COMPOSITIONS AND METHODS FOR TAXOL BIOSYNTHESIS

Abstract: The taxadiene synthase gene of Pacific yew has been cloned and its nucleic acid and polypeptide sequence is presented. Truncation or removal of the transit peptide increases expression of the cloned taxadiene synthase gene expression in E. coli cells.

Description: COMPOSITIONS AND METHODS FOR TAXOL BIOSYNTHESIS CROSS REFERENCE TO RELATED CASE This application claims the benefit of U.S. Provisional Application No. 60/015,993, filed April 15, 1996, incorporated herein by reference. TECHNICAL FIELD This invention is related to the field of detection of diterpenoid biosynthesis, panicularly to the biosynthesis of taxoid compounds such as Taxol . ACKNOWLEDGMENT OF GOVERNMENT SUPPORT This invention was made with government support under National Institutes of Health Grant No. CA-55254. The government has certain rights in this invention. BACKGROUND ART The highly functionalized diterpenoid Taxol (Wani et al. , J. Am. Chem. Soc. 93:2325-2327, 1971) is well-established as a potent chemotherapeutic agent (Holmes et al., in Taxane Anticancer Agents: Basic Science and Current Status, Georg et al. , eds., pp. 31-57, American Chemical Society, Washington, DC, 1995; Arbuck and Blaylock, in Taxol: Science and Applications, Suffness, ed., pp. 379-415, CRC Press, Boca Raton, FL, 1995). (Paclitaxel is the generic name for Taxol, a registered trademark of Bristol-Myers Squibb.) The supply of Taxol from the original source, the bark of the Pacific yew (Taxus brevifolia Nutt.; Taxaceae) is limited. As a result, there have been intensive efforts to develop alternate means of production, including isolation from the foliage and other renewable tissues of plantation-grown Taxus species, biosynthesis in tissue culture systems, and semisynthesis of Taxol and its analogs from advanced taxane diterpenoid (taxoid) metabolites that are more readily available (Cragg et al. , J. Nat. Prod. 56:1657-1668, 1993). Total synthesis of Taxol, at present, is not commercially viable (Borman, Chem. Eng. News 72(7):32-34, 1994), and it is clear that in the foreseeable future the supply of Taxol and its synthetically useful progenitors must rely on biological methods of production, either in Taxus plants or in cell cultures derived therefrom (Suffness, in Taxane Anticancer Agents: Basic Science and Current Status, Georg et al. , eds., American Chemical Society, Washington, DC, 1995, pp. 1-17). The biosynthesis of Taxol involves the initial cyclization of geranylgeranyl diphosphate, the universal precursor of diterpenoids (West, in Biosynthesis of Isoprenoid Compounds, Porter and Spurgeon, eds., vol. 1, pp. 375-411, Wiley & Sons, New York, NY, 1981), to taxa-4(5),l l(12)-diene (Koepp et al , J. Biol. Chem. 270:8686-8690, 1995) followed by extensive oxidative modification of this olefin (Koepp et al , J. Biol. Chem. 270:8686-8690, 1995; Croteau et al. , in Taxane Anticancer Agents: Basic Science and Current Status, Georg et al , eds., pp. 72-80, American Chemical Society, Washington, DC, 1995) and elaboration of the side chains (FIG. 1) (Floss and Mocek, in Taxol: Science and Applications , Suffness, ed., pp. 191-208, CRC Press, Boca Raton, FL, 1995). Taxa-4(5),l l(12)-diene synthase ("taxadiene synthase"), the enzyme responsible for the initial cyclization of geranylgeranyl diphosphate, to delineate the taxane skeleton, has been isolated from T. brevifolia stem tissue, partially purified, and characterized (Hezari et al. , Arch. Biochem. Biophys. 322:437-444, 1995). Although taxadiene synthase resembles other plant terpenoid cyclases in general enzymatic properties (Hezari et al., Arch. Biochem. Biophys. 322:437-444, 1995), it has proved extremely difficult to purify in sufficient amounts for antibody preparation or microsequencing, thwarting this approach toward cDNA cloning. SUMMARY OF THE INVENTION We have cloned and sequenced the taxadiene synthase gene of Pacific yew. One embodiment of the invention includes isolated polynucleotides comprising at least 15 consecutive nucleotides, preferably at least 20, more preferably at least 25, and most preferably at least 30 consecutive nucleotides of a native taxadiene synthase gene, e.g. , the taxadiene synthase gene of Pacific yew. Such polynucleotides are useful, for example, as probes and primers for obtaining homologs of the taxadiene synthase gene of Pacific yew by, for example, contacting a nucleic acid of a taxoid-producing organism with such a probe or primer under stringent hybridization conditions to permit the probe or primer to hybridize to a taxadiene synthase gene of the organism, then isolating the taxadiene synthase gene of the organism to which the probe or primer hybridizes. Another embodiment of the invention includes isolated polynucleotides comprising a sequence that encodes a polypeptide having taxadiene synthase biological activity. Preferably, the polypeptide-encoding sequence has at least 70 % , preferably at least 80 % , and more preferably at least 90 % nucleotide sequence similarity with a native Pacific yew taxadiene synthase polynucleotide gene. In preferred embodiments of such polynucleotides, the polypeptide-encoding sequence encodes a polypeptide having only conservative amino acid substitutions to the native Pacific yew taxadiene synthase polypeptide, except, in some embodiments, for amino acid substitutions at one or more of: cysteine residues 329, 650, 719, and 777; histidine residues 370, 415, 579, and 793; a DDXXD motif; a DXXDD motif; a conserved arginine; and a RWWK element. Preferably, the encoded polypeptide has only conservative amino acid substitutions to or is completely homologous with the native Pacific yew taxadiene synthase polypeptide. In addition, the encoded polypeptide preferably lacks at least part of the transit peptide. Also included are cells, particularly plant cells, and transgenic plants that include such polynucleotides and the encoded polypeptides. Another embodiment of the invention includes isolated polypeptides having taxadiene synthase activity, preferably having at least 70% , more preferably at least 80%, and most preferably at least 90% homology with a native taxadiene synthase polypeptide. Also included are isolated polypeptides that comprise at least 10, preferably at least 20, more preferably at least 30 consecutive amino acids of a native Pacific yew taxadiene synthase, and most preferably the mature Pacific yew taxadiene synthase polypeptide (i.e. , lacking only the transit peptide). Another embodiment of the invention includes antibodies specific for a native Pacific yew taxadiene synthase polypeptide. Another embodiment of the invention includes methods of expressing a taxadiene synthase polypeptide in a cell, e.g. , a taxoid-producing cell, by culturing a cell that includes an expressible polynucleotide encoding a taxadiene synthase polypeptide under conditions suitable for expression of the polypeptide, preferably resulting in the production of the taxoid at levels that are higher than would be expected from an otherwise similar cell that lacks the expressible polynucleotide. The foregoing and other objects and advantages of the invention will become more apparent from the following detailed description and accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows steps in the biosynthesis of Taxol, including the initial cyclization of geranylgeranyl diphosphate to taxa-4(5), l l(12)-diene, followed by extensive oxidative modification and elaboration of the side chains. FIG. 2 shows the nucleotide and predicted amino acid sequence of Pacific yew taxadiene synthase clone pTb 42.1. The start and stop codons are underlined. The locations of regions employed for primer synthesis are double underlined. The DDMAD and DSYDD motifs are in boldface. Conserved histidines (H) and cysteines (C) and an RWWK element are indicated by boxes. Truncation sites for removal of part or all of the transit peptide are indicated by a triangle (▼). