U.S. patent application number 09/742954 was filed with the patent office on 2001-06-28 for aromatic amino acid catabolism enzymes.
Invention is credited to Cahoon, Edgar B., Cahoon, Rebecca E., Falco, Saverio Carl, Hitz, William D., Kinney, Anthony J., Morgante, Michele, Rafalski, J. Antoni.
Application Number | 20010005749 09/742954 |
Document ID | / |
Family ID | 26789224 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010005749 |
Kind Code |
A1 |
Cahoon, Edgar B. ; et
al. |
June 28, 2001 |
Aromatic amino acid catabolism enzymes
Abstract
This invention relates to an isolated nucleic acid fragment
encoding an aromatic amino acid catabolic enzyme. The invention
also relates to the construction of a chimeric gene encoding all or
a portion of the aromatic amino acid catabolic enzyme, in sense or
antisense orientation, wherein expression of the chimeric gene
results in production of altered levels of the aromatic amino acid
catabolic enzyme in a transformed host cell.
Inventors: |
Cahoon, Edgar B.;
(Wilmington, DE) ; Cahoon, Rebecca E.;
(Wilmington, DE) ; Falco, Saverio Carl; (Arden,
DE) ; Hitz, William D.; (Wilmington, DE) ;
Kinney, Anthony J.; (Wilmington, DE) ; Morgante,
Michele; (Wilmington, DE) ; Rafalski, J. Antoni;
(Wilmington, DE) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY
LEGAL DEPARTMENT - PATENTS
1007 MARKET STREET
WILMINGTON
DE
19898
US
|
Family ID: |
26789224 |
Appl. No.: |
09/742954 |
Filed: |
December 21, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09742954 |
Dec 21, 2000 |
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09362473 |
Jul 28, 1999 |
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6218169 |
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60094783 |
Jul 31, 1998 |
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Current U.S.
Class: |
536/23.2 ;
435/189 |
Current CPC
Class: |
C12N 9/0069 20130101;
C12Y 307/01002 20130101; C12P 13/22 20130101; C12Y 113/11005
20130101; C12N 9/14 20130101; C12N 15/8251 20130101 |
Class at
Publication: |
536/23.2 ;
435/189 |
International
Class: |
C07H 021/04 |
Claims
What is claimed is:
1. An isolated nucleic acid fragment encoding a homogentisate
1,2-dioxygenase comprising a member selected from the group
consisting of: (a) an isolated nucleic acid fragment encoding the
amino acid sequence set forth in a member selected from the group
consisting of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6 and SEQ ID
NO:8; (b) an isolated nucleic acid fragment that is complementary
to (a).
2. The isolated nucleic acid fragment of claim 1 wherein nucleic
acid fragment is a functional RNA.
3. The isolated nucleic acid fragment of claim 1 wherein the
nucleotide sequence of the fragment comprises the sequence set
forth in a member selected from the group consisting of SEQ ID
NO:1, SEQ ID NO:3, SEQ ID NO:5 and SEQ ID NO:7
4. A chimeric gene comprising the nucleic acid fragment of claim 1
operably linked to suitable regulatory sequences.
5. A transformed host cell comprising the chimeric gene of claim
4.
6. An isolated nucleic acid fragment encoding a fumarylacetoacetase
comprising a member selected from the group consisting of: (a) an
isolated nucleic acid fragment encoding the amino acid sequence set
forth in a member selected from the group consisting of SEQ ID
NO:10, SEQ ID NO:12 and SEQ ID NO:14; (b) an isolated nucleic acid
fragment that is complementary to (a).
7. The isolated nucleic acid fragment of claim 6 wherein nucleic
acid fragment is a functional RNA.
8. The isolated nucleic acid fragment of claim 6 wherein the
nucleotide sequence of the fragment comprises the sequence set
forth in a member selected from the group consisting of SEQ ID
NO:9, SEQ ID NO:11 and SEQ ID NO:13.
9. A chimeric gene comprising the nucleic acid fragment of claim 6
operably linked to suitable regulatory sequences.
10. A transformed host cell comprising the chimeric gene of claim
9.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/094,783, filed Jul. 31, 1998.
FIELD OF THE INVENTION
[0002] This invention is in the field of plant molecular biology.
More specifically, this invention pertains to nucleic acid
fragments encoding enzymes involved in aromatic amino acid
catabolism in plants and seeds.
BACKGROUND OF THE INVENTION
[0003] In addition to their role as protein monomeric units, amino
acids are energy metabolites and precursors of many biologically
important nitrogen-containing compounds, such as heme,
physiologically active amines, glutathione, other amino acids,
nucleotides, and nucleotide coenzymes. Excess dietary amino acids
are neither stored for future use nor excreted. Rather they are
converted to common metabolic intermediates such as pyruvate,
oxaloacetate, and alpha-ketoglutarate. Consequently, amino acids
are also precursors of glucose, fatty acids, and ketone bodies and
are therefore metabolic fuels. The degradation of amino acids
converts them to citric acid cycle intermediates or their
precursors so that they can be metabolized to CO.sub.2 and water or
used in gluconeogenesis. Oxidative breakdown of amino acids
typically accounts for 10 to 15% of the metabolic energy generated
by animals.
[0004] The enzymes included in this application are involved in
catabolism of the aromatic amino acids. The first reaction in
phenylalanine degradation is its hydroxylation to tyrosine; thus a
single pathway is responsible for the breakdown of both of these
amino acids. 3,4-Dehydroxyphenyl acetate 2,3-dioxygenase is also
called homogentisate 1,2-dioxygenase (EC 1.13.11.15) and, in the
presence of oxygen, catalyzes the decyclization of homogentisic
acid (3,4-dihydroxyphenylacetate) into
2-Hydroxy-5-carboxymethylmuconate semialdehyde. Loss of
homogentisate 1,2 dioxygenase (HGO) activity is responsible for the
human metabolic disorder alkaptonuria. A large number of variant
forms of the human enzyme have been described which show the
clinical effect of single nucleotide changes on the activity of the
enzyme (Fernandez-Canon, J. M. et al. (1996). Nat Genet 14:19-24).
The gene Aspergillus nidulans homogentisate 1,2 dioxygenase has
been characterized and its disruption shown to induce secretion of
homogenistate ((1995) J Biol Chem 270:21199-21205). In the same
article, the authors searched the GenBank database with the
homogentisate 1,2 dioxygenase sequence and identified ESTs with
significant similarity to homogentisate 1,2 dioxygenase. These ESTs
were from tissues obtained from human, Arabidopsis thaliana, and
Ricinus communis.
[0005] Fumarylacetoacetase, also named fumarylacetoacetate
hydrolase (EC 3.7.1.2) catalyzes the last step in the
phenylalanine/tyrosine degradation catalyzing the conversion of
4-fumarylacetoacetate and water to acetoacetate and fumarate.
Debilitating mutations in this enzyme have been shown to be the
cause of hereditary tyrosinemia type 1 in humans (St-Louis, M. and
Tanguay, R. M. (1997) Hum. Mut. 9:291-299; Labelle, Y. et al.
(1993) Hum. Mol. Genet. 2:941-946).
SUMMARY OF THE INVENTION
[0006] The instant invention relates to isolated nucleic acid
fragments encoding enzymes involved in aromatic amino acid
catabolism. Specifically, this invention concerns an isolated
nucleic acid fragment encoding a homogentisate 1,2-dioxygenase or a
fumarylacetoacetase and an isolated nucleic acid fragment that is
substantially similar to an isolated nucleic acid fragment encoding
a homogentisate 1,2-dioxygenase or a fumarylacetoacetase. In
addition, this invention relates to a nucleic acid fragment that is
complementary to the nucleic acid fragment encoding homogentisate
1,2-dioxygenase or fumarylacetoacetase.
[0007] An additional embodiment of the instant invention pertains
to a polypeptide encoding all or a substantial portion of an enzyme
involved in aromatic amino acid catabolism selected from the group
consisting of homogentisate 1,2-dioxygenase and
fumarylacetoacetase.
[0008] In another embodiment, the instant invention relates to a
chimeric gene encoding a homogentisate 1,2-dioxygenase or a
fumarylacetoacetase, or to a chimeric gene that comprises a nucleic
acid fragment that is complementary to a nucleic acid fragment
encoding a homogentisate 1,2-dioxygenase or a fumarylacetoacetase,
operably linked to suitable regulatory sequences, wherein
expression of the chimeric gene results in production of levels of
the encoded protein in a transformed host cell that is altered
(i.e., increased or decreased) from the level produced in an
untransformed host cell.
[0009] In a further embodiment, the instant invention concerns a
transformed host cell comprising in its genome a chimeric gene
encoding a homogentisate 1,2-dioxygenase or a fumarylacetoacetase,
operably linked to suitable regulatory sequences. Expression of the
chimeric gene results in production of altered levels of the
encoded protein in the transformed host cell. The transformed host
cell can be of eukaryotic or prokaryotic origin, and include cells
derived from higher plants and microorganisms. The invention also
includes transformed plants that arise from transformed host cells
of higher plants, and seeds derived from such transformed
plants.
