TRYPTOPHAN BIOSYNTHETIC ENZYMES

Abstract: This invention relates to an isolated nucleic acid fragment encoding a tryptophan biosynthetic enzyme. The invention also relates to the construction of a chimeric gene encoding all or a portion of the tryptophan biosynthetic enzyme, in sense or antisense orientation, wherein expression of the chimeric gene results in production of altered levels of the tryptophan biosynthetic enzyme in a transformed host cell.

Description: TITLE TRYPTOPHAN BIOSYNTHETIC ENZYMES This application claims the benefit of U.S. Provisional Application No. 60/079,386, filed March 26, 1998. FIELD OF THE INVENTION This invention is in the field of plant molecular biology. More specifically, this invention pertains to nucleic acid fragments encoding enzymes involved in the biosynthesis of tyrotophan in plants and seeds. BACKGROUND OF THE INVENTION Many vertebrates, including man, lack the ability to manufacture a number of amino acids and therefore require these amino acids preformed in their diet. These are called essential amino acids. Plants are able to synthesize all twenty amino acids and serve as the ultimate source of the essential amino acids for humans and animals. Thus, the ability to manipulate the production and accumulation of the essential amino acids in plants is of considerable importance and value. Furthermore, the inability of animals to synthesize these amino acids provides a useful distinction between animal and plant cellular metabolism. This can be exploited for the discovery of herbicidal chemical compounds that target enzymes in the plant biosynthetic pathways of the essential amino acids and thus have low toxicity to animals. Tryptophan is an essential amino acid. In plants, the biosynthesis of tryptophan from chorismic acid (see Figure 1) requires five enzymatic steps catalyzed by anthranilate synthase (EC 4.1.3.27), anthranilate phosphoribosyl-transferase (EC 2.4.2.18), phosphoribosylanthranilate isomerase (EC 5.3.1.24), indole-3-glycerol phosphate synthase (EC 4.1.1.48) and tryptophan synthase (EC 4.2.1.20). The tryptophan pathway leads to the biosynthesis of many secondary metabolites including the hormone indole-3-acetic acid, antimicrobial phytoalexins, alkaloids and glucosinolates. The first enzyme in the tryptophan pathway from chorismate is anthranilate synthase. This enzyme is subject to feedback inhibition by tryptophan and is composed of two subunits. Even though the anthranilate synthase alpha subunits are encoded by duplicate genes, in Arabidopsis and Ruta graveolens it has been shown that only one gene is induced in response to wounding or pathogen attack (Bohlmann, J. et al. (1995) Plant J. 7:491-501 and Niyogi, K. K. and Fink, G. R. (1992) Plant Cell :721-733). Glutamine- and ammonia-dependent anthranilate synthase activities copurify in Ruta graveolens. The glutamine-dependent reaction of one of the alpha subunits requires a 60 to 65 kDa anthranilate synthase beta subunit (Bohlmann, J. et al. (1995) Plant J. 7:491-501) The beta-subunit is a highly asymmetric dimer with an apparent molecular weight of 200,000. Cleavage of the purified subunit with elastase, trypsin, or chymotrypsin results in fragments which retain enzyme activity. Elastase digestion results in a 30 kDa fragment and a 56 kDa fragment. The first fragment behaves as a monomer and interacts with free alpha subunit to produce the glutamine-dependent anthranilate synthase activity. The second fragment behaves as an asymmetric dimer and has N-(5'-phosphoribosyl) anthranilate isomerase and indole-3-glycerol phosphate synthase activity (Walker, M. S. and DeMoss, J. A. (1983) J Biol Chem 258:3571-3575). The two final reactions in tryptophan biosynthesis are catalyzed by tryptophan synthase. The 29 kDa alpha subunit is a bifunctional enzyme which cleaves indole-3- glycerol phosphate to produce indole and glyceraldehyde-3 -phosphate. The beta subunit joins indole with serine to form tryptophan. Either subunit alone is enzymatically active, but the rate of the reaction and affinity for the substrates increases when the subunits are forming a tetramer composed of two alpha subunits and two beta subunits (Radwanski, E. R. (1995) Mol Gen Genet 248:657-667). Few of the genes encoding enzymes from the tryptophan pathway in corn, soybeans, rice and wheat, have been isolated and sequenced. For example, no corn, soybean, rice or wheat genes have been reported for anthranilate synthase alpha or beta subunits and no soybean, rice or wheat genes have been reported for tryptophan synthase alpha subunit. A corn gene encoding tryptophan synthase has been identified and the instant invention describes a new corn tryptophan synthase homolog. Accordingly, the availability of nucleic acid sequences encoding all or a portion of these enzymes will facilitate studies to better understand cellular biosynthetic pathways, provide genetic tools for the manipulation of those pathways, provide a means to evaluate chemical compounds for their ability to inhibit the enzymatic activity of the compounds included in this application. SUMMARY OF THE INVENTION The instant invention relates to isolated nucleic acid fragments encoding tryptophan biosynthetic enzymes. Specifically, this invention concerns an isolated nucleic acid fragment encoding an anthranilate synthase alpha subunit, an anthranilate synthase beta subunit or a tryptophan synthase alpha subunit. In addition, this invention relates to a nucleic acid fragment that is complementary to the nucleic acid fragment encoding anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit. An additional embodiment of the instant invention pertains to a polypeptide encoding all or a substantial portion of a tryptophan biosynthetic enzyme selected from the group consisting of anthranilate synthase alpha subunit, anthranilate synthase beta subunit and tryptophan synthase alpha subunit. In another embodiment, the instant invention relates to a chimeric gene encoding an anthranilate synthase alpha subunit, an anthranilate synthase beta subunit or a tryptophan synthase alpha subunit, or to a chimeric gene that comprises a nucleic acid fragment that is complementary to a nucleic acid fragment encoding an anthranilate synthase alpha subunit, an anthranilate synthase beta subunit or a tryptophan synthase alpha subunit, 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. In a further embodiment, the instant invention concerns a transformed host cell comprising in its genome a chimeric gene encoding an anthranilate synthase alpha subunit, an anthranilate synthase beta subunit or a tryptophan synthase alpha subunit, 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. An additional embodiment of the instant invention concerns a method of altering the level of expression of an anthranilate synthase alpha subunit, an anthranilate synthase beta subunit or a tryptophan synthase alpha subunit in a transformed host cell comprising: a) transforming a host cell with a chimeric gene comprising a nucleic acid fragment encoding an anthranilate synthase alpha subunit, an anthranilate synthase beta subunit or a tryptophan synthase alpha subunit; 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 anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit in the transformed host cell. 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 an anthranilate synthase alpha subunit, an anthranilate synthase beta subunit or a tryptophan synthase alpha subunit. A further embodiment of the instant invention is a method for evaluating at least one compound for its ability to inhibit the activity of an anthranilate synthase alpha subunit, an anthranilate synthase beta subunit or a tryptophan synthase alpha subunit, the method comprising the steps of: (a) transforming a host cell with a chimeric gene comprising a nucleic acid fragment encoding an anthranilate synthase alpha subunit, an anthranilate synthase beta subunit or a tryptophan synthase alpha subunit, 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 anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit in the transformed host cell; (c) optionally purifying the anthranilate synthase alpha subunit, the anthranilate synthase beta subunit or the tryptophan synthase alpha subunit expressed by the transformed host cell; (d) treating the anthranilate synthase alpha subunit, the anthranilate synthase beta subunit or the tryptophan synthase alpha subunit with a compound to be tested; and (e) comparing the activity of the anthranilate synthase alpha subunit, the anthranilate synthase beta subunit or the tryptophan synthase alpha subunit that has been treated with a test compound to the activity of an untreated anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit, thereby selecting compounds with potential for inhibitory activity. BRIEF DESCRIPTION OF THE DRAWINGS AND SEQUENCE DESCRIPTIONS The invention can be more fully understood from the following detailed description and the accompanying drawings and Sequence Listing which form a part of this application. Figure 1 depicts biochemical pathway for the production of tryptophan from chorismate. Figure 2 shows an alignment of the amino acid sequences from Ruta graveolens