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A homology-based cloning strategy using the polymerase chain reaction (PCR) was employed to isolate a cDNA encoding taxadiene synthase. A set of degenerate primers was constructed based on consensus sequences of related monoterpene, sesquiterpene, and diterpene cyclases. Two of these primers amplified a 83 base pair (bp) fragment that was cyclase-like in sequence and that was employed as a hybridization probe to screen a cDNA library constructed from poly(A) + RNA extracted from Pacific yew stems. Twelve independent clones with insert size in excess of two kilobase pairs (kb) were isolated and partially sequenced. One of these cDNA isolates was functionally expressed in Escherichia coli, yielding a protein that was catalytically active in converting geranylgeranyl diphosphate to a diterpene olefin that was confirmed to be taxa-4(5), l l(12)-diene by combined capillary gas chromatography-mass spectrometry (Satterwhite and Croteau, J. Chromatography 452:61-73, 1988). The taxa-4(5), 1 l(12)-diene synthase cDNA sequence specifies an open reading frame of 2586 nucleotides. The deduced polypeptide sequence contains 862 amino acid residues and has a molecular weight of 98,303, compared to about 79,000 previously determined for the mature native enzyme. It therefore appears to be full-length and includes a long presumptive plastidial targeting peptide. Sequence comparisons with monoterpene, sesquiterpene, and diterpene cyclases of plant origin indicate a significant degree of similarity between these enzymes; the taxadiene synthase most closely resembles (46% identity, 67% similarity) abietadiene synthase, a diterpene cyclase from grand fir. Uses of the Taxadiene Synthase Gene Increasing Taxol Biosynthesis in Transformed Cells. The committed step of Taxol (paclitaxel) biosynthesis is the initial cyclization of geranylgeranyl diphosphate, a ubiquitous isoprenoid intermediate, catalyzed by taxadiene synthase, a diterpene cyclase. The product of this reaction is the parent olefin with a taxane skeleton, taxa-4(5), 1 l(12)-diene. For a review of taxoids and taxoid biochemistry, see, e.g. , Kingston et al , "The Taxane Diterpenoids," Progress in the Chemistry of Organic Natural Products, vol. 61 , Springer Verlag, New York, 1993, pp. 1-206. The committed cyclization step of the target pathway is a slow step in the extended biosynthetic sequence leading to Taxol and related taxoids (Koepp et al , J. Biol Chem. 270:8686- 8690, 1995; Hezari et al , Arch. Biochem. Biophys. 322:437-444, 1995). The yield of Taxol and related taxoids (e.g. , cephalomannine, baccatins, taxinines, among others) in cells of an organism capable of taxoid biosynthesis is increased by the expression in such cells of a recombinant taxadiene synthase gene. This approach to increasing taxoid biosynthesis can be used in any organism that is capable of taxoid biosynthesis. Taxol synthesis is known to take place, for example, in the Taxaceae, including Taxus species from all over the world (including, but not limited to, T. brevifolia, T. baccata, T. x media, T. cuspidata, T. canadensis, and T. chinensis), as well as in certain microorganisms. Taxol may also be produced by a fungus, Taxomyces andreanae (Stierle et al , Science 260:214, 1993). Agrobacterium tumefaciens-meάidΛcd transformation of Taxus species has been described and the resulting callus cultures shown to produce Taxol (Han et al. , Plant Sci. 95: 187-196, 1994). Taxol can be isolated from cells transformed with the taxadiene synthase gene by conventional methods. The production of callus and suspension cultures of Taxus, and the isolation of Taxol and related compounds from such cultures, has been described (for example, in Fett-Netto et al. , Bio/Technology 10: 1572-1575 , 1992). Biosynthesis of taxoids in microorganisms. As discussed below, taxadiene synthase activity was observed in transformed E. coli host cells expressing recombinant taxadiene synthase. Taxadiene synthase does not require extensive post-translational modification, as provided, for example, in mammalian cells, for enzymatic function. As a result, functional taxadiene synthase can be expressed in a wide variety of host cells. Geranylgeranyl diphosphate, a substrate of taxadiene synthase, is produced in a wide variety of organisms, including bacteria and yeast that synthesize carotenoid pigments (e.g. , Serratia spp. and Rhodotorula spp.). Introduction of vectors capable of expressing taxadiene synthase in such microorganisms permits the production of large amounts of taxa-4(5), l l(12)-diene and related compounds having the taxane backbone. The taxane backbone thus produced is useful as a chemical feedstock. Simple taxoids, for example, would be useful as perfume fixatives. Cloning taxadiene synthase homologs and related genes. The availability of the taxadiene synthase gene from Pacific yew makes possible the cloning of homologs of taxadiene synthase from other organisms capable of taxoid biosynthesis, particularly Taxus spp. Although the proportion of common taxoids varies with the species or cultivar of yew tested, apparently all Taxus species synthesize taxoids, including Taxol, to some degree (see, e.g. , Mattina and Palva, J. Environ. Hon. 10: 187-191 , 1992; Miller, J. Natural Products 43:425-437, 1980). Taxol may also be produced by a fungus, Taxomyces andreanae (Stierle et al. , Science 260:214, 1993). A taxadiene synthase gene can be isolated from any organism capable of producing Taxol or related taxoids by using primers or probes based on the Pacific yew taxadiene synthase gene sequence or antibodies specific for taxadiene synthase by conventional methods. Modified forms of taxadiene synthase gene and polypeptide. Knowledge of the taxadiene synthase gene sequence permits the modification of the sequence, as described more fully below, to produce variant forms of the gene and the polypeptide gene product. For example, the plastidial transit peptide can be removed and/or replaced by other transit peptides to allow the gene product to be directed to various intracellular compartments or exported from a host cell. DEFINITIONS AND METHODS The following definitions and methods are provided to better define the present invention and to guide those of ordinary skill in the art in the practice of the present invention. Definitions of common terms in molecular biology may also be found in Rieger et al , Glossary of Genetics: Classical and Molecular, 5th edition, Springer-Verlag, New York, 1991 ; and Lewin, Genes V, Oxford University Press, New York, 1994. The term "plant" encompasses any plant and progeny thereof. The term also encompasses parts of plants, including seed, cuttings, tubers, fruit, flowers, etc. A "reproductive unit" of a plant is any totipotent part or tissue of the plant from which one can obtain a progeny of the plant, including, for example, seeds, cuttings, buds, bulbs, somatic embryos, cultured cell (e.g. , callus or suspension cultures), etc. Nucleic Acids Nucleic acids (a term used interchangeably with "polynucleotides" herein) that are useful in the practice of the present invention include the isolated taxadiene synthase gene, its homologs in other plant species, and fragments and variants thereof. The term "taxadiene synthase gene" refers to a nucleic acid that contains a taxa-4(5), l 1(12)- diene synthase sequence, preferably a nucleic acid that encodes a polypeptide having taxadiene synthase enzymatic activity This term relates primarily to the isolated full-length taxadiene synthase cDNA from Pacific yew discussed above and shown in FIG 2 and the corresponding genomic sequence (including flanking or internal sequences operably linked thereto, including regulatory elements and/or mtron sequences) This term also encompasses alleles of the taxadiene synthase gene from Pacific yew "Native" The term "native" refers to a naturally-occurring ("wild-type") nucleic