[0010] An additional embodiment of the instant invention concerns a
method of altering the level of expression of a homogentisate
1,2-dioxygenase or a fumarylacetoacetase in a transformed host cell
comprising: a) transforming a host cell with a chimeric gene
comprising a nucleic acid fragment encoding a homogentisate
1,2-dioxygenase or a fumarylacetoacetase; and b) growing the
transformed host cell under conditions that are suitable for
expression of the chimeric gene wherein expression of the chimeric
gene results in production of altered levels of homogentisate
1,2-dioxygenase or fumarylacetoacetase in the transformed host
cell.
[0011] An addition embodiment of the instant invention concerns a
method for obtaining a nucleic acid fragment encoding all or a
substantial portion of an amino acid sequence encoding a
homogentisate 1,2-dioxygenase or a fumarylacetoacetase.
[0012] A further embodiment of the instant invention is a method
for evaluating at least one compound for its ability to inhibit the
activity of a homogentisate 1,2-dioxygenase or a
fumarylacetoacetase, the method comprising the steps of: (a)
transforming a host cell with a chimeric gene comprising a nucleic
acid fragment encoding a homogentisate 1,2-dioxygenase or a
fumarylacetoacetase, operably linked to suitable regulatory
sequences; (b) growing the transformed host cell under conditions
that are suitable for expression of the chimeric gene wherein
expression of the chimeric gene results in production of
homogentisate 1,2-dioxygenase or fumarylacetoacetase in the
transformed host cell; (c) optionally purifying the homogentisate
1,2-dioxygenase or the fumarylacetoacetase expressed by the
transformed host cell; (d) treating the homogentisate
1,2-dioxygenase or the fumarylacetoacetase with a compound to be
tested; and (e) comparing the activity of the homogentisate
1,2-dioxygenase or the fumarylacetoacetase that has been treated
with a test compound to the activity of an untreated homogentisate
1,2-dioxygenase or fumarylacetoacetase, thereby selecting compounds
with potential for inhibitory activity.
BRIEF DESCRIPTION OF THE SEQUENCE LISTING
[0013] The invention can be more fully understood from the
following detailed description and the accompanying Sequence
Listing which form a part of this application.
[0014] Table 1 lists the polypeptides that are described herein,
the designation of the cDNA clones that comprise the nucleic acid
fragments encoding polypeptides representing all or a substantial
portion of these polypeptides, and the corresponding identifier
(SEQ ID NO:) as used in the attached Sequence Listing. The sequence
descriptions and Sequence Listing attached hereto comply with the
rules governing nucleotide and/or amino acid sequence disclosures
in patent applications as set forth in 37 C.F.R.
.sctn.1.821-1.825.
1TABLE 1 Enzymes Involved in Aromatic Amino Acid Catabolism SEQ ID
NO: Protein Clone Designation (Nucleotide) (Amino Acid)
Homogentisate cbn2.pk0052.e6 1 2 1,2-Dioxygenase rls6.pk0027.h11 3
4 sfl1.pk0008.h2 5 6 wlk8.pk0020.a11 7 8 Fumarylacetoacetase Contig
of: 9 10 cc71se-b.pk0004.b5 cr1n.pk0107.d3 cr1n.pk0151.e7
rl0n.pk082.n4 11 12 sgs6c.pk001.h5 13 14
[0015] The Sequence Listing contains the one letter code for
nucleotide sequence characters and the three letter codes for amino
acids as defined in conformity with the IUPAC-IUBMB standards
described in Nucleic Acids Research 13:3021-3030 (1985) and in the
Biochemical Journal 219 (No. 2):345-373 (1984) which are herein
incorporated by reference. The symbols and format used for
nucleotide and amino acid sequence data comply with the rules set
forth in 37 C.F.R. .sctn.1.822.
DETAILED DESCRIPTION OF THE INVENTION
[0016] In the context of this disclosure, a number of terms shall
be utilized. As used herein, a "nucleic acid fragment" is a polymer
of RNA or DNA that is single- or double-stranded, optionally
containing synthetic, non-natural or altered nucleotide bases. A
nucleic acid fragment in the form of a polymer of DNA may be
comprised of one or more segments of cDNA, genomic DNA or synthetic
DNA.
[0017] As used herein, "contig" refers to a nucleotide sequence
that is assembled from two or more constituent nucleotide sequences
that share common or overlapping regions of sequence homology. For
example, the nucleotide sequences of two or more nucleic acid
fragments can be compared and aligned in order to identify common
or overlapping sequences. Where common or overlapping sequences
exist between two or more nucleic acid fragments, the sequences
(and thus their corresponding nucleic acid fragments) can be
assembled into a single contiguous nucleotide sequence.
[0018] As used herein, "substantially similar" refers to nucleic
acid fragments wherein changes in one or more nucleotide bases
results in substitution of one or more amino acids, but do not
affect the functional properties of the polypeptide encoded by the
nucleotide sequence. "Substantially similar" also refers to nucleic
acid fragments wherein changes in one or more nucleotide bases does
not affect the ability of the nucleic acid fragment to mediate
alteration of gene expression by gene silencing through for example
antisense or co-suppression technology. "Substantially similar"
also refers to modifications of the nucleic acid fragments of the
instant invention such as deletion or insertion of one or more
nucleotides that do not substantially affect the functional
properties of the resulting transcript vis-a-vis the ability to
mediate gene silencing or alteration of the functional properties
of the resulting protein molecule. It is therefore understood that
the invention encompasses more than the specific exemplary
nucleotide or amino acid sequences and includes functional
equivalents thereof.
[0019] For example, it is well known in the art that antisense
suppression and co-suppression of gene expression may be
accomplished using nucleic acid fragments representing less than
the entire coding region of a gene, and by nucleic acid fragments
that do not share 100% sequence identity with the gene to be
suppressed. Moreover, alterations in a nucleic acid fragment which
result in the production of a chemically equivalent amino acid at a
given site, but do not effect the functional properties of the
encoded polypeptide, are well known in the art. Thus, a codon for
the amino acid alanine, a hydrophobic amino acid, may be
substituted by a codon encoding another less hydrophobic residue,
such as glycine, or a more hydrophobic residue, such as valine,
leucine, or isoleucine. Similarly, changes which result in
substitution of one negatively charged residue for another, such as
aspartic acid for glutamic acid, or one positively charged residue
for another, such as lysine for arginine, can also be expected to
produce a functionally equivalent product. Nucleotide changes which
result in alteration of the N-terminal and C-terminal portions of
the polypeptide molecule would also not be expected to alter the
activity of the polypeptide. Each of the proposed modifications is
well within the routine skill in the art, as is determination of
retention of biological activity of the encoded products.
[0020] Moreover, substantially similar nucleic acid fragments may
also be characterized by their ability to hybridize. Estimates of
such homology are provided by either DNA-DNA or DNA-RNA
hybridization under conditions of stringency as is well understood
by those skilled in the art (Hames and Higgins, Eds. (1985) Nucleic
Acid Hybridisation, IRL Press, Oxford, U.K.). Stringency conditions
can be adjusted to screen for moderately similar fragments, such as
homologous sequences from distantly related organisms, to highly
similar fragments, such as genes that duplicate functional enzymes
from closely related organisms. Post-hybridization washes determine
stringency conditions. A set of preferred conditions uses a series
of washes starting with 6X SSC, 0.5% SDS at room temperature for 15
min, then repeated with 2X SSC, 0.5% SDS at 45.degree. C. for 30
min, and then repeated twice with 0.2X SSC, 0.5% SDS at 60.degree.
C. for 30 min. Another preferred set of highly stringent conditions
uses two final washes in 0.1X SSC, 0.1% SDS at 65.degree. C.
[0021] Substantially similar nucleic acid fragments of the instant
invention may also be characterized by the percent identity of the
amino acid sequences that they encode to the amino acid sequences
disclosed herein, as determined by algorithms commonly employed by
those skilled in this art. More preferred nucleic acid fragments
encode amino acid sequences that are 90% identical to the amino
acid sequences reported herein. Most preferred are nucleic acid
fragments that encode amino acid sequences that are 95% identical
to the amino acid sequences reported herein. Sequence alignments
and percent identity calculations were performed using the Megalign
program of the LASARGENE bioinformatics computing suite (DNASTAR
Inc., Madison, Wis.). Multiple alignment of the sequences was
performed using the Clustal method of alignment (Higgins and Sharp
(1989) CABIOS. 5:151-153) with the default parameters (GAP
PENALTY=10, GAP LENGTH PENALTY=10). Default parameters for pairwise
alignments using the Clustal method were KTUPLE 1, GAP PENALTY=3,
WINDOW=5 and DIAGONALS SAVED=5.