anthranilate synthase alpha subunit (SEQ ID NO:39) and the instant corn anthranilate synthase alpha subunit (contig of cdelc.pk004.g2, p0125.czabf83r, p0005.cbmew77r, p0096.cnamt32r, p0032.crcas26r, p0031 xcmar07r, p0015.cdpfkl2r, p0128.cpibl07r and ctaln.pk0051.g3; SEQ ID NO:2). Amino acid which are identical among both sequences are indicated with an asterisk (*) above the alignment. Dashes are used by the program to maximize alignment of the sequences. Figure 3 shows an alignment of the amino acid sequences from Arabidopsis thaliana anthranilate synthase beta subunit (SEQ ID NO:40), the instant corn anthranilate synthase beta subunit (contig of p0126.cnlcu75r, cen3n.pk0212.h6, ccoln.pk0038.bl, csiln.pk0017.d4, p0031.ccmai08r and ctnlc.pkOOl.llO; SEQ ID NO:20) and the instant rice anthranilate synthase beta subunit (contig of rds2c.pk004.i9, rls24.pk0002.e2 and rl0n.pkl 13.b4; SEQ ID NO:22). Amino acid which are identical among all sequences are indicated with an asterisk (*) above the alignment. Dashes are used by the program to maximize alignment of the sequences. Figure 4 shows an alignment of the amino acid sequences from Arabidopsis thaliana tryptophan synthase alpha subunit (SEQ ID NO:41), the instant corn tryptophan synthase alpha subunit (contig of chp2.pk0020.e2 and lkr.pk0013.gl; SEQ ID NO:26), the instant corn tryptophan synthase alpha subunit (crln.pk0052.b8; SEQ ID NO:28) and the instant rice tryptophan synthase alpha subunit (rrl .pk0038.h6; SEQ ID NO:32). Amino acid which are identical among all sequences are indicated with an asterisk (*) above the alignment. Dashes are used by the program to maximize alignment of the sequences. The following 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. §1.821-1.825. SEQ ID NO:l is the nucleotide sequence comprising the contig assembled from a portion of the cDNA inserts in clones cdelc.pk004.g2, p0125.czabf83r, p0005.cbmew77r, p0096.cnamt32r, p0032.crcas26r, p0031 xcmar07r, p0015.cdρfkl2r and p0128.cpibl07r and the entire cDNA insert in clone ctaln.pk0051.g3 encoding an entire corn anthranilate synthase alpha subunit. SEQ ID NO:2 is the deduced amino acid sequence of an entire corn anthranilate synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO: 1. SEQ ID NO: 3 is the nucleotide sequence comprising the contig assembled from a portion of the cDNA inserts in clones pOl 15.clsmj70r, cbn2.pk0049.d2, cpllc.pk008.m4 and p0102.ceraq01r and the entire cDNA insert in clone ceb3.pk001 l.cl2 encoding a portion of a corn anthranilate synthase alpha subunit. SEQ ID NO:4 is the deduced amino acid sequence of a portion of a corn anthranilate synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO:3. SEQ ID NO: 5 is the nucleotide sequence comprising a portion of the cDNA insert in clone rcaln.pk004.p22 encoding a portion of a rice anthranilate synthase alpha subunit. SEQ ID NO:6 is the deduced amino acid sequence of a portion of a rice anthranilate synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO:5. SEQ ID NO: 7 is the nucleotide sequence comprising a portion of the cDNA insert in clone sdp3c.pk019.c6 encoding a portion of a soybean anthranilate synthase alpha subunit. SEQ ID NO: 8 is the deduced amino acid sequence of a portion of a soybean anthranilate synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO:7. SEQ ID NO: 9 is the nucleotide sequence comprising a portion of the cDNA insert in clone sfll.pkl29.dl0 encoding a portion of a soybean anthranilate synthase alpha subunit. SEQ ID NO: 10 is the deduced amino acid sequence of a portion of a soybean anthranilate synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO:9. SEQ ID NO:l 1 is the nucleotide sequence comprising a portion of the 5' terminus of the cDNA insert in clone wleln.pk0075.b4 encoding a portion of a wheat anthranilate synthase alpha subunit. SEQ ID NO: 12 is the deduced amino acid sequence of a portion of a wheat anthranilate synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO: 11. SEQ ID NO: 13 is the nucleotide sequence comprising a portion of the 3' terminus of the cDNA insert in clone wleln.pk0075.b4 encoding a portion of a wheat anthranilate synthase alpha subunit. SEQ ID NO: 14 is the deduced amino acid sequence of a portion of a wheat anthranilate synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO: 13. SEQ ID NO: 15 is the nucleotide sequence comprising a portion of the cDNA insert in clone wlm96.pk026.j 1 encoding a portion of a wheat anthranilate synthase alpha subunit. SEQ ID NO: 16 is the deduced amino acid sequence of a portion of a wheat anthranilate synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO: 15. SEQ ID NO: 17 is the nucleotide sequence comprising the contig assembled from a portion of the cDNA insert in clones cen3n.pk0210.c3, cepe7.pk0012.g2 and cen3n.pk0047.g5 encoding a portion of a corn anthranilate synthase beta subunit. SEQ ID NO: 18 is the deduced amino acid sequence of a portion of a corn anthranilate synthase beta subunit derived from the nucleotide sequence of SEQ ID NO: 17. SEQ ID NO: 19 is the nucleotide sequence comprising the contig assembled from a portion of the cDNA inserts in clones p0126.cnlcu75r, cen3n.pk0212.h6, ccoln.pk0038.bl, csi 1 n.pkOO 17.d4, p0031.ccmai08r and ctnl c.pkOO 1.110 encoding an entire corn anthranilate synthase beta subunit. SEQ ID NO:20 is the deduced amino acid sequence of an entire corn anthranilate synthase beta subunit derived from the nucleotide sequence of SEQ ID NO: 19. SEQ ID NO:21 is the nucleotide sequence comprising the contig assembled from a portion of the cDNA inserts in clones rds2c.pk004.i9, rls24.pk0002.e2 and riOn.pkl 13.b4 encoding a a substantial portion of a rice anthranilate synthase beta subunit. SEQ ID NO:22 is the deduced amino acid sequence of a substantial portion of a rice anthranilate synthase beta subunit derived from the nucleotide sequence of SEQ ID NO:21. SEQ ID NO:23 is the nucleotide sequence comprising a portion of the cDNA insert in clone wlm96.pk045.cl 1 encoding a portion of a wheat anthranilate synthase beta subunit. SEQ ID NO:24 is the deduced amino acid sequence of a portion of a wheat anthranilate synthase beta subunit derived from the nucleotide sequence of SEQ ID NO:23. SEQ ID NO:25 is the nucleotide sequence comprising the contig assembled from a portion of the cDNA inserts in clones chp2.pk0020.e2 and lkr.pk0013.gl encoding an entire corn tryptophan synthase alpha subunit. SEQ ID NO:26 is the deduced amino acid sequence of an entire corn tryptophan synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO:25. SEQ ID NO:27 is the nucleotide sequence comprising a portion of the cDNA insert in clone crln.pk0052.b8 encoding an almost entire corn tryptophan synthase alpha subunit. SEQ ID NO:28 is the deduced amino acid sequence of an almost entire corn tryptophan synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO:27. SEQ ID NO:29 is the nucleotide sequence comprising the contig assembled from a portion of the cDNA inserts in clones reslc.pk004.c4 and rrl.pk0071.f2 encoding a portion of a rice tryptophan synthase alpha subunit. SEQ ID NO:30 is the deduced amino acid sequence of a portion of a rice tryptophan synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO:29. SEQ ID NO:31 is the nucleotide sequence comprising a portion of the cDNA insert in clone rrl.pk0038.h6 encoding an entire rice tryptophan synthase alpha subunit. SEQ ID NO:32 is the deduced amino acid sequence of an entire rice tryptophan synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO: 31. SEQ ID NO:33 is the nucleotide sequence comprising the contig assembled from a portion of the cDNA inserts in clones src3c.pk004.cl9and sls2c.pk025.dl 1 encoding a portion of a soybean tryptophan synthase alpha subunit. SEQ ID NO:34 is the deduced amino acid sequence of a portion of a soybean tryptophan synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO:33. SEQ ID NO:35 is the nucleotide sequence comprising the contig assembled from a portion of the cDNA inserts in clones wlm0.pk0028.f4 and wlm96.pk041.h3 encoding a portion of a wheat tryptophan synthase alpha subunit. SEQ ID NO:36 is the deduced amino acid sequence of a portion of a wheat tryptophan synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO:35. SEQ ID NO:37 is the nucleotide sequence comprising the contig assembled from a portion of the cDNA inserts in clones wlln.pk0109.d5 and wlln.pkOl lO.cl encoding a portion of a wheat tryptophan synthase alpha subunit. SEQ ID NO:38 is the deduced amino acid sequence of a portion of a wheat tryptophan synthase alpha subunit derived from the nucleotide sequence of SEQ ID NO:37. SEQ ID NO:39 is the amino acid sequence of a Ruta graveolens anthranilate synthase alpha subunit having an NCBI General Identifier No. 960289. SEQ ID NO:40 is the amino acid sequence of a Arabidopsis thaliana anthranilate synthase beta subunit having an NCBI General Identifier No. 541849. SEQ ID NO:41 is the amino acid sequence of a Arabidopsis thaliana tryptophan synthase alpha subunit having an NCBI General Identifier No. 2129755. 