acid or polypeptide "Homolog" A "homolog" of the taxadiene synthase gene is a gene sequence encoding a taxadiene synthase isolated from an organism other than Pacific yew "Isolated" An "isolated" nucleic acid is one that has been substantially separated or purified away from other nucleic acid sequences in the cell of the organism in which the nucleic acid naturally occurs, i e , other chromosomal and extrachromosomal DNA and RNA, by conventional nucleic acid purification methods The term also embraces recombinant nucleic acids and chemically synthesized nucleic acids Fragments, probes, and primers A fragment of a taxadiene synthase nucleic acid according to the present invention is a portion of the nucleic acid that is less than full-length and comprises at least a minimum length capable of hybridizing specifically with the taxadiene synthase nucleic acid of Figure 2 under stringent hybridization conditions The length of such a fragment is preferably 15- 17 nucleotides or more Nucleic acid probes and primers can be prepared based on the taxadiene synthase gene sequence provided in FIG 2 A "probe" is an isolated DNA or RNA attached to a detectable label or reporter molecule, e g , a radioactive isotope, ligand, chemiluminescent agent, or enzyme "Primers" are isolated nucleic acids, generally DNA oligonucleotides 15 nucleotides or more in length, that are annealed to a complementary target DNA strand by nucleic acid hybridization to form a hybrid between the primer and the target DNA strand, then extended along the target DNA strand by a polymerase, e g , a DNA polymerase Pnmer pairs can be used for amplification of a nucleic acid sequence, e g , by the polymerase chain reaction (PCR) or other conventional nucleic-acid amplification methods Methods for preparing and using probes and primers are descπbed, for example, in Sambrook et al , Molecular Cloning A Laboratory Manual, 2nd ed , vol 1-3, ed Sambrook et al , Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, 1989, Current Protocols in Molecular Biology, ed Ausubel et al , Greene Publishing and Wiley Interscience, New York, 1987 (with periodic updates), and Innis et al , PCR Protocols A Guide to Methods and Applications, Academic Press San Diego, 1990 PCR-primer pairs can be derived from a known sequence, for example, by using computer programs intended for that purpose such as Primer (Version 0 5, ® 1991 , Whitehead Institute for Biomedical Research, Cambridge, MA) Nucleotide sequence similarity. Nucleotide sequence "similarity" is a measure of the degree to which two polynucleotide sequences have identical nucleotide bases at corresponding positions in their sequence when optimally aligned (with appropriate nucleotide insertions or deletions) . Sequence similarity can be determined using sequence analysis software such as the Sequence Analysis Software Package of the Genetics Computer Group, University of Wisconsin Biotechnology Center, Madison, WI. Preferably, a variant form of a taxadiene synthase polynucleotide has at least 70%, more preferably at least 80%, and most preferably at least 90% nucleotide sequence similarity with a native taxadiene synthase gene, particularly with a native Pacific yew taxadiene synthase, as provided in FIG. 2. Operably linked. A first nucleic-acid sequence is "operably" linked with a second nucleic- acid sequence when the first nucleic-acid sequence is placed in a functional relationship with the second nucleic-acid sequence. For instance, a promoter is operably linked to a coding sequence if the promoter affects the transcription or expression of the coding sequence. Generally, operably linked DNA sequences are contiguous and, where necessary to join two protein coding regions, in reading frame. "Recombinant". A "recombinant" nucleic acid is an isolated polypeptide made by an artificial combination of two otherwise separated segments of sequence, e.g. , by chemical synthesis or by the manipulation of isolated segments of nucleic acids by genetic engineering techniques. Techniques for nucleic-acid manipulation are described generally in, for example, Sambrook et al. (1989) and Ausubel et al. (1987, with periodic updates). Methods for chemical synthesis of nucleic acids are discussed, for example, in Beaucage and Carruthers, Tetra. Letts. 22: 1859-1862, 1981 , and Matteucci et al , J. Am. Chem. Soc. 103:3185, 1981. Chemical synthesis of nucleic acids can be performed, for example, on commercial automated oligonucleotide synthesizers. Preparation of recombinant or chemically synthesized nucleic acids: vectors, transformation. host cells. Natural or synthetic nucleic acids according to the present invention can be incoφorated into recombinant nucleic-acid constructs, typically DNA constructs, capable of introduction into and replication in a host cell. Such a construct preferably is a vector that includes a replication system and sequences that are capable of transcription and translation of a polypeptide-encoding sequence in a given host cell. For the practice of the present invention, conventional compositions and methods for preparing and using vectors and host cells are employed, as discussed, inter alia, in > Sambrook et al, 1989, or Ausubel et al , 1987. A "transformed" or "transgenic" cell, tissue, organ, or organism is one into which a foreign nucleic acid, has been introduced. A "transgenic" or "transformed" cell or organism also includes (1) progeny of the cell or organism and (2) progeny produced from a breeding program employing a "transgenic" plant as a parent in a cross and exhibiting an altered phenotype resulting from the presence of the "transgene," i.e., the recombinant taxadiene synthase nucleic acid. Nucleic-Acid Hybridization: "Stringent Conditions"; "Specific". The nucleic-acid probes and primers of the present invention hybridize under stringent conditions to a target DNA sequence, e.g., to the taxadiene synthase gene. The term "stringent conditions" is functionally defined with regard to the hybridization of a nucleic-acid probe to a target nucleic acid (i.e. , to a particular nucleic-acid sequence of interest) by the hybridization procedure discussed in Sambrook et al, 1989, at 9.52-9.55. See also, Sambrook et al , 1989 at 9.47-9.52, 9.56-9.58; Kanehisa, Nucl. Acids Res. 12:203-213, 1984; and Wetmur and Davidson, J. Mol. Biol 31:349-370, 1968. Regarding the amplification of a target nucleic- acid sequence (e.g., by PCR) using a particular amplification primer pair, stringent conditions are conditions that permit the primer pair to hybridize only to the target nucleic-acid sequence to which a primer having the corresponding wild-type sequence (or its complement) would bind and preferably to produce a unique amplification product. The term "specific for (a target sequence)" indicates that a probe or primer hybridizes under stringent conditions only to the target sequence in a sample comprising the target sequence. Nucleic-acid amplification. As used herein, "amplified DNA" refers to the product of nucleic-acid amplification of a target nucleic-acid sequence. Nucleic-acid amplification can be accomplished by any of the various nucleic-acid amplification methods known in the art, including the polymerase chain reaction (PCR). A variety of amplification methods are known in the art and are described, inter alia, in U.S. Patent Nos. 4,683,195 and 4,683,202 and in PCR Protocols: A Guide to Methods and Applications , Innis et al, eds., Academic Press, San Diego, 1990. Methods of making cDN A clones encoding taxadiene synthase or homologs thereof. Based upon the availability of the taxadiene synthase cDNA as disclosed herein, other taxadiene synthase genes (e.g., alleles and homologs of taxadiene synthase) can be readily obtained from a wide variety of plants by cloning methods known in the art. One or more primer pairs based on the taxadiene synthase sequence can be used to amplify such taxadiene synthase genes or their homologs