[0022] A "substantial portion" of an amino acid or nucleotide
sequence comprises an amino acid or a nucleotide sequence that is
sufficient to afford putative identification of the protein or gene
that the amino acid or nucleotide sequence comprises. Amino acid
and nucleotide sequences can be evaluated either manually by one
skilled in the art, or by using computer-based sequence comparison
and identification tools that employ algorithms such as BLAST
(Basic Local Alignment Search Tool; Altschul et al. (1993) J. Mol.
Biol. 215:403-410; see also www.ncbi.nlm.nih.gov/BLAST- /). In
general, a sequence of ten or more contiguous amino acids or thirty
or more contiguous nucleotides is necessary in order to putatively
identify a polypeptide or nucleic acid sequence as homologous to a
known protein or gene. Moreover, with respect to nucleotide
sequences, gene-specific oligonucleotide probes comprising 30 or
more contiguous nucleotides may be used in sequence-dependent
methods of gene identification (e.g., Southern hybridization) and
isolation (e.g., in situ hybridization of bacterial colonies or
bacteriophage plaques). In addition, short oligonucleotides of 12
or more nucleotides may be used as amplification primers in PCR in
order to obtain a particular nucleic acid fragment comprising the
primers. Accordingly, a "substantial portion" of a nucleotide
sequence comprises a nucleotide sequence that will afford specific
identification and/or isolation of a nucleic acid fragment
comprising the sequence. The instant specification teaches amino
acid and nucleotide sequences encoding polypeptides that comprise
one or more particular plant proteins. The skilled artisan, having
the benefit of the sequences as reported herein, may now use all or
a substantial portion of the disclosed sequences for purposes known
to those skilled in this art. Accordingly, the instant invention
comprises the complete sequences as reported in the accompanying
Sequence Listing, as well as substantial portions of those
sequences as defined above.
[0023] "Codon degeneracy" refers to divergence in the genetic code
permitting variation of the nucleotide sequence without effecting
the amino acid sequence of an encoded polypeptide. Accordingly, the
instant invention relates to any nucleic acid fragment comprising a
nucleotide sequence that encodes all or a substantial portion of
the amino acid sequences set forth herein. The skilled artisan is
well aware of the "codon-bias" exhibited by a specific host cell in
usage of nucleotide codons to specify a given amino acid.
Therefore, when synthesizing a nucleic acid fragment for improved
expression in a host cell, it is desirable to design the nucleic
acid fragment such that its frequency of codon usage approaches the
frequency of preferred codon usage of the host cell.
[0024] "Synthetic nucleic acid fragments" can be assembled from
oligonucleotide building blocks that are chemically synthesized
using procedures known to those skilled in the art. These building
blocks are ligated and annealed to form larger nucleic acid
fragments which may then be enzymatically assembled to construct
the entire desired nucleic acid fragment. "Chemically synthesized",
as related to nucleic acid fragment, means that the component
nucleotides were assembled in vitro. Manual chemical synthesis of
nucleic acid fragments may be accomplished using well established
procedures, or automated chemical synthesis can be performed using
one of a number of commercially available machines. Accordingly,
the nucleic acid fragments can be tailored for optimal gene
expression based on optimization of nucleotide sequence to reflect
the codon bias of the host cell. The skilled artisan appreciates
the likelihood of successful gene expression if codon usage is
biased towards those codons favored by the host. Determination of
preferred codons can be based on a survey of genes derived from the
host cell where sequence information is available.
[0025] "Gene" refers to a nucleic acid fragment that expresses a
specific protein, including regulatory sequences preceding (5'
non-coding sequences) and following (3' non-coding sequences) the
coding sequence. "Native gene" refers to a gene as found in nature
with its own regulatory sequences. "Chimeric gene" refers any gene
that is not a native gene, comprising regulatory and coding
sequences that are not found together in nature. Accordingly, a
chimeric gene may comprise regulatory sequences and coding
sequences that are derived from different sources, or regulatory
sequences and coding sequences derived from the same source, but
arranged in a manner different than that found in nature.
"Endogenous gene" refers to a native gene in its natural location
in the genome of an organism. A "foreign" gene refers to a gene not
normally found in the host organism, but that is introduced into
the host organism by gene transfer. Foreign genes can comprise
native genes inserted into a non-native organism, or chimeric
genes. A "transgene" is a gene that has been introduced into the
genome by a transformation procedure.
[0026] "Coding sequence" refers to a nucleotide sequence that codes
for a specific amino acid sequence. "Regulatory sequences" refer to
nucleotide sequences located upstream (5' non-coding sequences),
within, or downstream (3' non-coding sequences) of a coding
sequence, and which influence the transcription, RNA processing or
stability, or translation of the associated coding sequence.
Regulatory sequences may include promoters, translation leader
sequences, introns, and polyadenylation recognition sequences.
[0027] "Promoter" refers to a nucleotide sequence capable of
controlling the expression of a coding sequence or functional RNA.
In general, a coding sequence is located 3' to a promoter sequence.
The promoter sequence consists of proximal and more distal upstream
elements, the latter elements often referred to as enhancers.
Accordingly, an "enhancer" is a nucleotide sequence which can
stimulate promoter activity and may be an innate element of the
promoter or a heterologous element inserted to enhance the level or
tissue-specificity of a promoter. Promoters may be derived in their
entirety from a native gene, or be composed of different elements
derived from different promoters found in nature, or even comprise
synthetic nucleotide segments. It is understood by those skilled in
the art that different promoters may direct the expression of a
gene in different tissues or cell types, or at different stages of
development, or in response to different environmental conditions.
Promoters which cause a nucleic acid fragment to be expressed in
most cell types at most times are commonly referred to as
"constitutive promoters". New promoters of various types useful in
plant cells are constantly being discovered; numerous examples may
be found in the compilation by Okamuro and Goldberg (1989)
Biochemistry of Plants 15:1-82. It is further recognized that since
in most cases the exact boundaries of regulatory sequences have not
been completely defined, nucleic acid fragments of different
lengths may have identical promoter activity.
[0028] The "translation leader sequence" refers to a nucleotide
sequence located between the promoter sequence of a gene and the
coding sequence. The translation leader sequence is present in the
fully processed mRNA upstream of the translation start sequence.
The translation leader sequence may affect processing of the
primary transcript to mRNA, mRNA stability or translation
efficiency. Examples of translation leader sequences have been
described (Turner and Foster (1995) Molecular Biotechnology
3:225).
[0029] The "3' non-coding sequences" refer to nucleotide sequences
located downstream of a coding sequence and include polyadenylation
recognition sequences and other sequences encoding regulatory
signals capable of affecting mRNA processing or gene expression.
The polyadenylation signal is usually characterized by affecting
the addition of polyadenylic acid tracts to the 3' end of the mRNA
precursor. The use of different 3' non-coding sequences is
exemplified by Ingelbrecht et al. (1989) Plant Cell 1:671-680.
[0030] "RNA transcript" refers to the product resulting from RNA
polymerase-catalyzed transcription of a DNA sequence. When the RNA
transcript is a perfect complementary copy of the DNA sequence, it
is referred to as the primary transcript or it may be a RNA
sequence derived from posttranscriptional processing of the primary
transcript and is referred to as the mature RNA. "Messenger RNA
(mRNA)" refers to the RNA that is without introns and that can be
translated into polypeptide by the cell. "cDNA" refers to a
double-stranded DNA that is complementary to and derived from mRNA.
"Sense" RNA refers to an RNA transcript that includes the mRNA and
so can be translated into a polypeptide by the cell. "Antisense
RNA" refers to an RNA transcript that is complementary to all or
part of a target primary transcript or mRNA and that blocks the
expression of a target gene (see U.S. Pat. No. 5,107,065,
incorporated herein by reference). The complementarity of an
antisense RNA may be with any part of the specific nucleotide
sequence, i.e., at the 5' non-coding sequence, 3' non-coding
sequence, introns, or the coding sequence. "Functional RNA" refers
to sense RNA, antisense RNA, ribozyme RNA, or other RNA that may
not be translated but yet has an effect on cellular processes.
[0031] The term "operably linked" refers to the association of two
or more nucleic acid fragments on a single nucleic acid fragment so
that the function of one is affected by the other. For example, a
promoter is operably linked with a coding sequence when it is
capable of affecting the expression of that coding sequence (i.e.,
that the coding sequence is under the transcriptional control of
the promoter). Coding sequences can be operably linked to
regulatory sequences in sense or antisense orientation.
[0032] The term "expression", as used herein, refers to the
transcription and stable accumulation of sense (mRNA) or antisense
RNA derived from the nucleic acid fragment of the invention.
Expression may also refer to translation of mRNA into a
polypeptide. "Antisense inhibition" refers to the production of
antisense RNA transcripts capable of suppressing the expression of
the target protein. "Overexpression" refers to the production of a
gene product in transgenic organisms that exceeds levels of
production in normal or non-transformed organisms. "Co-suppression"
refers to the production of sense RNA transcripts capable of
suppressing the expression of identical or substantially similar
foreign or endogenous genes (U.S. Pat. No. 5,231,020, incorporated
herein by reference).