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 73:3021-3030 (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. §1.822. DETAILED DESCRIPTION OF THE INVENTION In the context of this disclosure, a number of terms shall be utilized. As used herein, an "isolated 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. An isolated 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. As used herein, "contig" refers to an assemblage of overlapping nucleic acid sequences to form one contiguous nucleotide sequence. For example, several DNA sequences can be compared and aligned to identify common or overlapping regions. The individual sequences can then be assembled into a single contiguous nucleotide sequence. 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 protein encoded by the DNA 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 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 alteration of gene expression by antisense or co-suppression technology or alteration of the functional properties of the resulting protein molecule. It is therefore understood that the invention encompasses more than the specific exemplary sequences. 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 gene 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 protein, 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 protein molecule would also not be expected to alter the activity of the protein. 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. Moreover, the skilled artisan recognizes that substantially similar nucleic acid sequences encompassed by this invention are also defined by their ability to hybridize, under stringent conditions (0.1X SSC, 0.1% SDS, 65°C), with the sequences exemplified herein. 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. Preferred are those nucleic acid fragments whose nucleotide sequences encode amino acid sequences that are greater than 80% identical to the amino acid sequences reported herein. 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 similarity calculations were performed using the Megalign program of the LASARGENE bioinformatics computing suite (DNASTAR Inc., Madison, WI). Multiple alignment of the sequences was performed using the Clustal method of alignment (Higgins, D.G. and Sharp, P.M. (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. A "substantial portion" of an amino acid or nucleotide sequence comprises enough of the amino acid sequence of a polypeptide or the nucleotide sequence of a gene to afford putative identification of that polypeptide or gene, either by manual evaluation of the sequence by one skilled in the art, or by computer-automated sequence comparison and identification using algorithms such as BLAST (Basic Local Alignment Search Tool; Altschul, S. F., et al., (1993) J. Mol. Biol. 275:403-410; see also www.ncbi.nlm.nih.gov/BLAST/). In general, a sequence of ten or more contiguous amino acids or thirty or more 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 20-30 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 12-15 bases 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 enough of the sequence to afford specific identification and/or isolation of a nucleic acid fragment comprising the sequence. The instant specification teaches partial or complete amino acid and nucleotide sequences encoding 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. "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 that encodes all or a substantial portion of the amino acid sequence encoding the anthranilate synthase alpha subunit, the anthranilate synthase beta subunit or the tryptophan synthase alpha subunit proteins as set forth in SEQ ID NOs:2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36 and 38. 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 gene for improved expression in a host cell, it is desirable to design the gene such that its frequency of codon usage approaches the frequency of preferred codon usage of the host cell. "Synthetic genes" 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 gene segments which are then enzymatically assembled to construct the entire gene. "Chemically synthesized", as related to a sequence of DNA, means that the component nucleotides were assembled in vitro. Manual chemical synthesis of DNA 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 genes 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. "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. "Coding sequence" refers to a DNA 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. "Promoter" refers to a DNA 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 DNA 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 DNA 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 gene 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 75:1-82. It is further recognized that since in most cases the exact boundaries of regulatory sequences have not been completely defined, DNA fragments of different lengths may have identical promoter activity. The "translation leader sequence" refers to a DNA 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, R. and Foster, G. D. (1995) Molecular Biotechnology 3:225). The "3' non-coding sequences" refer to DNA 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 7:671-680. "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 protein by the cell. "cDNA" refers to a double-stranded DNA that is complementary to and derived from mRNA. "Sense" RNA refers to RNA transcript that includes the mRNA and so can be translated into protein by the cell. "Antisense RNA" refers to a 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 (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 gene transcript, 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. The term "operably linked" refers to the association of nucleic acid sequences 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. 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 ofthe 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). "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. "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. 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, J. J., (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: 627 -1632). "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) Nαtwre (London) 327:70-73; U.S. Pat. No. 4,945,050, incorporated herein by reference). Standard recombinant DNA and molecular cloning techniques used herein are well known in the art and are described more fully in Sambrook, J., Fritsch, E.F. and Maniatis, T. Molecular Cloning: A Laboratory Manual; Cold Spring Harbor Laboratory Press: Cold Spring Harbor, 1989 (hereinafter "Maniatis"). Nucleic acid fragments encoding at least a portion of several tryptophan biosynthetic enzymes 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. Table 1 lists the proteins that are described herein, and the designation ofthe cDNA clones that comprise the nucleic acid fragments encoding these proteins. TABLE 1 Tryptophan Biosynthetic Enzymes Enzyme Clone Plant Anthranilate Synthase Alpha Subunit Contig of: Corn cdelc.pk004.g2 p0125.czabf83r p0005.cbmew77r p0096.cnamt32r p0032.crcas26r p0031.ccmar07r p0015.cdpfkl2r p0128.cpibl07r ctaln.pk0051.g3 Contig of: Corn p0115.clsmj70r cbn2.pk0049.d2 cpllc.pk008.m4 p0102.ceraq01r ceb3.pk0011.cl2 rcaln.pk004.p22 Rice sdp3c.pk019.c6 Soybean sfll.pkl29.dl0 Soybean wleln.pk0075.b4 Wheat wlm96.pk026.jl Wheat Anthranilate Synthase Beta Subunit Contig of: Corn cen3n.pk0210.c3 cepe7.pk0012.g2 cen3n.pk0047.g5 Contig of: Corn p0126.cnlcu75r cen3n.pk0212.h6 ccoln.pk0038.bl csiln.pk0017.d4 p0031.ccmai08r ctnlc.pkOOl.llO Contig of: Rice rds2c.pk004.i9 rls24.pk0002.e2 rl0n.pkl l3.b4 wlm96.pk045.cll Wheat Enzyme Clone Plant Tryptophan Synthase Alpha Subunit Contig of: Corn chp2.pk0020.e2 lkr.pk0013.gl crln.pk0052.b8 Corn Contig of: Rice reslc.pk004.c4 rrl.pk0071.£2 rrl.pk0038.h6 Rice Contig of: Soybean src3c.pk004.cl9 sls2c.pk025.dl l Contig of: Wheat wlm0.pk0028.f4 wlm96.pk041.h3 Contig of: Wheat wlln.pk0109.d5 wlln.pk0110.cl The nucleic acid fragments ofthe 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). For example, genes encoding other anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit, either as cDNAs or genomic DNAs, could be isolated directly by using all or a portion ofthe 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. In addition, two short segments ofthe 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 ofthe other primer takes advantage ofthe presence ofthe polyadenylic acid tracts to the 3' end ofthe 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) PNAS USA 55:8998) to generate cDNAs by using PCR to amplify copies ofthe 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) PNAS 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, M. A. and Martin, G. R., (1989) Techniques 7:165). Availability ofthe instant nucleotide and deduced amino acid sequences facilitates immunological screening of cDNA expression libraries. Synthetic peptides representing portions ofthe 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, R. A. ( 984) Adv. Immunol. 