by the polymerase chain reaction (PCR) or other conventional amplification methods. Alternatively, the disclosed taxadiene synthase cDNA or fragments thereof can be used to probe a cDNA or genomic library made from a given plant species by conventional methods. Cloning ofthe taxadiene synthase genomic sequence and homologs Thereof. The availability of the taxadiene synthase cDNA sequence enables those skilled in the art to obtain a genomic clone corresponding to the taxadiene synthase cDNA (including the promoter and other regulatory regions and intron sequences) and the determination of its nucleotide sequence by conventional methods. Virtually all Taxus species synthesize taxoids, including Taxol, to some degree (see, e.g., Mattina and Palva, J. Environ. Hort. 10:187-191, 1992; Miller, J. Natural Products 43:425-437, 1980). Any organism that produces taxoids would be expected to express a homolog of taxadiene synthase. Taxadiene synthase genes can be obtained by hybridization of a Pacific yew taxadiene synthase probe to a cDNA or genomic library of a target species. Such a homolog can also be obtained by PCR or other amplification method from genomic DNA or RNA of a target species using pπmers based on the taxadiene synthase sequence shown in FIG 2 Genomic and cDNA libraries from yew or other plant species can be prepared by conventional methods Pnmers and probes based on the sequence shown in FIG 2 can be used to confirm (and, if necessary, to correct) the taxadiene synthase sequence by conventional methods Nucleotide-Sequence Variants of taxadiene synthase cDNA and Amino Acid Sequence Variants of taxadiene synthase Protein Using the nucleotide and the amino-acid sequence of the taxadiene synthase protein disclosed herein, those skilled in the art can create DNA molecules and polypeptides that have minor variations in their nucleotide or ammo acid sequence "Variant" DNA molecules are DNA molecules containing minor changes in the native taxadiene synthase sequence, / e , changes in which one or more nucleotides of a native taxadiene synthase sequence is deleted, added, and/or substituted, preferably while substantially maintaining taxadiene synthase activity Variant DNA molecules can be produced, for example, by standard DNA mutagenesis techniques or by chemically synthesizing the variant DNA molecule or a portion thereof Such variants preferably do not change the reading frame of the protein-coding region of the nucleic acid and preferably encode a protein having no change, only a minor reduction, or an increase m taxadiene synthase biological function Amino-acid substitutions are preferably substitutions of single amino-acid residues DNA insertions are preferably of about 1 to 10 contiguous nucleotides and deletions are preferably of about 1 to 30 contiguous nucleotides Insertions and deletions are preferably insertions or deletions from an end of the protein-coding or non-coding sequence and are preferably made in adjacent base pairs Substitutions, deletions, insertions or any combination thereof can be combined to arrive at a final construct Preferably, variant nucleic acids according to the present invention are "silent" or "conservative" variants "Silent" variants are variants of a native taxadiene synthase sequence or a homolog thereof in which there has been a substitution of one or more base pairs but no change in the amino-acid sequence of the polypeptide encoded by the sequence "Conservative" variants are variants of the native taxadiene synthase sequence or a homolog thereof in which at least one codon in the protein-coding region of the gene has been changed, resulting in a conservative change in one or more amino acid residues of the polypeptide encoded by the nucleic-acid sequence, i e , an amino acid substitution A number of conservative ammo acid substitutions are listed below In addition, one or more codons encoding cysteine residues can be substituted for, resulting in a loss of a cysteine residue and affecting disulfide linkages in the taxadiene synthase polypeptide Original Residue Conservative Substitutions Ala Ser Arg Lys Asn Gin, His Asp Glu Cys Ser Gin Asn Glu Asp Ile Leu, Val Leu Ile; Val Lys Arg; Gin; Glu Met Leu; Ile Phe Met; Leu; Tyr Ser Thr Thr Ser Trp Tyr Tyr Trp; Phe Val Ile, Leu Substantial changes in function are made by selecting substitutions that are less conservative than those listed above, e.g. , causing changes in: (a) the structure of the polypeptide backbone in the area of the substitution; (b) the charge or hydrophobicity of the polypeptide at the target site; or (c) the bulk of an ammo acid side chain Substitutions generally expected to produce the greatest changes in protem properties are those in which: (a) a hydrophilic residue, e.g. , seryl or threonyl, is substituted for (or by) a hydrophobic residue, e.g., leucyl, isoleucyl, phenylalanyl, valyl or alanyl, (b) a cysteine or proline is substituted for (or by) any other residue; (c) a residue having an electropositive side chain, e.g. , lysyl, argmyl, or histadyl, is substituted for (or by) an electronegative residue, e.g., glutamyl or aspartyl; or (d) a residue having a bulky side cham, e.g. , phenylalanine, is substituted for (or by) one not having a side chain, e.g., glycine. The taxadiene synthase gene sequence can be modified as follows: ( 1) To improve expression efficiency and redirect the targeting of the expressed polypeptide. For expression in non-plant hosts (or to direct the expressed polypeptide to a different intracellular compartment m a plant host), the native gene sequence can be truncated from the 5' end to remove the sequence encoding the plastidial transit peptide of approximately 137 ammo acids (i e. , to approximately 138S), leaving the sequence encoding the mature taxadiene synthase polypeptide of about 725 amino acids. In addition, one or more codons can be changed, for example, to conform the gene to the codon usage bias of the host cell for improved expression. Enzymatic stability can be altered by removing or adding one or more cysteine residues, thus removing or adding one or more disulfide bonds. (2) To alter catalytic efficiency As discussed below, the aspartate-πch to play a role in substrate binding, is also present in taxadiene synthase, as is a related DXXDD motif (FIG. 2). Histidine and cysteine residues have been implicated at the active sites of several terpenoid cyclases of plant origin. Histidines residues 370, 415 and 793 and cysteines at residues 329, 650 and 777 of taxadiene synthase are conserved among the plant terpenoid cyclase genes. One or more conserved histidine and cysteine residues (as discussed below), or semi- conserved residues such as conserved cysteine residues (e.g. , residues 329, 650, 719, and 777) and histidine residues (e.g. , residues 370, 415, 579, and 793), can be mutagenized to alter enzyme kinetics. In addition, residues adjacent to these conserved histidine and cysteine residues can also be altered to increase the cysteine or histidine content to improve charge stabilization. By increasing the aspartate content of the DDXXD and DXXDD motifs (where D is aspartate and X is any amino acid), which are likely to be involved in substrate/intermediate binding, it is also possible to increase the enzymatic rate (i.e. , the rate-limiting ionization step of the enzymatic reaction). Arginines have been implicated in binding or catalysis, and conserved arginine residues are also good targets for mutagenesis. Changing the conserved DDXXD and/or DXXDD motifs (e.g. , the aspartate residues thereof) by conventional site-directed mutagenesis methods to match those of other known enzymes can also lead to changes in the kinetics or substrate specificity of taxadiene synthase. Additionally, product formation can be altered by mutagenesis ofthe RWWK element (residues 564 to 567), which includes aromatic residues that may play a role in stabilizing carbocationic reaction intermediates. (3) To modify