid described in
Hamilton et al., 1989; and pMAL-C2, a plasmid used for expressing
the protein fused to MalE (New England BioLabs, Beverly, Mass.,
product #800). The plasmids have been introduced in wild-type E.
coli K12; AG102, a mar mutant described in George and Levy, 1983b;
AG1025, a marA::Tn5 mar revertant described in George and Levy
1983b; and CH164, a mar deleted strain described in Hachler, Cohen
and Levy, 1991.
EXAMPLE 4
[0095] Cloned copies of SEQ ID NO: 1 were digested with BspHI. A
resulting 1.24 kbp fragment, corresponding to nucleotides 1073 to
2314 of SEQ ID NO: 1 was used to produce .sup.32P labelled probes.
DNA extracted from a large number of bacterial species were tested
for homology to this probe under stringent DNA::DNA hybridization
techniques using dot blots and Southern hybridization methods, as
are well known to those of ordinary skill in the art (see, e.g.,
Maniatis, Fritch and Sambrook, 1982, incorporated herein by
reference). DNA from the following gram-negative genera were found
to hybridize with the probe: Citrobacter, Enterobacter,
Escherichia, Hafnia, Klebsiella, Salmonella, and Shigella. Two
species, Enterobacter aqglomerans and Salmonella sp., were further
tested. These were found to produce Mar phenotype mutants when
selected by the same regime employed with E. coli and to produce a
1.4 kb mRNA transcript at heightened levels. The 1.4 kb transcripts
were the same size as and homologous to the mRNA produced at
heightened levels in E. coli expressing the Mar phenotype.
EXAMPLE 5
[0096] The E. coli marA fragment described in Example 3 was cloned
into pMAL-C2, a vector bearing the maltose binding protein locus,
MalE. This vector is commercially available as part of a kit for
protein purification ("Protein fusion and purification system," New
England BioLabs, Beverly, Mass., product #800). The clone,
including the marA fragment, encoded a fusion product comprising
the mar activator protein and the maltose binding protein linked by
a peptide which is cleavable by protease Xa. The fusion protein was
made in E. coli TB 1 (ara, .DELTA.(lac pro AB) rpsL (.PHI.80 lacZ
.DELTA.MI5) hsdR). The fusion protein was then substantially
purified by amylose column chromatography. The peptide linking the
mar activator protein and the maltose binding protein was cleaved
and the substantially purified mar protein collected.
EXAMPLE 6
[0097] A 405 bp ThaI fragment containing the E. coli marO region
was ligated into the SmaI site of pMLB 1109, a lacZ transcriptional
fusion plasmid. The resulting plasmid construct had lacZ gene
expression under the control of the marO promoter. The fusion was
introduced into the chromosome of a wild-type cell and a mar operon
deleted strain. This was accomplished by first introducing the
marO-lacZ region of the fusion plasmids, by homologous
recombination, into the genome of phage .lambda.RZ5 by infecting an
E. coli K12 strain designated SPC103 (M4100 (.DELTA.lac U169 araD,
rpsL, relA, thi, fibB) deleted of 39 kbp surrounding the mar
operon) bearing one of the fusion plasmids, with .lambda.RZ5. The
resulting lysate was used to transduce plasmid-less SPC103.
Amp.sup.R, Lac.sup.+ lysogens were selected on LB agar containing
ampicillin (50 .mu.g/ml) and 5-bromo-4-chloro-3-indolyl
.beta.-D-galactosidase. Lysates from these purified lysogens were
then used to infect E. coli MC4100 or SPC103 and Amp.sup.R,
Lac.sup.+ lysogens were again isolated. The resulting strains,
SPC104, SPC105, SPC106, and SPC107 were confirmed to have a single
copy of the fusion region located in the same site on the
chromosome (likely the att site) by Southern hybridization of
PstI-digested chromosomal DNA from the strains with a 405 bp
EcoRI/BamHI fragment. Lysogens of MC4100 (SPC104, SPC105) had 2
bands which hybridized with a 9 kb fragment representing the
naturally occurring marO-marA sequence and a larger fragment
(>15 kb) representing the insertion of the marO-lacZ fusion
phage into the chromosome. The lysogens of the mar deletion strain
SPC103 (SPC106, SPC107) contained only chromosomal sequences
corresponding to the larger band. DNA manipulations and analyses
were performed according to Maniatis et al. (1982). Assays for
.beta.-galactosidase activity were performed on cells grown for one
hour to mid-logarithmic phase in LB broth in the presence of known
and potential inducers of the Mar phenotype at 30.degree. C. The
.beta.-galactosidase activity of these cells was compared to cells
grown similarly but in the absence of known inducers of the Mar
phenotype.
EXAMPLE 7
[0098] A 9 kbp PstI fragment of E. coli K12 chromosomal DNA
containing the mar operon was cloned into temperature-sensitive
plasmid pMAK705. The plasmid replicates at 30.degree. C. but is
lost from daughter cells during cell division at 42.degree. C.
Using BspHI, a 1.24 kbp deletion corresponding to nucleotide 1073
to nucleotide 2314 of SEQ ID NO: 1 and including all of the marO
region, the marR and marA loci, and part of the marB locus was made
within the mar operon on the plasmid. The plasmid was then
introduced within E. coli K12 AG100 (see George and Levy, 1983b for
genotype information). The plasmid and chromosome were then allowed
to undergo homologous recombination and the cells were cured of the
plasmid at 42.degree. C. A recombinant strain with the 1.24 kbp
deletion in the chromosomal mar operon was isolated and designated
AG100 .DELTA.15.