36:1; Maniatis). The nucleic acid fragments ofthe instant invention may be used to create transgenic plants in which the disclosed anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit 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 tryptophan in those cells. Manipulation ofthe levels of some ofthe antranilate synthase alpha subunits will also result in changes in the response to pathogen attack. Because this pathway is not followed for the production of tryptophan in higher animals, these enzymes are very good candidates for the discovery of herbicides and fungicides. Overexpression ofthe anthranilate synthase alpha subunit, the anthranilate synthase beta subunit or the tryptophan synthase alpha subunit proteins ofthe 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. 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 ofthe 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) EMBOJ 4:24 1-2418; De Almeida et al., (1989) Mol. Gen. Genetics 27δ: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. For some applications it may be useful to direct the instant tryptophan biosynthetic enzyme 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 anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit with appropriate intracellular targeting sequences such as transit sequences (Keegstra, K. (1989) Cell 5(5:247-253), signal sequences or sequences encoding endoplasmic reticulum localization (Chrispeels, J.J., (1991) Ann. Rev. Plant Phys. Plant Mol. Biol. 42:21-53), or nuclear localization signals (Raikhel, N. (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. It may also be desirable to reduce or eliminate expression of genes encoding anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit in plants for some applications. In order to accomplish this, a chimeric gene designed for co-suppression ofthe instant tryptophan biosynthetic enzyme can be constructed by linking a gene or gene fragment encoding an anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit to plant promoter sequences. Alternatively, a chimeric gene designed to express antisense RNA for all or part ofthe 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 ofthe corresponding endogenous genes are reduced or eliminated. The instant anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit (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 anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit in situ in cells or in vitro in cell extracts. Preferred heterologous host cells for production ofthe instant anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit 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 anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit. This chimeric gene could then be introduced into appropriate microorganisms via transformation to provide high level expression ofthe encoded tryptophan biosynthetic enzyme. An example of a vector for high level expression ofthe instant anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit in a bacterial host is provided (Example 7). Additionally, the instant anthranilate synthase alpha subunit, anthranilate synthase beta subunit or tryptophan synthase alpha subunit 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 anthranilate synthase alpha subunit, the anthranilate synthase beta subunit and the tryptophan synthase alpha subunit described herein catalyze various steps in tryptophan biosynthesis. Accordingly, inhibition ofthe activity of one or more ofthe enzymes described herein could lead to inhibition plant growth. Thus, the instant anthranilate synthase alpha subunit, anthranilate synthase beta subunit, tryptophan synthase alpha subunit could be appropriate for new herbicide discovery and design. All or a substantial portion ofthe nucleic acid fragments ofthe 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 ofthe instant invention. The resulting banding patterns may then be subjected to genetic analyses using computer programs such as MapMaker (Lander et at., (1987) Genomics 7:174-181) in order to construct a genetic map. In addition, the nucleic acid fragments ofthe 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 ofthe DNA polymorphisms is noted and used to calculate the position ofthe instant nucleic acid sequence in the genetic map previously obtained using this population (Botstein, D. et al., (1980) Am. J. Hum. Genet. 32:314-331). The production and use of plant gene-derived probes for use in genetic mapping is described in R. Bernatzky, R. and Tanksley, S. D. (1986) Plant Mol. Biol. Reporter 4(l):37-4l. 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. 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, J. D., et al., In: Nonmammalian Genomic Analysis: A Practical Guide, Academic press 1996, pp. 319-346, and references cited therein). 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, B. J. (1991) Trends Genet. 7:149-154). Although current methods of FISH mapping favor use of large clones (several to several hundred KB; see Laan, M. et al. (1995) Genome Research 5:13-20), improvements in sensitivity may allow performance of FISH mapping using shorter probes. 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, H. H. (1989) J Lab. Clin. Med. 114(2):95-96), polymorphism of PCR-amplified fragments (CAPS; Sheffield, V. C. et al. (1993) Genomics 75:325-332), allele-specific ligation (Landegren, U. et al. (1988) Science 247:1077-1080), nucleotide extension reactions (Sokolov, B. P. (1990) Nucleic Acid Res. 18:3671), Radiation Hybrid Mapping (Walter, M. A. et al. (1997) Nature Genetics 7:22-28) and Happy Mapping (Dear, P. H. and Cook, P. R. (1989) Nucleic Acid Res. 77:6795-6807). 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 ofthe mapping cross in the region corresponding to the instant nucleic acid sequence. This, however, is generally not necessary for mapping methods. 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 ofthe 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 ofthe mutation tag element in or near the plant gene encoding the anthranilate synthase alpha subunit, the anthranilate synthase beta subunit or the tryptophan synthase alpha subunit. 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 an anthranilate synthase alpha subunit, an anthranilate synthase beta subunit or a tryptophan synthase alpha subunit can be identified and obtained. This mutant plant can then be used to determine or confirm the natural function ofthe anthranilate synthase alpha subunit, the anthranilate synthase beta subunit or the tryptophan synthase alpha subunit gene product. EXAMPLES 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 ofthe 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 cDNA libraries representing mRNAs from various corn, rice, soybean and wheat tissues were prepared. The characteristics ofthe libraries are described below. TABLE 2 cDNA Libraries from Corn, Rice, Soybean and Wheat Library Tissue Clone cbn2 Corn Developing Kernel Two Days After cbn2.pk0049.d2 Pollination ceo 1 n Corn Cob of 67 Day Old Plants Grown in Green ceo 1 n.pk0038.b 1 House* cdelc Corn Developing Embryo 20 Days After cdelc.pk004.g2 Pollination ceb3 Corn Embryo 20 Days After Pollination ceb3.pk0011.c 12 cen3n Corn Endosperm 20 Days After Pollination* cen3n.pk0047.g5 cen3n.pk0210.c3 cen3n.pk0212.h6 cepe7 Corn 7 Day Old Epicotyl From Etiolated Seedling cepe7.pk0012.g2 chp2 Corn (B73 and MK593) 11 Day Old Leaf Treated chp2.pk0020.e2 24 Hours With Herbicides** cpllc Corn Pooled BMS Treated With Chemical cpllc.pk008.m4 Chelators*** crln Corn Root From 7 Day Old Seedlings* crln.pk0052.b8 Library Tissue Clone csi In Corn Silk* csiln.pk0017.d4 ctaln Corn Tassel* ctaln.pk0051.g3 ct 1 c Corn Tassel, Night Harvested ctnlc.pkOOl.llO lkr Corn 19 Day Old Seed lkr.pk0013.gl p0005 Corn Immature Ear p0005.cbmew77r pOO 15 Corn Embryo 13 Days After Pollination p0015.cdpfkl2r p0031 Corn (CM45) Shoot Culture p0031.ccmai08r p0031.ccmar07r p0032 Corn Regenerating Callus (Hi-II 223a and 1129e), p0032.crcas26r 10 and 14 Days After Auxin Removal, Pooled p0096 Corn Scutelum 2 and 3 Days After Germinating p0096.cnamt32r pO 102 Corn Early Meiosis Tassels* p0102.ceraq01r pO 115 Corn Leaf and Sheath Meristem Tissue Collected p0115.clsmj70r from 10th, 1 lth, and 12th Leaves, Pooled pO 125 Corn Anther Prophase I p0125.czabf83r p0126 Corn Leaf Tissue (V8-V10****), Night-Harvested ρ0126.cnlcu75r p0128 Corn Primary and Secondary Immature Ear p0128.cpibl07r rcaln Rice Callus* rcaln.pk004.p22 rds2c Rice Developing Seeds rds2c.pk004.i9 reslc Rice Etiolated Seedling reslc.pk004.c4 rlOn Rice 15 Day Old Leaf* rl0n.pkl l3.b4 rls24 Rice Leaf 15 Days After Germination, 24 Hours rls24.pk0002.e2 After Infection of Strain Magaporthe grisea 4360-R-67 (AVR2-YAMO); Susceptible rrl Rice Root of Two Week Old Developing Seedling rrl .pk0038.h6 rrl.pk0071.f2 sdp3c Soybean Developing Pods (8-9 mm) sdp3c.pk019.c6 sfll Soybean Immature Flower sfll.pkl29.dl0 sls2c Soybean Infected With Sclerotinia sclerotiorum sls2c.pk025.dl l mycelium src3c Soybean 8 Day Old Root Infected With Cyst src3c.pk004.cl9 Nematode wl 1 n Wheat Leaf From 7 Day Old Seedling* wlln.pk0109.d5 wlln.pk0110.cl wle In Wheat Leaf From 7 Day Old Etiolated Seedling* wleln.pk0075.b4 wlmO Wheat Seedlings 0 Hour After Inoculation With wlm0.pk0028.f4 Erysiphe graminisf sp tritici wlm96 Wheat Seedlings 96 Hours After Inoculation With wlm96.pk026.j