[0033] "Altered levels" refers to the production of gene product(s)
in transgenic organisms in amounts or proportions that differ from
that of normal or non-transformed organisms.
[0034] "Mature" protein refers to a post-translationally processed
polypeptide; i.e., one from which any pre- or propeptides present
in the primary translation product have been removed. "Precursor"
protein refers to the primary product of translation of mRNA; i.e.,
with pre- and propeptides still present. Pre- and propeptides may
be but are not limited to intracellular localization signals.
[0035] A "chloroplast transit peptide" is an amino acid sequence
which is translated in conjunction with a protein and directs the
protein to the chloroplast or other plastid types present in the
cell in which the protein is made. "Chloroplast transit sequence"
refers to a nucleotide sequence that encodes a chloroplast transit
peptide. A "signal peptide" is an amino acid sequence which is
translated in conjunction with a protein and directs the protein to
the secretory system (Chrispeels (1991) Ann. Rev. Plant Phys. Plant
Mol. Biol. 42:21-53). If the protein is to be directed to a
vacuole, a vacuolar targeting signal (supra) can further be added,
or if to the endoplasmic reticulum, an endoplasmic reticulum
retention signal (supra) may be added. If the protein is to be
directed to the nucleus, any signal peptide present should be
removed and instead a nuclear localization signal included (Raikhel
(1992) Plant Phys. 100:1627-1632).
[0036] "Transformation" refers to the transfer of a nucleic acid
fragment into the genome of a host organism, resulting in
genetically stable inheritance. Host organisms containing the
transformed nucleic acid fragments are referred to as "transgenic"
organisms. Examples of methods of plant transformation include
Agrobacterium-mediated transformation (De Blaere et al. (1987)
Meth. Enzymol. 143:277) and particle-accelerated or "gene gun"
transformation technology (Klein et al. (1987) Nature (London)
327:70-73; U.S. Pat. No. 4,945,050, incorporated herein by
reference).
[0037] Standard recombinant DNA and molecular cloning techniques
used herein are well known in the art and are described more fully
in Sambrook et al. Molecular Cloning: A Laboratory Manual; Cold
Spring Harbor Laboratory Press: Cold Spring Harbor, 1989
(hereinafter "Maniatis").
[0038] Nucleic acid fragments encoding at least a portion of
several enzymes involved in aromatic amino acid catabolism have
been isolated and identified by comparison of random plant cDNA
sequences to public databases containing nucleotide and protein
sequences using the BLAST algorithms well known to those skilled in
the art. The nucleic acid fragments of the instant invention may be
used to isolate cDNAs and genes encoding homologous proteins from
the same or other plant species. Isolation of homologous genes
using sequence-dependent protocols is well known in the art.
Examples of sequence-dependent protocols include, but are not
limited to, methods of nucleic acid hybridization, and methods of
DNA and RNA amplification as exemplified by various uses of nucleic
acid amplification technologies (e.g., polymerase chain reaction,
ligase chain reaction).
[0039] For example, genes encoding other homogentisate
1,2-dioxygenases, fumarylacetoacetases or nitrile hydratases,
either as cDNAs or genomic DNAs, could be isolated directly by
using all or a portion of the instant nucleic acid fragments as DNA
hybridization probes to screen libraries from any desired plant
employing methodology well known to those skilled in the art.
Specific oligonucleotide probes based upon the instant nucleic acid
sequences can be designed and synthesized by methods known in the
art (Maniatis). Moreover, the entire sequences can be used directly
to synthesize DNA probes by methods known to the skilled artisan
such as random primer DNA labeling, nick translation, or
end-labeling techniques, or RNA probes using available in vitro
transcription systems. In addition, specific primers can be
designed and used to amplify a part or all of the instant
sequences. The resulting amplification products can be labeled
directly during amplification reactions or labeled after
amplification reactions, and used as probes to isolate full length
cDNA or genomic fragments under conditions of appropriate
stringency.
[0040] In addition, two short segments of the instant nucleic acid
fragments may be used in polymerase chain reaction protocols to
amplify longer nucleic acid fragments encoding homologous genes
from DNA or RNA. The polymerase chain reaction may also be
performed on a library of cloned nucleic acid fragments wherein the
sequence of one primer is derived from the instant nucleic acid
fragments, and the sequence of the other primer takes advantage of
the presence of the polyadenylic acid tracts to the 3' end of the
mRNA precursor encoding plant genes. Alternatively, the second
primer sequence may be based upon sequences derived from the
cloning vector. For example, the skilled artisan can follow the
RACE protocol (Frohman et al. (1988) Proc. Natl. Acad. Sci. USA
85:8998) to generate cDNAs by using PCR to amplify copies of the
region between a single point in the transcript and the 3' or 5'
end. Primers oriented in the 3' and 5' directions can be designed
from the instant sequences. Using commercially available 3' RACE or
5' RACE systems (BRL), specific 3' or 5' cDNA fragments can be
isolated (Ohara et al. (1989) Proc. Natl. Acad. Sci. USA 86:5673;
Loh et al. (1989) Science 243:217). Products generated by the 3'
and 5' RACE procedures can be combined to generate full-length
cDNAs (Frohman and Martin (1989) Techniques 1:165).
[0041] Availability of the instant nucleotide and deduced amino
acid sequences facilitates immunological screening of cDNA
expression libraries. Synthetic peptides representing portions of
the instant amino acid sequences may be synthesized. These peptides
can be used to immunize animals to produce polyclonal or monoclonal
antibodies with specificity for peptides or proteins comprising the
amino acid sequences. These antibodies can be then be used to
screen cDNA expression libraries to isolate full-length cDNA clones
of interest (Lerner (1984) Adv. Immunol. 36:1; Maniatis).
[0042] The nucleic acid fragments of the instant invention may be
used to create transgenic plants in which the disclosed
polypeptides are present at higher or lower levels than normal or
in cell types or developmental stages in which they are not
normally found. This would have the effect of altering the level of
aromatic amino acids or their intermediates in those cells.
[0043] Overexpression of the proteins of the instant invention may
be accomplished by first constructing a chimeric gene in which the
coding region is operably linked to a promoter capable of directing
expression of a gene in the desired tissues at the desired stage of
development. For reasons of convenience, the chimeric gene may
comprise promoter sequences and translation leader sequences
derived from the same genes. 3' Non-coding sequences encoding
transcription termination signals may also be provided. The instant
chimeric gene may also comprise one or more introns in order to
facilitate gene expression.
[0044] Plasmid vectors comprising the instant chimeric gene can
then constructed. The choice of plasmid vector is dependent upon
the method that will be used to transform host plants. The skilled
artisan is well aware of the genetic elements that must be present
on the plasmid vector in order to successfully transform, select
and propagate host cells containing the chimeric gene. The skilled
artisan will also recognize that different independent
transformation events will result in different levels and patterns
of expression (Jones et al. (1985) EMBO J. 4:2411-2418; De Almeida
et al. (1989) Mol. Gen. Genetics 218:78-86), and thus that multiple
events must be screened in order to obtain lines displaying the
desired expression level and pattern. Such screening may be
accomplished by Southern analysis of DNA, Northern analysis of mRNA
expression, Western analysis of protein expression, or phenotypic
analysis.
[0045] For some applications it may be useful to direct the instant
polypeptides to different cellular compartments, or to facilitate
its secretion from the cell. It is thus envisioned that the
chimeric gene described above may be further supplemented by
altering the coding sequence to encode the instant polypeptides
with appropriate intracellular targeting sequences such as transit
sequences (Keegstra (1989) Cell 56:247-253), signal sequences or
sequences encoding endoplasmic reticulum localization (Chrispeels
(1991) Ann. Rev. Plant Phys. Plant Mol. Biol. 42:21-53), or nuclear
localization signals (Raikhel (1992) Plant Phys.100:1627-1632)
added and/or with targeting sequences that are already present
removed. While the references cited give examples of each of these,
the list is not exhaustive and more targeting signals of utility
may be discovered in the future.
[0046] It may also be desirable to reduce or eliminate expression
of genes encoding the instant polypeptides in plants for some
applications. In order to accomplish this, a chimeric gene designed
for co-suppression of the instant polypeptide can be constructed by
linking a gene or gene fragment encoding that polypeptide to plant
promoter sequences. Alternatively, a chimeric gene designed to
express antisense RNA for all or part of the instant nucleic acid
fragment can be constructed by linking the gene or gene fragment in
reverse orientation to plant promoter sequences. Either the
co-suppression or antisense chimeric genes could be introduced into
plants via transformation wherein expression of the corresponding
endogenous genes are reduced or eliminated.
[0047] Molecular genetic solutions to the generation of plants with
altered gene expression have a decided advantage over more
traditional plant breeding approaches. Changes in plant phenotypes
can be produced by specifically inhibiting expression of one or
more genes by antisense inhibition or cosuppression (U.S. Pat. Nos.