substrate utilization: The enzyme, particularly the active site, can be modified to allow the enzyme to bind shorter (e.g., C l0 ) or longer (e.g., C^) chains than geranylgeranyl diphosphate. Substrate size utilization can be altered by increasing or decreasing the size of the hydrophobic patches to modify the size of the hydrophobic pocket of the enzyme. Similar effects can be achieved by domain swapping. (4) To change product outcome: Directed mutagenesis of conserved aspartate and arginine residues can be used to permit the enzyme to produce different diterpene skeletons with, for example, one, two, or three rings. See, e.g. , Cane et al. , Biochemistry 34:2480-2488, 1995; Joly and Edwards, J. Biol. Chem. 268:26983-26989, 1993; Marrero et al. , J. Biol. Chem. 267:533-536, 1992; and Song and Poulter, Proc. Natl. Acad. Sci. USA 91:3044-3048, 1994). Expression of taxadiene synthase nucleic acids in host cells. DNA constructs incoφorating a taxadiene synthase gene or fragment thereof according to the present invention preferably place the taxadiene synthase protein coding sequence under the control of an operably linked promoter that is capable of expression in a host cell . Various promoters suitable for expression of heterologous genes in plant cells are known in the art, including constitutive promoters, e.g. the cauliflower mosaic virus (CaMV) 35S promoter, which is expressed in many plant tissues, organ- or tissue-specific promoters, and promoters that are inducible by chemicals such as methyl jasminate, salicylic acid, or safeners, for example. A variety of other promoters or other sequences useful in constructing expression vectors are available for expression in bacterial, yeast, mammalian, insect, amphibian, avian, or other host cells Nucleic acids attached to a solid support The nucleic acids of the present invention can be free in solution or attached by conventional means to a solid support, such as a hybridization membrane (e g , nitrocellulose or nylon), a bead, or other solid supports known in the art Polypeptides The term "taxadiene synthase protein" (or polypeptide) refers to a protein encoded by a taxadiene synthase gene, including alleles, homologs, and variants thereof, for example A taxadiene synthase polypeptide can be produced by the expression of a recombinant taxadiene synthase nucleic acid or be chemically synthesized Techniques for chemical synthesis of polypeptides are described, for example, in Merπfield, J Amer Chem Soc 85.2149-2156, 1963 Polypeptide sequence identity and similarity Ordinarily, taxadiene synthase polypeptides encompassed by the present invention have at least about 70% ammo acid sequence "identity' (or homology) compared with a native taxadiene synthase polypeptide, preferably at least about 80% identity, and more preferably at least about 90% identity to a native taxadiene synthase polypeptide Preferably, such polypeptides also possess characteristic structural features and biological activity of a native taxadiene synthase polypeptide Amino acid sequence "similarity" is a measure of the degree to which aligned amino acid sequences possess identical ammo acids or conservative amino acid substitutions at corresponding positions A taxadiene synthase "biological activity" includes taxadiene synthase enzymatic activity as determined by conventional protocols (e g , the protocol described m Hezari et al , Arch Biochem Biophys 322 437-444, 1995, incorporated herein by reference) Other biological activities of taxadiene synthase include, but are not limited to substrate binding, immunological activity (including the capacity to elicit the production of antibodies that are specific for taxadiene synthase), etc Polypeptide identity (homology) or similarity is typically analyzed using sequence analysis software such as the Sequence Analysis Software Package of the Genetics Computer Group, University of Wisconsin Biotechnology Center, Madison, WI) Polypeptide sequence analysis software matches polypeptide sequences usmg measures of identity assigned to vaπous substitutions, deletions, substitutions, and other modifications "Isolated, " "Puπfied. " "Homogeneous" Polypeptides A polypeptide is "isolated" if it has been separated from the cellular components (nucleic acids, lipids, carbohydrates, and other polypeptides) that naturally accompany it Such a polypeptide can also be referred to as "pure' or "homogeneous" or "substantially" pure or homogeneous Thus, a polypeptide which is chemically synthesized or recombinant (i e , the product of the expression of a recombinant nucleic acid, even if expressed in a homologous cell type) is considered to be isolated A monomeric polypeptide is isolated when at least 60-90 % by weight of a sample is composed of the polypeptide, preferably 95 % or more, and more preferably more than 99% Protein purity or homogeneity is indicated, for example, by polyacrylamide gel electrophoresis of a protein sample, followed by visualization of a single polypeptide band upon staining the polyacrylamide gel; high pressure liquid chromatography; or other conventional methods. Protein purification. The polypeptides of the present invention can be purified by any of the means known in the art. Various methods of protein purification are described, e.g. , in Guide to Protein Purification, ed. Deutscher, Meth. Enzymol. 185, Academic Press, San Diego, 1990; and Scopes, Protein Purification: Principles and Practice, Springer Verlag, New York, 1982. Variant forms of taxadiene synthase polypeptides: labeling. Encompassed by the taxadiene synthase polypeptides according to an embodiment of the present invention are variant polypeptides in which there have been substitutions, deletions, insertions or other modifications of a native taxadiene synthase polypeptide. The variants substantially retain structural and/or biological characteristics and are preferably silent or conservative substitutions of one or a small number of contiguous amino acid residues. Preferably, such variant polypeptides are at least 70% , more preferably at least 80%, and most preferably at least 90% homologous to a native taxadiene synthase polypeptide. The native taxadiene synthase polypeptide sequence can be modified by conventional methods, e.g. , by acetylation, carboxylation, phosphorylation, glycosylation, ubiquitination, and labeling, whether accomplished by in vivo or in vitro enzymatic treatment of a taxadiene synthase polypeptide or by the synthesis of a taxadiene synthase polypeptide using modified amino acids. There are a variety of conventional methods and reagents for labeling polypeptides and fragments thereof. Typical labels include radioactive isotopes, ligands or ligand receptors, fluorophores, chemiluminescent agents, and enzymes. Methods for labeling and guidance in the choice of labels appropriate for various puφoses are discussed, e.g. , in Sambrook et al. (1989) and Ausubel et al. (1987 with periodic updates). Polypeptide Fragments. The present invention also encompasses fragments of taxadiene synthase polypeptides that lack at least one residue of a native full-length taxadiene synthase polypeptide yet retain at least one of the biological activities characteristic of taxadiene synthase, e.g. , taxadiene synthase enzymatic activity or possession of a characteristic immunological determinant. As an additional example, an immunologically active fragment of a taxadiene synthase polypeptide is capable of raising taxadiene synthase-specific antibodies in a target immune system (e.g. , murine or rabbit) or of competing with taxadiene synthase for binding to taxadiene synthase-specific antibodies, and is thus useful in immunoassays for the presence of taxadiene synthase polypeptides in a biological sample. Such immunologically active fragments typically have a minimum size of 7 to 17 amino acids. Fragments preferably comprise at least 10, more preferably at least 20, and most preferably at least 30 