l Erysiphe graminisf. sp tritici wlm96.pk041.h3 wlm96.pk045.cl l *These libraries were normalized essentially as described in U.S. Pat. No. 5,482,845 ** Application of 2-[(2,4-dihydro-2,6,9-trimethyl[l]benzothiopyrano[4,3-c]pyrazol-8- yl)carbonyl]-l,3-cyclohexanedione S,S-dioxide (synthesis and methods of using this compound are described in WO 97/19087, incorporated herein by reference) and 2- [(2,3-dihydro-5,8-dimetiιylspiro[4H-l-benzothiopyran-4,2'-[l,3]dioxolan]-6- yl)carbonyl]-l,3-cyclohexanedione S,5-dioxide (also named 2-[(2,3-dihydro-5,8- dimethylspiro[4H-l-benzothiopyran-4,2'-[l,3]dioxolan]-6-yl)carbonyl]-3-hydroxy-2- cyclohexen-1-one S,S-dioxide; synthesis and methods of using this compound are described in WO 97/01550, incorporated herein by reference). ***Chemicals used included nitrilotriacetic acid, mercaptobenzothiazole, diethyldithio- carbamate ****For description of corn stages see Iowa State University Cooperative Extension Service Special Report No. 48. cDNA libraries were prepared in Uni-ZAP™ XR vectors according to the manufacturer's protocol (Stratagene Cloning Systems, La Jolla, CA). Conversion ofthe Uni-ZAP™ XR libraries into plasmid libraries was accomplished according to the protocol provided by Stratagene. Upon conversion, cDNA inserts were contained in the plasmid vector pBluescript. cDNA inserts from randomly picked bacterial colonies containing recombinant pBluescript plasmids were amplified via polymerase chain reaction using primers specific for vector sequences flanking the inserted cDNA sequences or plasmid DNA was prepared from cultured bacterial cells. Amplified insert DNAs or plasmid DNAs were sequenced in dye-primer sequencing reactions to generate partial cDNA sequences (expressed sequence tags or "ESTs"; see Adams, M. D. et al., (1991) Science 252:1651). The resulting ESTs were analyzed using a Perkin Elmer Model 377 fluorescent sequencer. EXAMPLE 2 Identification of cDNA Clones ESTs encoding tryptophan biosynthetic enzymes were identified by conducting BLAST (Basic Local Alignment Search Tool; Altschul, S. F., et al., (1993) J. Mol. Biol. 275:403-410; 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 ofthe 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, W. and States, D. J. (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 ofthe logarithm ofthe 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 Anthranilate Synthase The BLASTX search using the EST sequences from several com, rice and wheat clones revealed similarity ofthe proteins encoded by the cDNAs to anthranilate synthase from Arabidopsis thaliana and Ruta graveolens. The BLASTX search using the nucleotide sequence from clone wreln.pk0036.gl0 also revealed similarity ofthe protein encoded by the cDNA to para-aminobenzoate synthase from Streptomyces pristinaespiralis (GenBank Accession No.U60417; pLog = 27.39). In the process of comparing the com ESTs it was found that clones cen3n.pk0210.c3, ctaln.pk0025.f8 and cen3n.pk0047.g5 had overlapping regions of homology. A comparison ofthe com ESTs from clones cen3n.pk0212.h6 and csiln.pk0017.d4 also had overlapping regions of homology. Using this homology it was possible to align the ESTs and assemble two contigs (a contig is an assemblage of overlapping nucleic acid sequences to form one contiguous nucleotide sequence). The individual sequences were assembled into unique contiguous nucleotide sequences encoding anthranilate synthase from com. The database accession numbers and BLAST results for each of these ESTs and contigs are shown in Table 3: TABLE 3 BLAST Results for Clones Encoding Polypeptides Homologous to Anthranilate Synthase Database Clone Organism Accession No Blast Score pLog Contig of: A. thaliana GenBank L22585 70.69 cen3n.pk0210.c3b ctaln.pk0025.f8 cen3n.pk0047.g5b ceb3.pk0011.cl2 R. graveolens GenBank L34344 48.15 ctaln.pk0051.g3 R. graveolens GenBank L34344 32.22 cbn2.pk0049.d2 R. graveolens GenBank L34344 35.00 Contig of: A. thaliana GenBank L22585 34.00 cen3n.pk0212.h6 csiln.pk0017.d4 ccoln.pk0038.bl A. thaliana GenBank L22585 17.69 rls24.pk0002.e2 A. thaliana GenBank L22585 20.69 wleln.pk0075.b4 R. graveolens GenBank L34343 38.00 Characterization ofcDNA Clones Encoding Anthranilate Synthase Alpha Subunit The sequence ofthe entire cDNA insert in clone ctaln.pk0051.g3 was determined. A contig was assembled with this sequence and sequences from portions ofthe cDNA inserts in clones cdelc.pk004.g2, p0125.czabf83r, p0005.cbmew77r, p0096.cnamt32r, p0032.crcas26r, p0031.ccmar07r, p0015.cdpfkl2r and p0128.cpibl07r. The sequence ofthe entire cDNA insert in clone ctaln.pk0051.g3 was determined. A contig was assembled with this sequence and sequences from portions ofthe cDNA in clones pOl 15.clsmj70r, cbn2.pk0049.d2, cpllc.pk008.m4 and p0102.ceraq01r. The BLASTX search using these contig sequences and the EST sequences from clones rcaln.pk004.p22, sdp3c.pk019.c6, sfll.pkl29.dl0, wleln.pk0075.b4, the 5' sequence from clone wlm96.pk026.j 1 and the 3' sequence from clone wlm96.pk026.jl revealed similarity ofthe protein encoded by the cDNA to anthranilate synthase alpha subunit from Ruta graveolens (NCBI General Identifier No. 960289). The BLAST results for each of these sequences are shown in Table 4: TABLE 4 BLAST Results for Clones Encoding Polypeptides Homologous to Anthranilate Synthase Alpha Subunit BLAST pLog Score Clone NCBI General Identifier No. 960289 Contig of: >254 cdelc.pk004.g2 p0125.czabf83r p0005.cbmew77r p0096.cnamt32r p0032.crcas26r p0031.ccmar07r p0015.cdpfkl2r p0128.cpibl07r ctaln.pk0051.g3 Contig of: >254 p0115.clsmj70r cbn2.pk0049.d2 cρllc.ρk008.m4 p0102.ceraq01r ceb3.pk0011.cl2 rcaln.pk004.p22 20.15 sdp3c.pk019.c6 48.52 sfll.pkl29.dl0 34.70 wleln.pk0075.b4 (5 'end) 57.00 wleln.pk0075.b4(3'end) 35.70 wlm96.pk026.jl 5.40 The sequence ofthe contig assembled from the entire cDNA insert from clone ctaln.pk0051.g3 and a portion ofthe cDNA insert in clones cdelc.pk004.g2, p0125.czabf83r, p0005.cbmew77r, p0096.cnamt32r, p0032.crcas26r, p0031.ccmar07r, p0015.cdpfkl2r and p0128.cpibl07r is shown in SEQ ID NO:l; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:2. The amino acid sequence set forth in SEQ ID NO:2 was evaluated by BLASTP, yielding a pLog value >254 versus the Ruta graveolens sequence. The sequence ofthe contig assembled from the entire cDNA insert in clone ctaln.pk0051.g3 and a portion ofthe cDNA in clones pOl 15.clsmj70r, cbn2.pk0049.d2, cpllc.pk008.m4 and p0102.ceraq01r is shown in SEQ ID NO:3; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:4. The sequence of a portion ofthe cDNA insert from clone rcaln.pk004.p22 is shown in SEQ ID NO:5; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:6. The sequence of a portion ofthe cDNA insert from clone sdp3c.pk019.c6 is shown in SEQ ID NO:7; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:8. The sequence of a portion ofthe cDNA insert from clone sfll.pkl29.dl0 is shown in SEQ ID NO:9; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO: 10. The sequence of the 5' terminal portion ofthe cDNA insert from clone wleln.pk0075.b4 is shown in SEQ ID NO:l 1; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO: 12. The sequence ofthe 3' terminal portion ofthe cDNA insert from clone wleln.pk0075.b4 is shown in SEQ ID NO: 13; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:14. The sequence of a portion ofthe cDNA insert from clone wlm96.pk026.jl is shown in SEQ ID NO: 15; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO: 16. Figure 2 presents an alignment ofthe amino acid sequence set forth in SEQ ID NO:2 and the Ruta graveolens anthranilate synthase alpha subunit sequence (SEQ ID NO:39). The amino acid sequence set forth in SEQ ID NO:2 is 64.7% similar to the Ruta graveolens sequence. Sequence alignments and percent similarity calculations were performed using the Megalign program ofthe LASARGENE bioinformatics computing suite (DNASTAR Inc., Madison, WI). Pairwise alignment ofthe sequences was performed using the Clustal method of alignment (Higgins, D.G. and Sharp, P.M. (1989) CABIOS. 5:151-153) with the default parameters (KTUPLE 1, GAP PENALTY=3, WINDOW=5 and DIAGONALS SAVED=5). Sequence alignments and BLAST scores and probabilities indicate that the instant nucleic acid fragments encode an entire com anthranilate synthase alpha subunit and portions of com, rice, soybean and wheat anthranilate synthase alpha subunits. These sequences represent the first com, rice, soybean and wheat sequences encoding anthranilate synthase alpha subunit. Characterization ofcDNA Clones Encoding Anthranilate Synthase Beta Subunit The sequence of a larger portion ofthe cDNA insert in clone cen3n.pk0210.c3 was determined, this sequence includes the sequence from clone ctaln.pk0025.f8. A contig was assembled with the sequence from clone cen3n.pk0210.c3 and sequence from a portion of the cDNA in clones cepe7.pk0012.g2 and cen3n.pk0047.g5. The BLASTX search using these contig sequences, the sequences from the contig assembled of clones p0126.cnlcu75r, cen3n.pk0212.h6, ccoln.pk0038.bl, csiln.pk0017.d4, p0031.ccmai08r and ctnlc.pkOOl.llO, the contig assembled from clones rds2c.pk004.i9, rls24.pk0002.e2 and riOn.pkl 13.b4 and the EST sequences from clone wlm96.pk045.cl 1 revealed similarity ofthe proteins encoded by the cDNAs to anthranilate synthase beta subunit from Arabidopsis thaliana (NCBI General Identifier No. 541849). The BLAST results for each of these ESTs are shown in Table 5: TABLE 5 BLAST Results for Clones Encoding Polypeptides Homologous to Anthranilate Synthase Beta Subunit BLAST pLog Score Clone NCBI General Identifier No. 541849 Contig of: 84.52 cen3n.