5,190,931, 5,107,065 and 5,283,323). An antisense or cosuppression
construct would act as a dominant negative regulator of gene
activity. While conventional mutations can yield negative
regulation of gene activity these effects are most likely
recessive. The dominant negative regulation available with a
transgenic approach may be advantageous from a breeding
perspective. In addition, the ability to restrict the expression of
specific phenotype to the reproductive tissues of the plant by the
use of tissue specific promoters may confer agronomic advantages
relative to conventional mutations which may have an effect in all
tissues in which a mutant gene is ordinarily expressed.
[0048] The person skilled in the art will know that special
considerations are associated with the use of antisense or
cosuppresion technologies in order to reduce expression of
particular genes. For example, the proper level of expression of
sense or antisense genes may require the use of different chimeric
genes utilizing different regulatory elements known to the skilled
artisan. Once transgenic plants are obtained by one of the methods
described above, it will be necessary to screen individual
transgenics for those that most effectively display the desired
phenotype. Accordingly, the skilled artisan will develop methods
for screening large numbers of transformants. The nature of these
screens will generally be chosen on practical grounds, and is not
an inherent part of the invention. For example, one can screen by
looking for changes in gene expression by using antibodies specific
for the protein encoded by the gene being suppressed, or one could
establish assays that specifically measure enzyme activity. A
preferred method will be one which allows large numbers of samples
to be processed rapidly, since it will be expected that a large
number of transformants will be negative for the desired
phenotype.
[0049] The instant polypeptides (or portions thereof) may be
produced in heterologous host cells, particularly in the cells of
microbial hosts, and can be used to prepare antibodies to the these
proteins by methods well known to those skilled in the art. The
antibodies are useful for detecting the polypeptides of the instant
invention in situ in cells or in vitro in cell extracts. Preferred
heterologous host cells for production of the instant polypeptides
are microbial hosts. Microbial expression systems and expression
vectors containing regulatory sequences that direct high level
expression of foreign proteins are well known to those skilled in
the art. Any of these could be used to construct a chimeric gene
for production of the instant polypeptides. This chimeric gene
could then be introduced into appropriate microorganisms via
transformation to provide high level expression of the encoded
aromatic amino acid catabolic enzymes. An example of a vector for
high level expression of the instant polypeptides in a bacterial
host is provided (Example 7).
[0050] Additionally, the instant polypeptides can be used as
targets to facilitate design and/or identification of inhibitors of
those enzymes that may be useful as herbicides. This is desirable
because the polypeptides described herein catalyze various steps in
aromatic amino acid catabolism. Accordingly, inhibition of the
activity of one or more of the enzymes described herein could lead
to inhibition plant growth. Thus, the instant polypeptides could be
appropriate for new herbicide discovery and design.
[0051] All or a substantial portion of the nucleic acid fragments
of the instant invention may also be used as probes for genetically
and physically mapping the genes that they are a part of, and as
markers for traits linked to those genes. Such information may be
useful in plant breeding in order to develop lines with desired
phenotypes. For example, the instant nucleic acid fragments may be
used as restriction fragment length polymorphism (RFLP) markers.
Southern blots (Maniatis) of restriction-digested plant genomic DNA
may be probed with the nucleic acid fragments of the instant
invention. The resulting banding patterns may then be subjected to
genetic analyses using computer programs such as MapMaker (Lander
et al. (1987) Genomics 1:174-181) in order to construct a genetic
map. In addition, the nucleic acid fragments of the instant
invention may be used to probe Southern blots containing
restriction endonuclease-treated genomic DNAs of a set of
individuals representing parent and progeny of a defined genetic
cross. Segregation of the DNA polymorphisms is noted and used to
calculate the position of the instant nucleic acid sequence in the
genetic map previously obtained using this population (Botstein et
al. (1980) Am. J. Hum. Genet. 32:314-331).
[0052] The production and use of plant gene-derived probes for use
in genetic mapping is described in Bernatzky and Tanksley (1986)
Plant Mol. Biol. Reporter 4(1):37-41. Numerous publications
describe genetic mapping of specific cDNA clones using the
methodology outlined above or variations thereof. For example, F2
intercross populations, backcross populations, randomly mated
populations, near isogenic lines, and other sets of individuals may
be used for mapping. Such methodologies are well known to those
skilled in the art.
[0053] Nucleic acid probes derived from the instant nucleic acid
sequences may also be used for physical mapping (i.e., placement of
sequences on physical maps; see Hoheisel et al. In: Nonmammalian
Genomic Analysis: A Practical Guide, Academic press 1996, pp.
319-346, and references cited therein).
[0054] In another embodiment, nucleic acid probes derived from the
instant nucleic acid sequences may be used in direct fluorescence
in situ hybridization (FISH) mapping (Trask (1991) Trends Genet.
7:149-154). Although current methods of FISH mapping favor use of
large clones (several to several hundred KB; see Laan et al. (1995)
Genome Research 5:13-20), improvements in sensitivity may allow
performance of FISH mapping using shorter probes.
[0055] A variety of nucleic acid amplification-based methods of
genetic and physical mapping may be carried out using the instant
nucleic acid sequences. Examples include allele-specific
amplification (Kazazian (1989) J. Lab. Clin. Med. 114(2):95-96),
polymorphism of PCR-amplified fragments (CAPS; Sheffield et al.
(1993) Genomics 16:325-332), allele-specific ligation (Landegren et
al. (1988) Science 241:1077-1080), nucleotide extension reactions
(Sokolov (1990) Nucleic Acid Res. 18:3671), Radiation Hybrid
Mapping (Walter et al. (1997) Nature Genetics 7:22-28) and Happy
Mapping (Dear and Cook (1989) Nucleic Acid Res. 17:6795-6807). For
these methods, the sequence of a nucleic acid fragment is used to
design and produce primer pairs for use in the amplification
reaction or in primer extension reactions. The design of such
primers is well known to those skilled in the art. In methods
employing PCR-based genetic mapping, it may be necessary to
identify DNA sequence differences between the parents of the
mapping cross in the region corresponding to the instant nucleic
acid sequence. This, however, is generally not necessary for
mapping methods.
[0056] Loss of function mutant phenotypes may be identified for the
instant cDNA clones either by targeted gene disruption protocols or
by identifying specific mutants for these genes contained in a
maize population carrying mutations in all possible genes
(Ballinger and Benzer (1989) Proc. Natl. Acad. Sci USA 86:9402;
Koes et al. (1995) Proc. Natl. Acad. Sci USA 92:8149; Bensen et al.
(1995) Plant Cell 7:75). The latter approach may be accomplished in
two ways. First, short segments of the instant nucleic acid
fragments may be used in polymerase chain reaction protocols in
conjunction with a mutation tag sequence primer on DNAs prepared
from a population of plants in which Mutator transposons or some
other mutation-causing DNA element has been introduced (see Bensen,
supra). The amplification of a specific DNA fragment with these
primers indicates the insertion of the mutation tag element in or
near the plant gene encoding the instant polypeptides.
Alternatively, the instant nucleic acid fragment may be used as a
hybridization probe against PCR amplification products generated
from the mutation population using the mutation tag sequence primer
in conjunction with an arbitrary genomic site primer, such as that
for a restriction enzyme site-anchored synthetic adaptor. With
either method, a plant containing a mutation in the endogenous gene
encoding the instant polypeptides can be identified and obtained.
This mutant plant can then be used to determine or confirm the
natural function of the instant polypeptides disclosed herein.
EXAMPLES
[0057] The present invention is further defined in the following
Examples, in which all parts and percentages are by weight and
degrees are Celsius, unless otherwise stated. It should be
understood that these Examples, while indicating preferred
embodiments of the invention, are given by way of illustration
only. From the above discussion and these Examples, one skilled in
the art can ascertain the essential characteristics of this
invention, and without departing from the spirit and scope thereof,
can make various changes and modifications of the invention to
adapt it to various usages and conditions.
Example 1
Composition of cDNA Libraries; Isolation and Sequencing of cDNA
Clones
[0058] cDNA libraries representing mRNAs from various corn, rice,
soybean and wheat tissues were prepared. The characteristics of the
libraries are described below.
2TABLE 2 cDNA Libraries from Corn, Rice, Soybean and Wheat Library
Tissue Clone cbn2 Corn Developing Kernel Two Days cbn2.pk0052.e6
After Pollination cc71se-b Corn Callus Type II Tissue, Somatic
cc71se-b.pk0004.b5 Embryo Formed cr1n Corn Root From 7 Day Old
Seedlings* cr1n.pk0107.d3 cn1n.pk0151.e7 r10n Rice 15 Day Old Leaf*
r10n.pk082.n4 r1s6 Rice Leaf 15 Days After Germination,
r1s6.pk0027.h11 6 Hours After Infection of Strain Magaporthe grisea
4360-R-67 (AVR2-YAMO); Susceptible sfl1 Soybean Immature Flower
sfl1.pk0008.h2 sgs6c Soybean Seeds 8 Days After sgs6c.pk001.h5
Germination wlk8 Wheat Seedlings 8 Hours After wlk8.pk0020.a11
Treatment With Herbicide** *These libraries were normalized
essentially as described in U.S. Pat. No. 5,482,845, incorporated
herein by reference. **Application of
6-iodo-2-propoxy-3-propyl-4(3H)-quinazolinone; synthesis and
methods of using this compound are described in USSN 08/545,827,
incorporated herein by reference.