consecutive amino acids of a native taxadiene synthase polypeptide. Fusion polypeptides. The present invention also provides fusion polypeptides including, for example, heterologous fusion polypeptides, i.e. , a taxadiene synthase polypeptide sequence or fragment thereof and a heterologous polypeptide sequence, e.g. , a sequence from a different polypeptide. Such heterologous fusion polypeptides thus exhibit biological properties (such as ligand- binding, catalysis, secretion signals, antigenic determinants, etc.) derived from each of the fused sequences. Fusion partners include, for example, immunoglobulins, betagalactosidase, tφE, protein A, beta lactamase, alpha amylase, alcohol dehydrogenase, yeast alpha mating factor, and various signal and leader sequences which, e.g. , can direct the secretion of the polypeptide. Fusion polypeptides are typically made by the expression of recombinant nucleic acids or by chemical synthesis. Polypeptide sequence determination. The sequence of a polypeptide of the present invention can be determined by various methods known in the art. In order to determine the sequence of a polypeptide, the polypeptide is typically fragmented, the fragments separated, and the sequence of each fragment determined. To obtain fragments of a taxadiene synthase polypeptide, the polypeptide can be digested with an enzyme such as trypsin, clostripain, or Staphylococcus protease, or with chemical agents such as cyanogen bromide, o-iodosobenzoate, hydroxylamine or 2-nitro-5-thiocyanobenzoate. Peptide fragments can be separated, e.g., by reversed-phase high-performance liquid chromatography (HPLC) and analyzed by gas-phase sequencing. Antibodies The present invention also encompasses polyclonal and/or monoclonal antibodies that are specific for taxadiene synthase, i.e. , bind to taxadiene synthase and are capable of distinguishing the taxadiene synthase polypeptide from other polypeptides under standard conditions. Such antibodies are produced and assayed by conventional methods. For the preparation and use of antibodies according to the present invention, including various immunoassay techniques and applications, see, e.g. , Goding, Monoclonal Antibodies: Principles and Practice, 2d ed, Academic Press, New York, 1986; and Harlow and Lane, Antibodies: A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY, 1988. Taxadiene synthase-specific antibodies are useful, for example in: purifying taxadiene synthase polypeptides; cloning taxadiene synthase homologs from Pacific yew or other plant species from an expression library; antibody probes for protein blots and immunoassays; etc. Taxadiene synthase polypeptides and antibodies can be labeled by conventional techniques. Suitable labels include radionuclides, enzymes, substrates, cofactors, inhibitors, fluorescent agents, chemiluminescent agents, magnetic particles, etc. Plant transformation and regeneration. Any well-known method can be employed for plant cell transformation, culture, and regeneration can be employed in the practice of the present invention. Methods for introduction of foreign DNA into plant cells include, but are not limited to: transfer involving the use of Agrobacterium tumefaciens and appropriate Ti vectors, including binary vectors; chemically induced transfer (e.g. , with polyethylene glycol); biolistics; and microinjection. See, e.g. , An et al , Plant Molecular Biology Manual A3:l-19, 1988. The invention will be better understood by reference to the following Examples, which are intended to merely illustrate the best mode now known for practicing the invention. The scope of the invention is not to be considered limited thereto, however. EXAMPLE 1: Cloning and Sequencing of a cDNA encoding Taxa-4(5).l l(12)-diene Synthase Materials and Methods Plants, Substrates, and Standards. Four-year-old T. brevifolia saplings in active growth were maintained in a greenhouse. [1- 3 H] Geranylgeranyl diphosphate (120 Ci/mol) was prepared as described previously (LaFever et al , Arch. Biochem. Biophys. 313: 139-149, 1994). and authentic (±)-taxa-4(5),l l(12)-diene was prepared by total synthesis (Rubenstein, J. Org. Chem. 60:7215- 7223, 1995). Library Construction. Total RNA was extracted from T. brevifolia stem using the procedures of Lewinsohn and associates (Lewinsohn et al, Plant Mol. Biol. Rep. 12:20-25, 1991) developed for woody gymnosperm tissue. Poly(A) + RNA was purified by chromatography on oligo(dT)-cellulose (Pharmacia) and 5 μg of the resulting mRNA was utilized to construct a λZAP II cDNA library according to the manufacturer's instructions (Stratagene). PCR-Based Probe Generation and Library Screening. Comparison of six available sequences for monoteφene, sesquiterpene, and diteφene cyclases from higher plants (Facchini and Chappell, Proc. Natl. Acad. Sci. USA 89: 11088- 11092, 1992; Colby et al. , J. Biol. Chem. 268:23016-23024, 1993; Mau and West, Proc. Natl. Acad. Sci. USA 91:8497-8501, 1994; Back and Chappell, J. Biol. Chem. 270:7375-7381, 1995; Sun and Karmiya, Plant Cell 6: 1509-1518, 1994; and Bensen et al, Plant Cell 7:75-84, 1995) allowed definition of eleven homologous regions for which consensus degenerate primers were synthesized. All twenty primers (the most carboxy terminal primer, the most amino terminal primer, and nine internal primers in both directions) were deployed in all possible combinations with a broad range of amplification conditions using CsCl-purified T. brevifolia stem library phage DNA as template (Innis and Gelfand, in PCR Protocols (Innis et al , eds), pp. 3-12, 253-258, Academic Press, San Diego, CA, 1990; Sambrook et al, 1989). Analysis of PCR products by gel electrophoresis (Sambrook et al. , 1989) indicated that only the combination of primers CC7.2F and CC3R (see FIG. 2) generated a specific DNA fragment ( ~80 bp). This DNA fragment was cloned into pT7Blue (Novagen) and sequenced (DyeDeoxy Terminator Cycle Sequencing, Applied Biosystems), and shown to be 83 bp in length. PCR was used to prepare approximately 1 μg of mis material for random hexamer labeling with [α- 32 P]dATP (Tabor et al. , in Current Protocols in Molecular Biology, Ausubel et al , Sections 3.5.9-3.5.10, 1987) and use as a hybridization probe to screen filter lifts of 3 x 10 5 plaques grown in E. coli LE392 using standard protocols (Britten and Davidson, in Nucleic Acid Hybridisation, Hames and Higgins, eds., pp. 3-14, IRL Press, Oxford, 1988). Ofthe plaques affording positive signals (102 total), 50 were purified through two additional cycles of hybridization. Thirty-eight pure clones were in vivo excised as Bluescript phagemids. The insert size was determined by PCR using T3 and T7 promoter primers, and the twelve largest clones (insert > 2 kb) were partially sequenced. cDNA Expression in E. coli. All of the partially sequenced, full-length inserts were either out of frame or bore premature stop sites immediately upstream of the presumptive methionine start codon. The latter complication likely resulted from haiφin-primed second-strand cDNA synthesis (Old and Primrose, Principles of Gene Manipulation, 4th ed., pp. 3435, Blackwell Scientific, London, 1989). The 2.7-kb insert from pTb42 was cloned into frame by PCR using the thermostable, high fidelity, blunting polymerase Pful (Stratagene) and the FRM42 primer (downstream of false stop codons) and T7 promoter primer. The resulting blunt fragment was ligated into EcoRV -digested pBluescript SK(-) (Stratagene), yielding pTb42.1, and transformed into E. coli XLl -Blue (Stratagene). To evaluate functional expression of teφene cyclase activity, E. coli XLl-Blue cells harboring pTb42.1 were grown (to A^ = 0.4) on 5 ml LB medium supplemented with 100 μg/ml ampicillin and 12.5 μg/ml tetracycline before induction with 200 μM IPTG and subsequent growth for 4 h at 25°C Bacteria were harvested by centrifugation (1800g, 10 min), resuspended in taxadiene synthase assay buffer (Hezari et al. , Arch. Biochem. Biophys. 