ρk0210.c3 cepe7.pk0012.g2 cen3n.pk0047.g5 Contig of: 110.0 p0126.cnlcu75r cen3n.pk0212.h6 ccoln.pk0038.bl csiln.pk0017.d4 p0031.ccmai08r ctnlc.pkOOl.llO Contig of: 20.15 rds2c.ρk004.i9 rls24.pk0002.e2 rl0n.pkl l3.b4 The sequence ofthe contig assembled from a portion ofthe cDNA insert from clones cen3n.pk0210.c3, cepe7.pk0012.g2 and cen3n.pk0047.g5 is shown in SEQ ID NO:17; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO: 18. The sequence of the contig assembled from a portion ofthe cDNA insert from clones p0126.cnlcu75r, cen3n.pk0212.h6, ccoln.pk0038.bl, csi 1 n.pkOO 17.d4, p0031.ccmai08r and ctnlc.pkOOl.llO is shown in SEQ ID NO: 17; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO: 18. The amino acid sequence set forth in SEQ ID NO: 18 was evaluated by BLASTP, yielding a pLog value of 92.22 versus the Arabidopsis thaliana sequence. The sequence ofthe contig assembled from a portion ofthe cDNA insert from clones rds2c.pk004.i9, rls24.pk0002.e2 and riOn.pkl 13. b4 is shown in SEQ ID NO: 19; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO: 20. The sequence of a portion ofthe cDNA insert from clone wlm96.pk045.cl 1 is shown in SEQ ID NO:21; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:22. Figure 3 presents an alignment ofthe amino acid sequences set forth in SEQ ID NOs:20 and 22 and the Arabidopsis thaliana anthranilate synthase beta subunit sequence (SEQ ID NO:40). The amino acid sequences set forth in SEQ ID NO:20 are 58.0% similar to the Arabidopsis thaliana sequence while the amino acid sequences set forth in SEQ ID NO:22 are 57.1%) similar to the Arabidopsis thaliana sequence. Sequence alignments and percent similarity calculations were performed using the Megalign program ofthe LASARGENE bioinformatics computing suite (DNASTAR Inc., Madison, WI). Multiple alignment ofthe sequences was performed using the Clustal method of alignment (Higgins, D.G. and Sharp, P.M. ( 1989) CABIOS. 5:151-153) with the default parameters (GAP PENALTY=10, GAP LENGTH PENALTY=10). Sequence alignments and BLAST scores and probabilities indicate that the instant nucleic acid fragments encode an entire com anthranilate synthase beta subunit, an almost entire rice anthranilate synthase beta subunit and portions of com and wheat anthranilate synthase beta subunit. These sequences represent the first com, rice and wheat sequences encoding anthranilate synthase beta subunit. EXAMPLE 4 Characterization of cDNA Clones Encoding Tryptophan Synthase The BLASTX search using the EST sequences from several com, rice, soybean and wheat clones revealed similarity ofthe proteins encoded by the cDNAs to tryptophan synthase from Zea mays and Arabidopsis thaliana. In the process of comparing the rice ESTs it was found that clones wlln.pk0109.d5 and wlln.pk01 lO.cl had overlapping regions of homology. A comparison ofthe com ESTs from clones chp2.pk0020.e2, cen3n.pk0147.d4, lkr.pk0013.gl, csl/pk0089.a8, ctaln.pk0057.d6, cen3n.pk0014.c5, cen3n.pk0193.c5 and cen3n.pk0135.dl also revealed overlapping regions of homology. A comparison ofthe com ESTs from clones m.l5.6.g07.sk20, m.l5.4.g07.sk20 and ccoln.pk0030.g7 revealed overlapping regions of homology. Using this homology it was possible to align the ESTs and assemble four contigs (a contig is an assemblage of overlapping nucleic acid sequences to form one contiguous nucleotide sequence). The individual sequences were assembled into unique contiguous nucleotide sequences encoding tryptophan synthase from com, rice, soybean and wheat. The database accession numbers and BLAST results for each of these ESTs and contigs are shown in Table 6: TABLE 6 BLASTX Results for Clones Encoding Polypeptides Homologous to Trypotophan Synthase Database Clone Organism Accession No Blast Score pLog rrl.pk0038.h6 A. thaliana GenBank U 18993 26.00 Contig of: Z. mays GenBank X76713 52.39 wlln.pk0109.d5 wlln.pk0110.cl wlln.pk0067.f2 A. thaliana GenBank Ul 8993 19.69 crln.pk0033.f8 Z. mays GenBank X76713 47.69 crln.pk0052.b8 Z. mays GenBank X76713 25.70 Database Clone Organism Accession No Blast Score pLog Contig of: Z mays GenBank X76713 83.40 chp2.pk0020.e2 cen3n.pk0147.d4 lkr.pk0013.gl csl.pk0089.a8 ctaln.pk0057.d6 cen3n.pk0014.c5 cen3n.pk0193.c5 cen3n.pk0135.dl Contig of: Z. mays GenBank X76713 25.52 m.l5.6.g07.sk20 ml5.4.g07.sk20 ccoln.pk0030.g7 Characterization ofcDNA Clones Encoding Tryptophan Synthase Alpha Subunit The sequence ofthe entire cDNA insert in clone lkr.pk0013.gl and a larger portion of the sequence from clone chp2.pk0020.e2 have been determined. A contig has been assembled with these two sequences and they have been found to include the sequences from clones ccoln.pk0030.g7, m.l5.6.g07.sk20, cen3n.ρk0014.c5, cen3n.pk0135.dl, cen3n.pk0147.d4, cen3n.pk0193 5, ctaln.pk0057.d6, csl.pk0089.a8 and m.l5.6.g07.sk20. The sequence from the entire cDNA insert in clone crln.pk0052.b8 has been determined, it includes the sequences from clone crln.pk0033.f8. The BLASTX search using these sequences, the sequence from the contig assembled of clones reslc.pk004.c4 and rrl.pk0071.f2, the EST sequences from clone rrl.pk0038.h6 and the sequence ofthe contig assembled from clones src3c.pk004.cl9 and sls2c.pk025.dl 1 revealed similarity ofthe proteins encoded by the cDNAs to tryptophan synthase alpha subunit from Arabidopsis thaliana (NCBI General Identifier No. 2129755). The BLASTX search using the sequence ofthe contig assembled from clones wll .pkOl 09. d5 and wlln.pkOl 10. cl revealed similarity ofthe proteins encoded by the cDNAs to tryptophan synthase alpha subunit from Zea mays (NCBI General Identifier No. 1174783). The BLAST results for each of these sequences are shown in Table 7: TABLE 7 BLAST Results for Clones Encoding Polypeptides Homologous to Tryptophan Synthase Alpha Subunit Clone NCBI General Identifier No . BLAST pLog Score Contig of: 2129755 124.0 chp2.pk0020.e2 lkr.pk0013.gl crln.pk0052.b8 2129755 120.0 Contig of: 2129755 78.70 reslc.pk004.c4 rrl.pk0071.f2 rrl.pk0038.h6 2129755 125.0 Contig of: 2129755 49.00 src3c.pk004.cl9 sls2c.pk025.dl l Contig of: 1174783 33.70 wlm0.pk0028.f4 wlm96.pk041.h3 Contig of: 1174783 85.70 wlln.pk0109.d5 wlln.pk0110.cl The sequence ofthe contig assembled from the entire cDNA insert in clone lkr.pk0013.gl and a large portion ofthe cDNA insert in clone chp2.pk0020.e2 is shown in SEQ ID NO:25; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:26. The amino acid sequence set forth in SEQ ID NO:26 was evaluated by BLASTP, yielding a pLog value of 105.0 versus the Arabidopsis thaliana sequence. The sequence ofthe entire cDNA insert in clone crln.pk0052.b8 was determined and is shown in SEQ ID NO:27; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:28. The amino acid sequence set forth in SEQ ID NO:28 was evaluated by BLASTP, yielding a pLog value of 98.22 versus the Arabidopsis thaliana sequence. The sequence ofthe contig assembled from a portion ofthe cDNA insert in clones reslc.pk004.c4 and rrl .pk0071.f2 is shown in SEQ ID NO:29; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:30. The sequence ofthe entire cDNA insert in clone rrl.pk0038.h6 was determined and is shown in SEQ ID NO:31; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:32. The amino acid sequence set forth in SEQ ID NO:32 was evaluated by BLASTP, yielding a pLog value of 106.0 versus the Arabidopsis thaliana sequence. The sequence ofthe contig assembled from a portion ofthe cDNA insert from clones src3c.pk004.cl9 and sls2c.pk025.dl 1 is shown in SEQ ID NO:33; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:34. The sequence ofthe contig assembled from a portion of the cDNA insert from clones wlm0.pk0028.f4 and wlm96.pk041.h3 is shown in SEQ ID NO:35; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:36. The sequence ofthe contig assembled from a portion ofthe cDNA insert from clones wlln.pk0109.d5 and wlln.pkOl lO.cl is shown in SEQ ID NO:37; the deduced amino acid sequence of this cDNA is shown in SEQ ID NO:38. Figure 3 presents an alignment ofthe amino acid sequences set forth in SEQ ID NOs:26, 28 and 32 and the Arabidopsis thaliana sequence (SEQ ID NO:41). The data in Table 8 represents a calculation ofthe percent identity ofthe amino acid sequences set forth in SEQ ID NOs:26, 28 and 32 and the Arabidopsis thaliana tryptophan synthase alpha subunit sequence. TABLE 8 Percent Identity of Amino Acid Sequences Deduced From the Nucleotide Sequences of cDNA Clones Encoding Polypeptides Homologous to Tryptophan Synthase Alpha Subunit Percent Identity to Clone SEQ ID NO. 2129755 Contig of: 26 59.0 Chp2.pk0020.e2 lkr.pk0013.gl crln.pk0052.b8 28 56.7 irl.pk0038.h6 32 59.9 Sequence alignments and percent similarity calculations were performed using the Megalign program ofthe LASARGENE bioinformatics computing suite (DNASTAR Inc., Madison, WT). Multiple alignment ofthe sequences was performed using the Clustal method of alignment (Higgins, D.G. and Sharp, P.M. (1989) CABIOS. 