[0059] cDNA libraries may be prepared by any one of many methods
available. For example, the cDNAs may be introduced into plasmid
vectors by first preparing the cDNA libraries in Uni-ZAP.TM. XR
vectors according to the manufacturer's protocol (Stratagene
Cloning Systems, La Jolla, Calif.). The Uni-ZAP.TM. XR libraries
are converted into plasmid libraries according to the protocol
provided by Stratagene. Upon conversion, cDNA inserts will be
contained in the plasmid vector pBluescript. In addition, the cDNAs
may be introduced directly into precut Bluescript II SK(+) vectors
(Stratagene) using T4 DNA ligase (New England Biolabs), followed by
transfection into DH10B cells according to the manufacturer's
protocol (GIBCO BRL Products). Once the cDNA inserts are in plasmid
vectors, plasmid DNAs are prepared from randomly picked bacterial
colonies containing recombinant pBluescript plasmids, or the insert
cDNA sequences are amplified via polymerase chain reaction using
primers specific for vector sequences flanking the inserted cDNA
sequences. Amplified insert DNAs or plasmid DNAs are sequenced in
dye-primer sequencing reactions to generate partial cDNA sequences
(expressed sequence tags or "ESTs"; see Adams et al., (1991)
Science 252:1651). The resulting ESTs are analyzed using a Perkin
Elmer Model 377 fluorescent sequencer.
Example 2
Identification of cDNA Clones
[0060] cDNA clones encoding enzymes involved in aromatic amino acid
catabolism were identified by conducting BLAST (Basic Local
Alignment Search Tool; Altschul et al. (1993) J. Mol. Biol.
215:403-410; see also www.ncbi.nlm.nih.gov/BLAST/) searches for
similarity to sequences contained in the BLAST "nr" database
(comprising all non-redundant GenBank CDS translations, sequences
derived from the 3-dimensional structure Brookhaven Protein Data
Bank, the last major release of the SWISS-PROT protein sequence
database, EMBL, and DDBJ databases). The cDNA sequences obtained in
Example 1 were analyzed for similarity to all publicly available
DNA sequences contained in the "nr" database using the BLASTN
algorithm provided by the National Center for Biotechnology
Information (NCBI). The DNA sequences were translated in all
reading frames and compared for similarity to all publicly
available protein sequences contained in the "nr" database using
the BLASTX algorithm (Gish and States (1993) Nature Genetics
3:266-272) provided by the NCBI. For convenience, the P-value
(probability) of observing a match of a cDNA sequence to a sequence
contained in the searched databases merely by chance as calculated
by BLAST are reported herein as "pLog" values, which represent the
negative of the logarithm of the reported P-value. Accordingly, the
greater the pLog value, the greater the likelihood that the cDNA
sequence and the BLAST "hit" represent homologous proteins.
Example 3
Characterization of cDNA Clones Encoding Homogentisate
1,2-Dioxygenase
[0061] The BLASTX search using the EST sequences from clones listed
in Table 3 revealed similarity of the polypeptides encoded by the
cDNAs to homogentisate 1,2-dioxygenase from Arabidopsis thaliana
(NCBI General Identifier No. 4098647). Shown in Table 3 are the
BLAST results for the sequences of the entire cDNA inserts
comprising the indicated cDNA clones ("FIS"):
3TABLE 3 BLAST Results for Sequences Encoding Polypeptides
Homologous to Homogentisate 1,2-Dioxygenase BLAST pLog Score Clone
Status 4098647 cbn2.pk0052.e6 FIS 254.00 r1s6.pk0027.h11 FIS 254.00
sfl1.pk0008.h2 FIS 254.00 wlk8.pk0020.a11 FIS 133.00
[0062] Nucleotides 952 through 1656 from clone cbn2.pk0052.e6 are
99% identical to nucleotides 1 through 705 of a 719 nt rice EST
having NCBI General Identifier No. 4714775. Nucleotides 255 through
613 are 85% identical to nucleotides 1 through 359 of a 719 nt rice
EST having NCBI General Identifier No. 4714775. The data in Table 4
represents a calculation of the percent identity of the amino acid
sequences set forth in SEQ ID NOs:2, 4, 6 and 8 and the Arabidopsis
thaliana sequence (SEQ ID NO:9).
4TABLE 4 Percent Identity of Amino Acid Sequences Deduced From the
Nucleotide Sequences of cDNA Clones Encoding Polypeptides
Homologous to Homogentisate 1,2-Dioxygenase Percent Identity to SEQ
ID NO. 4098647 2 73.2 4 72.7 6 76.3 8 76.2
[0063] Sequence alignments and percent identity calculations were
performed using the Megalign program of the LASARGENE
bioinformatics computing suite (DNASTAR Inc., Madison, Wis.).
Multiple alignment of the sequences was performed using the Clustal
method of alignment (Higgins and Sharp (1989) CABIOS. 5:151-153)
with the default parameters (GAP PENALTY=10, GAP LENGTH
PENALTY=10). Default parameters for pairwise alignments using the
Clustal method were KTUPLE 1, GAP PENALTY=3, WINDOW=5 and DIAGONALS
SAVED=5. Sequence alignments and BLAST scores and probabilities
indicate that the nucleic acid fragments comprising the instant
cDNA clones encode entire corn, rice and soybean homogentisate
1,2-dioxygenase and a substantial portion of a wheat homogentisate
1,2-dioxygenase. These sequences represent the first corn, rice,
soybean and wheat sequences encoding homogentisate
1,2-dioxygenase.
Example 4
Characterization of cDNA Clones Encoding Fumarylacetoacetase
[0064] The BLASTX search using the EST sequences from clones listed
in Table 5 revealed similarity of the polypeptides encoded by the
cDNAs to fumarylacetoacetase from Arabidopsis thaliana (NCBI
General Identifier No. 3157928). Shown in Table 5 are the BLAST
results for the sequences of the entire cDNA inserts comprising the
indicated cDNA clones ("FIS"), or contigs assembled from two or
more ESTs ("Contig"):
5TABLE 5 BLAST Results for Sequences Encoding Polypeptides
Homologous to Fumarylacetoacetase BLAST pLog Score Clone Status
3157928 Contig of: Contig 107.00 cc71se-b.pk0004.b5 cr1n.pk0107.d3
cr1n.pk0151.e7 rl0n.pk082.n4 FIS 169.00 sgs6c.pk001.h5 FIS
180.00
[0065] Nucleotides 457 through 842 from the corn contig (SEQ ID
NO:9) are 92% identical to nucleotides 599 through 212 of a corn
EST having NCBI General Identifier No. 4730752. Nucleotides 414
through 782 from rice clone r10n.pk082.n4 are 99% identical to
nucleotides 1 through 369 of a 382 nt rice EST having NCBI General
Identifier No. 4969133. Nucleotides 53 to 329 from the same clone
are 97% identical to nucleotides 1 through 277 of a 277 nt rice EST
having NCBI General Identifier No. 2312281. Nucleotides 993 through
1310 from clone sgs6c.pk001.h5 are 94% identical to nucleotides 156
through 473 of a 474 nt soybean EST having NCBI General Identifier
No.4292828. Nucleotides 1117 through 1304 from the same clone are
92% identical to nucleotides 1 through 189 or a 393 soybean EST
having NCBI General Identifier No. 9397647. The data in Table 6
represents a calculation of the percent identity of the amino acid
sequences set forth in SEQ ID NOs: 11, 13 and 15 and the
Arabidopsis thaliana sequence (SEQ ID NO:16).
6TABLE 6 Percent Identity of Amino Acid Sequences Deduced From the
Nucleotide Sequences of cDNA Clones Encoding Polypeptides
Homologous to Fumarylacetoacetase Percent Identity to SEQ ID NO.
3157928 10 63.0 12 65.0 14 71.1
[0066] Sequence alignments and percent identity calculations were
performed using the Megalign program of the LASARGENE
bioinformatics computing suite (DNASTAR Inc., Madison, Wis.).
Multiple alignment of the sequences was performed using the Clustal
method of alignment (Higgins and Sharp (1989) CABIOS. 5:151-153)
with the default parameters (GAP PENALTY=10, GAP LENGTH
PENALTY=10). Default parameters for pairwise alignments using the
Clustal method were KTUPLE 1, GAP PENALTY=3, WINDOW=5 and DIAGONALS
SAVED=5. Sequence alignments and BLAST scores and probabilities
indicate that the nucleic acid fragments comprising the instant
cDNA clones encode a substantial portion of a corn
fumarylacetoacetase and entire rice and soybean
fumarylacetoacetase. These sequences represent the first corn, rice
and soybean sequences encoding fumarylacetoacetase.