322:437-444, 1995), disrupted by brief sonication at 0-4°C, and the resulting suspension centrifuged (18,000g, 10 min) to pellet debris. The supernatant was assayed for taxadiene synthase activity by an established protocol (Hezari et al. , Arch. Biochem. Biophys. 322:437-444, 1995) in the presence of 15 μM [ 1 - 3 H]geranylgeranyl diphosphate and 1 mM MgCl 2 , with incubation at 31°C for 4 h. The reaction products were extracted with pentane and the extract purified by column chromatography on silica gel as previously described (Hezari et al , Arch. Biochem. Biophys. 322:437-444, 1995) to afford the olefin fraction, an aliquot of which was counted by liquid scintillation spectrometry to determine 3 H incoφoration. Control experiments with transformed E. coli bearing the plasmid with out-of-frame inserts were also carried out. The identity ofthe olefin product of the recombinant enzyme was verified by capillary radio- gas chromatography ("capillary radio-GC") (Croteau and Satterwhite, J. Chromatogr. 500:349-354, 1990) as well as capillary gas chromatography-tnass spectrum/ spectrometry ("capillary GC-MS") using methods described previously (Koepp et al , J. Biol Chem. 270:8686-8690, 1995) and authentic taxa-4(5),l l(12)-diene (Rubenstein, J. Org. Chem. 60:7215-7223, 1995). For GC-MS analysis (Hewlett-Packard 6890 GC-MSD), selected diagnostic ions were monitored: m/z 272 [P + ]; 257 [P + -15(CH 3 )]; 229 [P + -43(C 3 H 7 )]; 121 , 122, 123 [C-ring fragment cluster]; and 107 [m/z 122 base peak - 15(CH 3 )] . The origin of the highly characteristic C-ring double cleavage fragment ion [base peak, m/z 122(0^,,,)] has been described (Koepp et al. , J. Biol. Chem. 270:8686-8690, 1995). RESULTS AND DISCUSSION cDNA Isolation and Characterization. In general characteristics (molecular weight, divalent metal ion requirement, kinetic constants, etc.), taxadiene synthase resembles other teφenoid cyclases from higher plants; however, the low tissue titers of the enzyme and its instability under a broad range of fractionating conditions impeded purification of the protein to homogeneity (Hezari et al. , Arch. Biochem. Biophys. 322:437-444, 1995). A 10 μg sample of the electrophoretically-purified cyclase, prepared by standard analytical procedures (Schagger and von Jagow, Anal Biochem. 166:368-379, 1987; Towbin et al. , Proc. Natl. Acad. Sci. USA 76:350-4354, 1979), failed to provide amino-terminal sequence via Edman degradation. Repeated attempts at trypsinization and CNBr cleavage of comparable protein samples also failed to provide sequenceable peptides, in large part because of very low recoveries. As an alternate approach to cDNA library screening using protein-based oligonucleotide probes, a PCR-based strategy was developed that was founded on a set of degenerate primers for PCR amplification designed to recognize highly-conserved regions of six higher plant teφene cyclases whose nucleotide sequences are known. Three of these cyclases, (-)-limonene synthase (a monoteφene cyclase from spearmint) (Colby et al , J. Biol. Chem. 268:23016-23024, 1993), epi- aristolochene synthase (a sesquiterpene cyclase from tobacco) (Facchini and Chappell, Proc. Natl. Acad. Sci. USA 89: 11088-11092, 1992; Back and Chappell, J. Biol. Chem. 270:7375-7381 , 1995), and casbene synthase (a diteφene cyclase from castor bean) (Mau and West, Proc. Natl Acad. Sci. USA 91:8497-8501 , 1994), exploit reaction mechanisms similar to taxadiene synthase in the cyclization of the respective geranyl (C !0 ), farnesyl (C 15 ), and geranylgeranyl (C 20 ) diphosphate substrates (Lin et al., Biochemistry, in press). Kaurene synthase A from Arabidopsis thaliana (Sun and Karmiya, Plant Cell 6: 1509-1518, 1994) and maize (Bensen et al. , Plant Cell 7:75-84, 1995) and (-)-abietadiene synthase from grand fir (Abies grandis; Stofer Vogel, Wildung, Vogel, and Croteau, manuscript in preparation) exploit a quite different mechanism that involves protonation of the terminal double bond of geranylgeranyl diphosphate to initiate cyclization to the intermediate copalyl diphosphate followed, in the case of abietadiene synthase, by the more typical ionization of the diphosphate ester function to initiate a second cyclization sequence to the product olefin (LaFever et al.. Arch. Biochem. Biophys. 313: 139-149, 1994). The latter represents the only gymnosperm teφene cyclase sequence presently available. Comparison of deduced amino acid sequences between all of the cyclases targeted eleven regions for PCR primer construction. Testing of all twenty primers in all combinations under a broad range of amplification conditions, followed by product analysis by gel electrophoresis, revealed that only one combination of primers [CC7.2 (forward) with CC3 (reverse), see FIG. 2 for locations] yielded a specific DNA fragment (83 bp) using T. brevifolia library phage as template. Primer CC3 delineates a region of strong homology between (-)-limonene synthase (Colby et al , J. Biol Chem. 268:23016-23024, 1993), epi-aristolochene synthase (Facchini and Chappell, Proc. Natl. Acad. Sci. USA 89: 1 1088-11092, 1992) and casbene synthase (Mau and West, Proc. Natl. Acad. Sci. USA 91:8497-8501, 1994). Primer CC7.2 was selected based on sequence comparison of the angiosperm diteφene cyclases (Mau and West, Proc. Natl. Acad. Sci. USA 91:8497-8501, 1994; Sun and Karmiya, Plant Cell 6: 1509-1518, 1994; Bensen et al , Plant Cell 7:75-84, 1995) to the recently acquired cDNA clone encoding a gymnosperm diteφene cyclase, (-)-abietadiene synthase from grand fir (Stofer Vogel, Wildung, Vogel, and Croteau, manuscript in preparation). The 83 bp fragment was cloned and sequenced, and thus demonstrated to be cyclase-like. This PCR product was 32 P-labeled for use as a hybridization probe and employed in high stringency screening of 3 x 10 5 plaques which yielded 102 positive signals. Fifty of these clones were purified through two additional rounds of screening, in vivo excised and the inserts sized. The twelve clones bearing the largest inserts ( > 2.0 kb) were partially sequenced, indicating that they were all representations of the same gene. Four of these inserts appeared to be full-length. cDNA Expression in E. coli. All four of the full-length clones that were purified were out of frame or had stop sites immediately upstream of the starting methionine codon resulting from haiφin-primed second strand cDNA synthesis. The insert from pTb42 was cloned into frame by PCR methods, the blunt fragment was ligated into the EcoRV-site of pBluescript SK(-), yielding pTb42.1, and transformed into E. coli XLl -Blue. Transformed E. coli were grown in LB medium supplemented with antibiotics and induced with IPTG. The cells were harvested and homogenized, and the extracts were assayed for taxadiene synthase activity using standard protocols with [1- 3 H] geranylgeranyl diphosphate as substrate (Hezari et al. Arch. Biochem. Biophys. 