5:151-153) with the default parameters (GAP PENALTY=10, GAP LENGTH PENALTY=10). Sequence alignments and BLAST scores and probabilities indicate that the instant nucleic acid fragments encode entire or nearly entire com and rice tryptophan synthase alpha subunit and portions of rice, soybean and wheat tryptophan synthase alpha subunit. These sequences represent the first rice, soybean and wheat and variant com sequences encoding tryptophan synthase alpha subunit. EXAMPLE 5 Expression of Chimeric Genes in Monocot Cells A chimeric gene comprising a cDNA encoding a tryptophan biosynthetic enzyme 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) ofthe cDNA clone using appropriate oligonucleotide primers. Cloning sites (Ncol or Smal) 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 Ncol and Smal and fractionated on an agarose gel. The appropriate band can be isolated from the gel and combined with a 4.9 kb Ncol-Smal fragment ofthe plasmid pML103. Plasmid pML103 has been deposited under the terms ofthe 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 Sall-Ncol promoter fragment ofthe maize 27 kD zein gene and a 0.96 kb Smal-Sall fragment from the 3' end ofthe maize 10 kD zein gene in the vector pGem9Zf(+) (Promega). Vector and insert DNA can be ligated at 15°C overnight, essentially as described (Maniatis). The ligated DNA may then be used to transform E. coli XL 1 -Blue (Εpicurian Coli XL-1 Blue™; 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™ 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 a tryptophan biosynthetic enzyme, and the 10 kD zein 3' region. The chimeric gene described above can then be introduced into corn cells by the following procedure. Immature com embryos can be dissected from developing caryopses derived from crosses ofthe inbred com 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°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. The plasmid, p35S/Ac (obtained from Dr. Peter Εckes, 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 ofthe 35S promoter from Cauliflower Mosaic Vims (Odell et al. (1985) Nature 313:810-812) and the 3' region ofthe nopaline synthase gene from the T-DNA ofthe Ti plasmid of Agrobacterium tumefaciens. 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 μm in diameter) are coated with DNA using the following technique. Ten μg of plasmid DNAs are added to 50 μL of a suspension of gold particles (60 mg per mL). Calcium chloride (50 μL of a 2.5 M solution) and spermidine free base (20 μL 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 μL 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 μL of ethanol. An aliquot (5 μL) ofthe DNA-coated gold particles can be placed in the center of a Kapton™ flying disc (Bio-Rad Labs). The particles are then accelerated into the com tissue with a Biolistic™ PDS-1000 He (Bio-Rad Instruments, Hercules CA), using a helium pressure of 1000 psi, a gap distance of 0.5 cm and a flying distance of 1.0 cm. 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. 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 ofthe plates containing the glufosinate- supplemented medium. These calli may continue to grow when sub-cultured on the selective medium. 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 5:833-839). EXAMPLE 6 Expression of Chimeric Genes in Dicot Cells A seed-specific expression cassette composed ofthe promoter and transcription terminator from the gene encoding the β subunit ofthe seed storage protein phaseolin from the bean Phaseolus vulgaris (Doyle et al. (1986) J. Biol. Chem. 261 :9228-9238) can be used for expression ofthe instant tryptophan biosynthetic enzyme 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. 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. Soybean embroys may then be transformed with the expression vector comprising sequences encoding a tryptophan biosynthetic enzyme. To induce somatic embryos, cotyledons, 3-5 mm in length dissected from surface sterilized, immature seeds ofthe soybean cultivar A2872, can be cultured in the light or dark at 26°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. Soybean embryogenic suspension cultures can maintained in 35 mL liquid media on a rotary shaker, 150 rpm, at 26°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. Soybean embryogenic suspension cultures may then be transformed by the method of particle gun bombardment (Kline et al. (1987) Nature (London) 327:70, U.S. Patent No. 4,945,050). A DuPont Biolistic™ PDS1000/HE instrument (helium retrofit) can be used for these transformations. A selectable marker gene which can be used to facilitate soybean transformation is a chimeric gene composed ofthe 35S promoter from Cauliflower Mosaic Virus (Odell et al.(1985) Nature 373:810-812), the hygromycin phosphotransferase gene from plasmid pJR225 (from E. coli; Gritz et al.(1983) Gene 25:179-188) and the 3' region ofthe nopaline synthase gene from the T-DNA ofthe Ti plasmid of Agrobacterium tumefaciens. The seed expression cassette comprising the phaseolin 5' region, the fragment encoding the tryptophan biosynthetic enzyme and the phaseolin 3' region can be isolated as a restriction fragment. This fragment can then be inserted into a unique restriction site ofthe vector carrying the marker gene. To 50 μL of a 60 mg/mL 1 μm gold particle suspension is added (in order): 5 μL DNA (1 μg/μL), 20 μl spermidine (0.1 M), and 50 μL CaCl 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 μL 70% ethanol and resuspended in 40 μL of anhydrous ethanol. The DNA/particle suspension can be sonicated three times for one second each. Five μL ofthe DNA-coated gold particles are then loaded on each macro carrier disk. Approximately 300-400 mg of a two-week-old suspension culture is placed in an empty 60x15 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. 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 The cDNAs encoding the instant tryptophan biosynthetic enzymes can be inserted into the T7 E. coli expression vector pBT430. This vector is a derivative of pET-3a (Rosenberg et al. (1987) Gene 5(5:125-135) 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. 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™ low melting agarose gel (FMC). Buffer and agarose contain 10 μg/ml ethidium bromide for visualization ofthe DNA fragment. The fragment can then be purified from the agarose gel by digestion with GELase™ (Epicentre Technologies) according to the manufacturer's instructions, ethanol precipitated, dried and resuspended in 20 μL of water. Appropriate oligonucleotide adapters may be ligated to the fragment using T4 DNA ligase (New England Biolabs, Beverly, MA). 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°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 μg/mL ampicillin. Transformants containing the gene encoding the tryptophan biosynthetic enzyme are then screened for the correct orientation with respect to the T7 promoter by restriction enzyme analysis. 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°C. At an optical density at 600 nm of approximately 1, IPTG (isopropylthio-β-galactoside, the inducer) can be added to a final concentration of 0.4 mM and incubation can be continued for 3 h at 25°. Cells are then harvested by centrifugation and re-suspended in 50 μL 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 ofthe supernatant determined. One μg of protein from the soluble fraction ofthe 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 Tryptophan Biosynthetic Enzymes The tryptophan biosynthetic enzymes 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, inplanta, and using viral expression systems in suitably infected organisms or cell lines. The instant tryptophan biosynthetic enzymes may be expressed either as mature forms ofthe 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)^"). 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. Purification ofthe instant tryptophan biosynthetic enzymes, 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 tryptophan biosynthetic enzymes 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, a tryptophan biosynthetic enzyme may be expressed as a fusion protein coupled to the C-terminus of thioredoxin. In addition, a (His)6 peptide may be engineered into the N-terminus ofthe 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 β-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 ofthe 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™ affinity resin or other resin. 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 ofthe tryptophan biosynthetic enzymes disclosed herein. Assays may be conducted under well known experimental conditions which permit optimal enzymatic activity. For example, assays for anthranilate synthase alpha subunit are presented by Bohlmann J et al. (1995) Plant J 7:491-501. Assays for anthranilate synthase beta subunit are presented by Walker M. S. and DeMoss J. A. (1983) JBiol Chem 258:3571-3575. Assays for tryptophan synthase alpha subunit are presented by Zhao J and Last R. L. (1995) JBiol Chem 270:6081-7.