Example 5
Expression of Chimeric Genes in Monocot Cells
[0067] A chimeric gene comprising a cDNA encoding the instant
polypeptides in sense orientation with respect to the maize 27 kD
zein promoter that is located 5' to the cDNA fragment, and the 10
kD zein 3' end that is located 3' to the cDNA fragment, can be
constructed. The cDNA fragment of this gene may be generated by
polymerase chain reaction (PCR) of the cDNA clone using appropriate
oligonucleotide primers. Cloning sites (NcoI or SmaI) can be
incorporated into the oligonucleotides to provide proper
orientation of the DNA fragment when inserted into the digested
vector pML103 as described below. Amplification is then performed
in a standard PCR. The amplified DNA is then digested with
restriction enzymes NcoI and SmaI and fractionated on an agarose
gel. The appropriate band can be isolated from the gel and combined
with a 4.9 kb NcoI-SmaI fragment of the plasmid pML103. Plasmid pML
103 has been deposited under the terms of the Budapest Treaty at
ATCC (American Type Culture Collection, 10801 University Blvd.,
Manassas, Va. 20110-2209), and bears accession number ATCC 97366.
The DNA segment from pML103 contains a 1.05 kb SalI-NcoI promoter
fragment of the maize 27 kD zein gene and a 0.96 kb SmaI-SalI
fragment from the 3' end of the maize 10 kD zein gene in the vector
pGem9Zf(+) (Promega). Vector and insert DNA can be ligated at
15.degree. C. overnight, essentially as described (Maniatis). The
ligated DNA may then be used to transform E. coli XL1-Blue
(Epicurian Coli XL-1 Blue.TM.; Stratagene). Bacterial transformants
can be screened by restriction enzyme digestion of plasmid DNA and
limited nucleotide sequence analysis using the dideoxy chain
termination method (Sequenase.TM. DNA Sequencing Kit; U.S.
Biochemical). The resulting plasmid construct would comprise a
chimeric gene encoding, in the 5' to 3' direction, the maize 27 kD
zein promoter, a cDNA fragment encoding the instant polypeptides,
and the 10 kD zein 3' region.
[0068] The chimeric gene described above can then be introduced
into corn cells by the following procedure. Immature corn embryos
can be dissected from developing caryopses derived from crosses of
the inbred corn lines H99 and LH132. The embryos are isolated 10 to
11 days after pollination when they are 1.0 to 1.5 mm long. The
embryos are then placed with the axis-side facing down and in
contact with agarose-solidified N6 medium (Chu et al. (1975) Sci.
Sin. Peking 18:659-668). The embryos are kept in the dark at
27.degree. C. Friable embryogenic callus consisting of
undifferentiated masses of cells with somatic proembryoids and
embryoids borne on suspensor structures proliferates from the
scutellum of these immature embryos. The embryogenic callus
isolated from the primary explant can be cultured on N6 medium and
sub-cultured on this medium every 2 to 3 weeks.
[0069] The plasmid, p35S/Ac (obtained from Dr. Peter Eckes, Hoechst
Ag, Frankfurt, Germany) may be used in transformation experiments
in order to provide for a selectable marker. This plasmid contains
the Pat gene (see European Patent Publication 0 242 236) which
encodes phosphinothricin acetyl transferase (PAT). The enzyme PAT
confers resistance to herbicidal glutamine synthetase inhibitors
such as phosphinothricin. The pat gene in p35S/Ac is under the
control of the 35S promoter from Cauliflower Mosaic Virus (Odell et
al. (1985) Nature 313:810-812) and the 3' region of the nopaline
synthase gene from the T-DNA of the Ti plasmid of Agrobacterium
tumefaciens.
[0070] The particle bombardment method (Klein et al. (1987) Nature
327:70-73) may be used to transfer genes to the callus culture
cells. According to this method, gold particles (1 .mu.m in
diameter) are coated with DNA using the following technique. Ten
.mu.g of plasmid DNAs are added to 50 .mu.L of a suspension of gold
particles (60 mg per mL). Calcium chloride (50 .mu.L of a 2.5 M
solution) and spermidine free base (20 .mu.L of a 1.0 M solution)
are added to the particles. The suspension is vortexed during the
addition of these solutions. After 10 minutes, the tubes are
briefly centrifuged (5 sec at 15,000 rpm) and the supernatant
removed. The particles are resuspended in 200 .mu.L of absolute
ethanol, centrifuged again and the supernatant removed. The ethanol
rinse is performed again and the particles resuspended in a final
volume of 30 .mu.L of ethanol. An aliquot (5 .mu.L) of the
DNA-coated gold particles can be placed in the center of a
Kapton.TM. flying disc (Bio-Rad Labs). The particles are then
accelerated into the corn tissue with a Biolistic.TM. PDS-1000/He
(Bio-Rad Instruments, Hercules, Calif.), using a helium pressure of
1000 psi, a gap distance of 0.5 cm and a flying distance of 1.0
cm.
[0071] For bombardment, the embryogenic tissue is placed on filter
paper over agarose-solidified N6 medium. The tissue is arranged as
a thin lawn and covered a circular area of about 5 cm in diameter.
The petri dish containing the tissue can be placed in the chamber
of the PDS-1000/He approximately 8 cm from the stopping screen. The
air in the chamber is then evacuated to a vacuum of 28 inches of
Hg. The macrocarrier is accelerated with a helium shock wave using
a rupture membrane that bursts when the He pressure in the shock
tube reaches 1000 psi.
[0072] Seven days after bombardment the tissue can be transferred
to N6 medium that contains gluphosinate (2 mg per liter) and lacks
casein or proline. The tissue continues to grow slowly on this
medium. After an additional 2 weeks the tissue can be transferred
to fresh N6 medium containing gluphosinate. After 6 weeks, areas of
about 1 cm in diameter of actively growing callus can be identified
on some of the plates containing the glufosinate-supplemented
medium. These calli may continue to grow when sub-cultured on the
selective medium.
[0073] Plants can be regenerated from the transgenic callus by
first transferring clusters of tissue to N6 medium supplemented
with 0.2 mg per liter of 2,4-D. After two weeks the tissue can be
transferred to regeneration medium (Fromm et al. (1990)
Bio/Technology 8:833-839).
Example 6
Expression of Chimeric Genes in Dicot Cells
[0074] A seed-specific expression cassette composed of the promoter
and transcription terminator from the gene encoding the .beta.
subunit of the seed storage protein phaseolin from the bean
Phaseolus vulgaris (Doyle et al. (1986) J. Biol. Chem.
261:9228-9238) can be used for expression of the instant
polypeptides in transformed soybean. The phaseolin cassette
includes about 500 nucleotides upstream (5') from the translation
initiation codon and about 1650 nucleotides downstream (3') from
the translation stop codon of phaseolin. Between the 5' and 3'
regions are the unique restriction endonuclease sites Nco I (which
includes the ATG translation initiation codon), Sma I, Kpn I and
Xba I. The entire cassette is flanked by Hind III sites.
[0075] The cDNA fragment of this gene may be generated by
polymerase chain reaction (PCR) of the cDNA clone using appropriate
oligonucleotide primers. Cloning sites can be incorporated into the
oligonucleotides to provide proper orientation of the DNA fragment
when inserted into the expression vector. Amplification is then
performed as described above, and the isolated fragment is inserted
into a pUC18 vector carrying the seed expression cassette.
[0076] Soybean embroys may then be transformed with the expression
vector comprising sequences encoding the instant polypeptides. To
induce somatic embryos, cotyledons, 3-5 mm in length dissected from
surface sterilized, immature seeds of the soybean cultivar A2872,
can be cultured in the light or dark at 26.degree. C. on an
appropriate agar medium for 6-10 weeks. Somatic embryos which
produce secondary embryos are then excised and placed into a
suitable liquid medium. After repeated selection for clusters of
somatic embryos which multiplied as early, globular staged embryos,
the suspensions are maintained as described below.
[0077] Soybean embryogenic suspension cultures can maintained in 35
mL liquid media on a rotary shaker, 150 rpm, at 26.degree. C. with
florescent lights on a 16:8 hour day/night schedule. Cultures are
subcultured every two weeks by inoculating approximately 35 mg of
tissue into 35 mL of liquid medium.
[0078] Soybean embryogenic suspension cultures may then be
transformed by the method of particle gun bombardment (Klein et al.
(1987) Nature (London) 327:70, U.S. Pat. No. 4,945,050). A DuPont
Biolistic.TM. PDS1000/HE instrument (helium retrofit) can be used
for these transformations.