EXAMPLE 8
[0099] About 10.sup.8 E. coli which were not expressing the Mar
phenotype were grown at 30.degree. C., collected at the end of
logarithmic phase and resuspended in fresh broth containing
salicylate, a known mar operon inducer, for one hour at 30.degree.
C. The mRNA was extracted from these cells and separated by gel
electrophoresis. A 1.24 kb BspHI fragment from within the mar
operon which includes the marA locus was labeled with .sup.32P by
random priming and used as a probe in a Northern hybridization to
assay for increased expression of the marA locus.
EXAMPLE 9
[0100] TnphoA and TnlacZ were used to mutagenize a recA E. coli
strain which had been deleted of the mar operon and transformed
with a temperature-sensitive (curable) plasmid containing the
constitutively expressed mar operon. From a total of 2100 fusions,
5 mar-regulated mutants were identified. Two lacZ fusions showed
loss of LacZ activity upon loss of the plasmid at 42.degree. C.,
while three phoA fusions showed an increase in PhoA activity with
plasmid loss. The DNA sequence of the chromosomal DNA proximal to
each of the fusions did not show homology with any known genes of
E. coli. The lacZ fusions were at 31.5 and 14 min; two of the phoA
fusions were at 77 min and one was at 51.6 min. In one of the two
phoA fusions at 77 min, PhoA activity was associated with the
membranes. This approach has identified new genes in E. coli which
are regulated by the marRAB operon and involved in the Mar
phenotype.
EXAMPLE 10
[0101] Based on sequencing of Mar mutants, the protein products of
both ORF 125 and ORF 144 were considered candidates for the mar
repressor. To investigate this, fusion proteins of the maltose
binding protein (MBP) and each of the two potential repressors,
MarR125 and MarR144, were produced. MBP-MarR144, but not
MBP-MarR125, repressed expression of LacZ from a marO-lacZ fusion.
The fusion proteins MBP-MarR125 and MBP-MarR144 were purified by
amylose affinity chromatography. Gel retardation studies showed
that purified MBP-MarR144 bound to marO with an affinity of
5.times.10.sup.9M. No binding was seen with MBP-MarR125. Therefore
the N-terminal amino acid residues lacking in MBP-MarR125 are
required for marO binding. Structurally unrelated compounds
(tetracycline, chloramphenicol, ampicillin, DNP and salicylate) at
different concentrations caused reversal of the binding of MarR
(i.e., MBP-MarR144) to marO.
EXAMPLE 11
[0102] In conjunction with SEQ ID NO: 1, the polymerase chain
reaction (PCR) was employed to amplify the coding sequences of
Region I and Region II. PCR primers were created to flank the
coding regions of ORF 156/157, ORF 64, Region I, Region II, and
Regions I and II together. For cloning ORF 156/157, the PCR primers
were nucleotides 570-587 at the 5' end of SEQ ID NO: 1 and
nucleotides 1022-1039 at the 3' end with either PstI or EcoRI
linkers at the ends. For cloning ORF 64, the PCR primers were
nucleotides 1039-1056 at the 5' end and nucleotides 1216-1233 at
the 3' end with either PstI or EcoRI linkers at the ends. For
cloning the entire Region I sequence, the PCR primers were
nucleotides 163-180 at the 5' end and nucleotides 1216-1233 at the
3' end with either PstI or EcoRI linkers at the ends. For cloning
Regions I and II together, the PCR primers were nucleotides 163-180
at the 5' end and nucleotides 2575-2592 at the 3' end with PstI
linkers at both ends. The PCR synthesized sequences were cloned
into several plasmid vectors at appropriate restriction enzyme
sites: pMAK705, a temperature-sensitive low copy-number plasmid
(Hamilton et al. 1989) and pMAL-C2, a plasmid used for expressing
the protein fused to MalE (New England BioLabs, Beverly,
Mass.).
[0103] Plasmid constructs containing different PCR fragments of
Region I and Region II were used in complementation analyses to
define the genes required to restore multidrug resistance (to
tetracycline, chloramphenicol, nalidixic acid, norfloxacin, and
rifampicin) in mar deletion and inactivated strains. In two
deletion mutants (.DELTA.39 kb including mar-MCH164; .DELTA.1.2 kb
in the mar operon-WY100) plasmids containing marA alone restored
wild-type MICs to tetracycline, nalidixic acid, and
chloramphenicol. The addition of marB to marA increased the
resistance to the drugs 19-46% (depending upon the drug tested),
suggesting that both marA and marB are associated with intrinsic
drug susceptibility/resistance.
[0104] Plasmids containing both Region I and Region II in the same
mutant strains further increased antibiotic resistance levels 2-3
fold to levels comparable with Mar mutants. These findings indicate
that both Region I and Region II are involved with the multiple
antibiotic resistance phenotype. More detailed results are shown
below for complementation tests with nalidixic acid (nal),
tetracycline (tet) and chloramphenicol (cml).
1 MIC (.mu.g/ml) E. coli strain nal Tet clm AG100 (wild-type) 4.2
3.3 4.6 MCH164 (.DELTA.39 kb) 2.2 1.9 0.9 MCH164pMAL-marA 5.3 3.4
4.9 MCH164pMAL-marB 2.1 2.1 1.0 MCH164pMAL-marAB 6.3 4.7 7.2
MCH164pMAK-Region II 6.8 4.7 N/A* MCH164pMAK-Regions I & II
11.3 10.4 N/A* MCH164pHH193-SEQ ID NO: 1 13.6 13.1 N/A* AG102 (Mar
mutant) 14.5 13.8 >25 *Plasmid confers resistance to cml.
[0105]
Sequence CWU 1
1
* * * * *