Claims: CLAIMS What is claimed is: 1. An isolated nucleic acid fragment encoding all or a substantial portion of an anthranilate synthase alpha subunit comprising a member selected from the group consisting of: (a) an isolated nucleic acid fragment encoding all or a substantial portion of 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, SEQ ID NO:8, SEQ ID NO:10, SEQID NO:12, SEQ ID NO:14 and SEQ ID NO:16; (b) an isolated nucleic acid fragment that is substantially similar to an isolated nucleic acid fragment encoding all or a substantial portion of 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, SEQ ID NO:8, SEQ ID NO: 10, SEQID NO: 12, SEQ ID NO: 14 and SEQ ID NO: 16; and (c) an isolated nucleic acid fragment that is complementary to (a) or (b). 2. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:2. 3. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:4. 4. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:6. 5. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:8. 6. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO: 10. 7. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO: 12. 8. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:14. 9. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO: 16. 10. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:2. 11. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment is at least 80%) similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:4. 12. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:6. 13. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:8. 14. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:10. 15. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO: 12. 16. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment is at least 80%) similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO: 14. 17. The isolated nucleic acid fragment of Claim 1 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO: 16. 18. The isolated nucleic acid fragment of Claim 1 wherein the nucleotide sequence ofthe fragment comprises all or a portion ofthe sequence set forth in a member selected from the group consisting of SEQ ID NO:l, SEQ ID NO:3, SEQ ID NO:5, SEQ ID NO:7, SEQ ID NO:9, SEQID NO:l l, SEQ ID NO:13 and SEQ ID NO:15. 19. A chimeric gene comprising the nucleic acid fragment of Claim 1 operably linked to suitable regulatory sequences. 20. A transformed host cell comprising the chimeric gene of Claim 19. 21. An anthranilate synthase alpha subunit polypeptide comprising all or a substantial portion ofthe 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, SEQ ID NO:8, SEQ ID NO: 10, SEQID NO: 12, SEQ ID NO: 14 and SEQ ID NO: 16. 22. An isolated nucleic acid fragment encoding all or a substantial portion of an anthranilate synthase beta subunit comprising a member selected from the group consisting of: (a) an isolated nucleic acid fragment encoding all or a substantial portion of the amino acid sequence set forth in a member selected from the group consisting of SEQ ID NO: 18, SEQ ID NO:20, SEQ ID NO:22 and SEQ ID NO:24; (b) an isolated nucleic acid fragment that is substantially similar to an isolated nucleic acid fragment encoding all or a substantial portion of the amino acid sequence set forth in a member selected from the group consisting of SEQ ID NO: 18, SEQ ID NO:20, SEQ ID NO:22 and SEQ ID NO:24; and (c) an isolated nucleic acid fragment that is complementary to (a) or (b). 23. The isolated nucleic acid fragment of Claim 22 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO: 18. 24. The isolated nucleic acid fragment of Claim 22 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:20. 25. The isolated nucleic acid fragment of Claim 22 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:22. 26. The isolated nucleic acid fragment of Claim 22 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:24. 27. The isolated nucleic acid fragment of Claim 22 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO: 18. 28. The isolated nucleic acid fragment of Claim 22 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:20. 29. The isolated nucleic acid fragment of Claim 22 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:22. 30. The isolated nucleic acid fragment of Claim 22 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:24. 31. The isolated nucleic acid fragment of Claim 22 wherein the nucleotide sequence ofthe fragment comprises all or a portion ofthe sequence set forth in a member selected from the group consisting of SEQ ID NO: 17, SEQ ID NO:19, SEQ ID NO:21 and SEQ ID NO:23. 32. A chimeric gene comprising the nucleic acid fragment of Claim 22 operably linked to suitable regulatory sequences. 33. A transformed host cell comprising the chimeric gene of Claim 32. 34. An anthranilate synthase beta subunit polypeptide comprising all or a substantial portion ofthe amino acid sequence set forth in a member selected from the group consisting of SEQ ID NO: 18, SEQ ID NO:20, SEQ ID NO:22 and SEQ ID NO:24. 35. An isolated nucleic acid fragment encoding all or a substantial portion of a tryptophan synthase alpha subunit comprising a member selected from the group consisting of: (a) an isolated nucleic acid fragment encoding all or a substantial portion of the amino acid sequence set forth in a member selected from the group consisting of SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, SEQ ID NO:32, SEQ ID NO:34, SEQ ID NO:36 and SEQ ID NO:38; (b) an isolated nucleic acid fragment that is substantially similar to an isolated nucleic acid fragment encoding all or a substantial portion of the amino acid sequence set forth in a member selected from the group consisting of SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, SEQ ID NO:32, SEQ ID NO:34, SEQ ID NO:36 and SEQ ID NO:38; and (c) an isolated nucleic acid fragment that is complementary to (a) or (b). 36. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:26. 37. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:28. 38. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:30. 39. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:32. 40. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:34. 41. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:36. 42. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment hybridizes under stringent conditions to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:38. 43. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:26. 44. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:28. 45. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:30. 46. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:32. 47. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:34. 48. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:36. 49. The isolated nucleic acid fragment of Claim 35 wherein the isolated nucleic acid fragment is at least 80% similar to an isolated nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence set forth in SEQ ID NO:38. 50. The isolated nucleic acid fragment of Claim 35 wherein the nucleotide sequence ofthe fragment comprises all or a portion ofthe sequence set forth in a member selected from the group consisting of SEQ ID NO:25, SEQ ID NO:27, SEQ ID NO:29, SEQ ID NO:31, SEQ ID NO:33, SEQ ID NO:35 and SEQ ID NO:37. 51. A chimeric gene comprising the nucleic acid fragment of Claim 35 operably linked to suitable regulatory sequences. 52. A transformed host cell comprising the chimeric gene of Claim 51. 53. A tryptophan synthase alpha subunit polypeptide comprising all or a substantial portion ofthe amino acid sequence set forth in a member selected from the group consisting of SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, SEQ ID NO:32, SEQ ID NO:34, SEQ ID NO:36 and SEQ ID NO:38. 54. A method of altering the level of expression of a tryptophan biosynthetic enzyme in a host cell comprising: (a) transforming a host cell with the chimeric gene of any of Claims 19, 32 and 51 ; and (b) growing the transformed host cell produced in step (a) under conditions that are suitable for expression ofthe chimeric gene wherein expression ofthe chimeric gene results in production of altered levels of a tryptophan biosynthetic enzyme in the transformed host cell. 55. A method of obtaining a nucleic acid fragment encoding all or a substantial portion ofthe amino acid sequence encoding a tryptophan biosynthetic enzyme comprising: (a) probing a cDNA or genomic library with the nucleic acid fragment of any of Claims 1, 22 and 35; (b) identifying a DNA clone that hybridizes with the nucleic acid fragment of any of Claims 1, 22 and 35; (c) isolating the DNA clone identified in step (b); and (d) sequencing the cDNA or genomic fragment that comprises the clone isolated in step (c) wherein the sequenced nucleic acid fragment encodes all or a substantial portion ofthe amino acid sequence encoding a tryptophan biosynthetic enzyme. 56. A method of obtaining a nucleic acid fragment encoding a substantial portion of an amino acid sequence encoding a tryptophan biosynthetic enzyme comprising: (a) synthesizing an oligonucleotide primer corresponding to a portion ofthe sequence set forth in any of SEQ ID NOs:l, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35 and 37; and (b) amplifying a cDNA insert present in a cloning vector using the oligonucleotide primer of step (a) and a primer representing sequences ofthe cloning vector wherein the amplified nucleic acid fragment encodes a substantial portion of an amino acid sequence encoding a tryptophan biosynthetic enzyme. 57. The product ofthe method of Claim 55. 58. The product ofthe method of Claim 56. 59. A method for evaluating at least one compound for its ability to inhibit the activity of a tryptophan biosynthetic enzyme, the method comprising the steps of: (a) transforming a host cell with a chimeric gene comprising a nucleic acid fragment encoding a tryptophan biosynthetic enzyme, operably linked to suitable regulatory sequences; (b) growing the transformed host cell under conditions that are suitable for expression ofthe chimeric gene wherein expression ofthe chimeric gene results in production ofthe tryptophan biosynthetic enzyme encoded by the operably linked nucleic acid fragment in the transformed host cell; (c) optionally purifying the tryptophan biosynthetic enzyme expressed by the transformed host cell; (d) treating the tryptophan biosynthetic enzyme with a compound to be tested; and (e) comparing the activity ofthe tryptophan biosynthetic enzyme that has been treated with a test compound to the activity of an untreated tryptophan biosynthetic enzyme, thereby selecting compounds with potential for inhibitory activity.

Inventors: BROGLIE RICHARD M, BRYAN GREGORY T, CAHOON REBECCA E, FALCO SAVERIO CARL, RAFALSKI J ANTONI, VOLLMER STEVEN J, BROGLIE, RICHARD, M., BRYAN, GREGORY, T., CAHOON, REBECCA, E., FALCO, SAVERIO, CARL, RAFALSKI, J., ANTONI, VOLLMER, STEVEN, J.
Applicants: BROGLIE RICHARD M, BRYAN GREGORY T, CAHOON REBECCA E, DU PONT, FALCO SAVERIO CARL, RAFALSKI J ANTONI, VOLLMER STEVEN J, BROGLIE, RICHARD, M., BRYAN, GREGORY, T., CAHOON, REBECCA, E., E.I. DU PONT DE NEMOURS AND COMPANY, FALCO, SAVERIO, CARL, RAFALSKI, J., ANTONI, VOLLMER, STEVEN, J.
Application year: 1999
Main classification: C12N15
IPC codes: C12N1, C12N9, C12N15