[0079] A selectable marker gene which can be used to facilitate
soybean transformation is a chimeric gene composed of the 35S
promoter from Cauliflower Mosaic Virus (Odell et al. (1985) Nature
313:810-812), the hygromycin phosphotransferase gene from plasmid
pJR225 (from E. coli; Gritz et al.(1983) Gene 25:179-188) and the
3' region of the nopaline synthase gene from the T-DNA of the Ti
plasmid of Agrobacterium tumefaciens. The seed expression cassette
comprising the phaseolin 5' region, the fragment encoding the
instant polypeptides and the phaseolin 3' region can be isolated as
a restriction fragment. This fragment can then be inserted into a
unique restriction site of the vector carrying the marker gene.
[0080] To 50 .mu.L of a 60 mg/mL 1 .mu.m gold particle suspension
is added (in order): 5 .mu.L DNA (1 .mu.g/.mu.L), 20 .mu.l
spermidine (0.1 M), and 50 .mu.L CaCl.sub.2 (2.5 M). The particle
preparation is then agitated for three minutes, spun in a microfuge
for 10 seconds and the supernatant removed. The DNA-coated
particles are then washed once in 400 .mu.L 70% ethanol and
resuspended in 40 .mu.L of anhydrous ethanol. The DNA/particle
suspension can be sonicated three times for one second each. Five
.mu.L of the DNA-coated gold particles are then loaded on each
macro carrier disk.
[0081] Approximately 300-400 mg of a two-week-old suspension
culture is placed in an empty 60.times.15 mm petri dish and the
residual liquid removed from the tissue with a pipette. For each
transformation experiment, approximately 5-10 plates of tissue are
normally bombarded. Membrane rupture pressure is set at 1100 psi
and the chamber is evacuated to a vacuum of 28 inches mercury. The
tissue is placed approximately 3.5 inches away from the retaining
screen and bombarded three times. Following bombardment, the tissue
can be divided in half and placed back into liquid and cultured as
described above.
[0082] Five to seven days post bombardment, the liquid media may be
exchanged with fresh media, and eleven to twelve days post
bombardment with fresh media containing 50 mg/mL hygromycin. This
selective media can be refreshed weekly. Seven to eight weeks post
bombardment, green, transformed tissue may be observed growing from
untransformed, necrotic embryogenic clusters. Isolated green tissue
is removed and inoculated into individual flasks to generate new,
clonally propagated, transformed embryogenic suspension cultures.
Each new line may be treated as an independent transformation
event. These suspensions can then be subcultured and maintained as
clusters of immature embryos or regenerated into whole plants by
maturation and germination of individual somatic embryos.
Example 7
Expression of Chimeric Genes in Microbial Cells
[0083] The cDNAs encoding the instant polypeptides can be inserted
into the T7 E. coli expression vector pBT430. This vector is a
derivative of pET-3a (Rosenberg et al. (1987) Gene 56:125-135)
which employs the bacteriophage T7 RNA polymerase/T7 promoter
system. Plasmid pBT430 was constructed by first destroying the EcoR
I and Hind III sites in pET-3a at their original positions. An
oligonucleotide adaptor containing EcoR I and Hind III sites was
inserted at the BamH I site of pET-3a. This created pET-3aM with
additional unique cloning sites for insertion of genes into the
expression vector. Then, the Nde I site at the position of
translation initiation was converted to an Nco I site using
oligonucleotide-directed mutagenesis. The DNA sequence of pET-3aM
in this region, 5'-CATATGG, was converted to 5'-CCCATGG in
pBT430.
[0084] Plasmid DNA containing a cDNA may be appropriately digested
to release a nucleic acid fragment encoding the protein. This
fragment may then be purified on a 1% NuSieve GTG.TM. low melting
agarose gel (FMC). Buffer and agarose contain 10 .mu.g/ml ethidium
bromide for visualization of the DNA fragment. The fragment can
then be purified from the agarose gel by digestion with GELase.TM.
(Epicentre Technologies) according to the manufacturer's
instructions, ethanol precipitated, dried and resuspended in 20
.mu.L of water. Appropriate oligonucleotide adapters may be ligated
to the fragment using T4 DNA ligase (New England Biolabs, Beverly,
Mass.). The fragment containing the ligated adapters can be
purified from the excess adapters using low melting agarose as
described above. The vector pBT430 is digested, dephosphorylated
with alkaline phosphatase (NEB) and deproteinized with
phenol/chloroform as described above. The prepared vector pBT430
and fragment can then be ligated at 16.degree. C. for 15 hours
followed by transformation into DH5 electrocompetent cells (GIBCO
BRL). Transformants can be selected on agar plates containing LB
media and 100 .mu.g/mL ampicillin. Transformants containing the
gene encoding the instant polypeptides are then screened for the
correct orientation with respect to the T7 promoter by restriction
enzyme analysis.
[0085] For high level expression, a plasmid clone with the cDNA
insert in the correct orientation relative to the T7 promoter can
be transformed into E. coli strain BL21(DE3) (Studier et al. (1986)
J. Mol. Biol. 189:113-130). Cultures are grown in LB medium
containing ampicillin (100 mg/L) at 25.degree. C. At an optical
density at 600 nm of approximately 1, IPTG
(isopropylthio-.beta.-galactoside, the inducer) can be added to a
final concentration of 0.4 mM and incubation can be continued for 3
h at 25.degree.. Cells are then harvested by centrifugation and
re-suspended in 50 .mu.L of 50 mM Tris-HCl at pH 8.0 containing 0.1
mM DTT and 0.2 mM phenyl methylsulfonyl fluoride. A small amount of
1 mm glass beads can be added and the mixture sonicated 3 times for
about 5 seconds each time with a microprobe sonicator. The mixture
is centrifuged and the protein concentration of the supernatant
determined. One .mu.g of protein from the soluble fraction of the
culture can be separated by SDS-polyacrylamide gel electrophoresis.
Gels can be observed for protein bands migrating at the expected
molecular weight.
Example 8
Evaluating Compounds for Their Ability to Inhibit the Activity of
Aromatic Amino Acid Catabolism Enzymes
[0086] The polypeptides described herein may be produced using any
number of methods known to those skilled in the art. Such methods
include, but are not limited to, expression in bacteria as
described in Example 7, or expression in eukaryotic cell culture,
in planta, and using viral expression systems in suitably infected
organisms or cell lines. The instant polypeptides may be expressed
either as mature forms of the proteins as observed in vivo or as
fusion proteins by covalent attachment to a variety of enzymes,
proteins or affinity tags. Common fusion protein partners include
glutathione S-transferase ("GST"), thioredoxin ("Trx"), maltose
binding protein, and C- and/or N-terminal hexahistidine polypeptide
("(His).sub.6"). The fusion proteins may be engineered with a
protease recognition site at the fusion point so that fusion
partners can be separated by protease digestion to yield intact
mature enzyme. Examples of such proteases include thrombin,
enterokinase and factor Xa. However, any protease can be used which
specifically cleaves the peptide connecting the fusion protein and
the enzyme.
[0087] Purification of the instant polypeptides, if desired, may
utilize any number of separation technologies familiar to those
skilled in the art of protein purification. Examples of such
methods include, but are not limited to, homogenization,
filtration, centrifugation, heat denaturation, ammonium sulfate
precipitation, desalting, pH precipitation, ion exchange
chromatography, hydrophobic interaction chromatography and affinity
chromatography, wherein the affinity ligand represents a substrate,
substrate analog or inhibitor. When the instant polypeptides are
expressed as fusion proteins, the purification protocol may include
the use of an affinity resin which is specific for the fusion
protein tag attached to the expressed enzyme or an affinity resin
containing ligands which are specific for the enzyme. For example,
the instant polypeptides may be expressed as a fusion protein
coupled to the C-terminus of thioredoxin. In addition, a
(His).sub.6 peptide may be engineered into the N-terminus of the
fused thioredoxin moiety to afford additional opportunities for
affinity purification. Other suitable affinity resins could be
synthesized by linking the appropriate ligands to any suitable
resin such as Sepharose-4B. In an alternate embodiment, a
thioredoxin fusion protein may be eluted using dithiothreitol;
however, elution may be accomplished using other reagents which
interact to displace the thioredoxin from the resin. These reagents
include .beta.-mercaptoethanol or other reduced thiol. The eluted
fusion protein may be subjected to further purification by
traditional means as stated above, if desired. Proteolytic cleavage
of the thioredoxin fusion protein and the enzyme may be
accomplished after the fusion protein is purified or while the
protein is still bound to the ThioBond.TM. affinity resin or other
resin.
[0088] Crude, partially purified or purified enzyme, either alone
or as a fusion protein, may be utilized in assays for the
evaluation of compounds for their ability to inhibit enzymatic
activation of the instant polypeptides disclosed herein. Assays may
be conducted under well known experimental conditions which permit
optimal enzymatic activity. For example, assays for homogentisate
1,2-dioxygenase are presented by Schmidt S. R. (1995) Eur. J.
Biochem. 1995 228:425-430. Assays for fumarylacetoacetase are
presented by Nagainis M. P. et al. (1981) Biochim. Biophys. Acta
657:203-211.
* * * * *
References