322:437-444, 1995). The olefin fraction isolated from the reaction mixture contained a radioactive product ( - 1 nmol) that was coincident on capillary radio-GC with authentic taxa-4(5),ll(12)-diene (Rt = 19.40 ± 0.13 min). The identification of this diteφene olefin was confirmed by capillary GC-MS analysis. The retention time (12.73 min. vs. 12.72 min.) and selected ion mass spectrum (Table I) of the diteφene olefin product was identical to that of authentic (+)-taxa-4(5),l l(12)-diene (Rubenstein, J. Org. Chem. 60:7215-7223, 1995). The origin of the selected diagnostic ions shown in Table I, which account for most of full spectrum abundance, are described herein and elsewhere (Koepp et al. , J. Biol. Chem. 270:8686-8690, 1995). Because of different sample sizes, the total abundance of the authentic standard (2.96 E 5 ) was approximately twice that of the biosynthetic olefin ( 1.42 E 5 ). This , and variation in background between runs, probably account for minor differences in relative abundances of the high mass fragments. Table 1 : GC-MS Analysis of the Diterpene Olefin Synthesized by Recombinant Taxadiene Synthase ("Product") Compared to Authentic Taxa-4(5).l l(12)-diene ("Standard") Relative Abundance (%) m/z Product Standard 107 15.3 15.3 121 14.3 14.3 122 58.1 57.8 123 10.2 10.3 229 0.56 0.71 257 0.35 0.45 272 1.19 1.17 Since identically prepared extracts of control cultures of E. coli that were transformed with pBluescript bearing an out-of-frame insert were incapable of transforming geranylgeranyl diphosphate to detectable levels of diteφene olefin, these results confirm that clone pTb42.1 encodes the taxadiene synthase from Pacific yew. Sequence Analysis. Both strands of the inserts from pTb42 and ρTb42.1 were sequenced. No mistakes were incoφorated by Pfu polymerase. The pTb42.1 taxadiene synthase cDNA is 2700 nucleotides in length and contains a complete open reading frame of 2586 nucleotides (FIG. 2). The deduced amino acid sequence indicates the presence of a putative plastidial transit peptide of approximately 137 amino acids and a mature protein of about 725 residues ( -82.5 kDa), based on the size of the native (mature) enzyme ( — 79 kDa) as estimated by gel permeation chromatography and sodium dodecyl sulfate- polyacrylamide gel electrophoresis ("SDS-PAGE") (Hezari et al. , Arch. Biochem. Biophys. 322:437-444, 1995), the characteristic amino acid content and structural features of such aminoterminal targeting sequences, and their cleavage sites (Keegstra et al. , Annu. Rev. Plant Physiol. Plant Mol. Biol. 40:471-501 , 1989; von Heijne <?/ α/. , £«r. J. Biochem. 180:535-545, 1989), and the fact that diteφene biosynthesis is localized exclusively within plastids (West et al , Rec. Adv. Phytochem. 13:163-198, 1979; Kleinig, Λ/ιnκ. Rev. Plant Physiol. Plant Mol. Biol. 40:39-59, 1989). The transit peptide/mature protein junction and thus the exact lengths of both moieties are unknown, because the amino terminus of the mature protein is apparently blocked and has not yet been identified. Pairwise sequence comparison (Feng and Doolittle, Methods Enzymol. 183:375-387, 1990; Genetics Computer Group, Program Manual for the Wisconsin Packet, Version 8, Genetics Computer Group, Madison, WI, 1994) with other terpene cyclases from higher plants revealed a significant degree of sequence similarity at the amino acid level. The taxadiene synthase from yew showed 32% identity and 55 % similarity to (-)-limonene synthase from spearmint (Colby et al. , J. Biol. Chem. 268.23016-23024, 1993), 30% identity and 54% similarity to epi-aπstolochene synthase from tobacco (Facchini and Chappell, Proc. Natl Acad. Sci USA 89 11088-11092, 1992), 31% identity and 56% similarity to casbene synthase from castor bean (Mau and West, Proc. Natl Acad Sci. USA 91.8497- 8501, 1994), and 33% identity and 56% similarity to kaurene synthase A from Arabidopsis thaliana and maize (Sun and Karmiya, Plant Cell 6:1509-1518, 1994, Bensen et al , Plant Cell 7 75-84, 1995), and 45% identity and 67% similarity to (-)-abιetadιene synthase from grand fir (Stofer Vogel, Wildung, Vogel, and Croteau, manuscript in preparation) Pairwise comparison of other members withm this group show roughly comparable levels of identity (30-40%) and similarity (50-60%) These teφenoid synthases represent a broad range of cyclase types from diverse plant families, supporting the suggestion of a common ancestry for this class of enzymes (Colby et al. , J Biol Chem. 268 23016-23024, 1993, Mau and West, Proc Natl. Acad Sci USA 91:8497-8501 , 1994, Back and Chappell, 7 Biol. Chem 270 7375-7381, 1995, McGarvey and Croteau, Plant Cell 7 1015- 1026, 1995, Chappell, Annu. Rev. Plant Physiol. Plant Mol. Biol 46:521-547, 1995) The amino acid sequence of taxadiene synthase does not closely resemble (identity -20%, similarity -40%) that of any of the microbial sesquiteφene cyclases that have been determined recently (Hohn and Beremand, Gene (Amst.) 79:131-136, 1989; Proctor and Hohn, J. Biol Chem 268 4543-4548, 1993; Cane et al , Biochemistry 33:5846-5857, 1994), nor does the taxadiene synthase sequence resemble any of the published sequences for prenyltransferases (Chen et al , Protein Sci 3:600-607, 1994; Scolmk and Bartley, Plant Physiol 104: 1469-1470, 1994; Attucci et al Arch. Biochem. Biophys. 321:493-500, 1995), a group of enzymes that, like the terpenoid cyclases, employ allylic diphosphate substrates and exploit similar electrophilic reaction mechanisms (Poulter and Rilling, in Biosynthesis of Isoprenoid Compounds, Porter and Spurgeon, eds , vol 1 , pp 161-224, Wiley & Sons, New York, NY, 1981) The aspartate-nch (I,L,V)XDDXX(XX)D motιf(s) found in most prenyltransferases and teφenoid cyclases (Facchini and Chappell, Proc Natl Acad. Sci USA 89: 11088-11092, 1992, Colby et al, J Biol Chem. 268.23016-23024, 1993, Mau and West, Proc. Natl Acad Sa USA 91:8497-8501, 1994, Back and Chappell, J Biol Chem 270 7375-7381 , 1995; Hohn and Beremand, Gene (Amst.) 79:131-136, 1989; Proctor and Hohn, J Biol. Chem. 268:4543-4548, 1993; Cane et al. Biochemistry 33.5846-5857, 1994; Chen et al , Protein Sa 3:600-607, 1994, Scolmk and Bartley, Plant Physiol 104: 1469-1470, 1994; Attucci et al Arch. Biochem. Biophys. 321:493-500, 1995, Abe and Prestwich, /. Biol. Chem. 269.802-804, 1994), and thought to play a role in substrate binding (Chen et al. Protein Sci. 3:600-607, 1994, Abe and Prestwich, J Biol Chem. 269 802-804, 1994, Marrero et al , J. Biol Chem 267 21873- 21878, 1992; Joly and Edwards, J. Biol Chem. 268:26983-26989, 1993, Tarshis et al., Biochemistry 33- 10871-10877, 1994), is also present in taxadiene synthase, as is a related DXXDD motif (FIG 2) Histidine and cysteine residues have been implicated at the active sites of several teφenoid cyclases of plant origin (Rajaonaπvony et al., Arch. Biochem. Biophys. 299 77-82, 1992, Savage et al , Arch Biochem. Biophys. 320:257-265, 1995) A search of the aligned sequences revealed that three histidines (at positions 370, 415 and 793) and three cysteines (at positions 329, 650 and 777) of taxadiene synthase are conserved among the plant teφenoid cyclase genes. The taxadiene synthase from yew most closely resembles the abietadiene synthase from grand fir rather than the casbene synthase from castor bean (Mau and West, Proc. Natl. Acad. Sci. USA 91:8497-8501, 1994), which catalyzes a similar type of cyclization reaction but is phylogenetically quite distant. The abietadiene synthase from grand fir is the only other teφenoid cyclase sequence from a gymnosperm now available (Stofer Vogel, Wildung, Vogel, and Croteau, in preparation), and these two diteφene cyclases from the coniferales share several regions of significant sequence homology, one of which was fortuitously chosen for primer construction and proved to be instrumental in the acquisition of a PCR-derived probe that led to the cloning of taxadiene synthase. EXAMPLE 3: Expression of Taxadiene Synthase Genes Truncated to Remove Transit Peptide Sequences The native taxadiene synthase gene sequence was truncated from the 5' end to removing part or all of the sequence that encodes the plastidial transit peptide of approximately 137 amino acids (the mature taxadiene synthase polypeptide is about 725 amino acids.) Deletion mutants were produced that remove amino acid residues from the amino terminus up to residue 31 (Glu), 39 (Ser), 49 (Ser), 54 (Gly), 79 (Val), or 82 (Ile). These mutants were expressed in E. coli calls and cell extracts were assayed for taxadiene synthase activity as described above. In preliminary experiments, expression of truncation mutants up was increased over wild-type taxadiene synthase by up to about 50%, with further truncation past residues 83-84 apparently decreasing taxadiene synthase activity. Truncation of at least part of the plastidial transit peptide improves taxadiene synthase expression. Moreover, removing this sequence improves purification of taxadiene synthase, since the transit peptide is recognized by E. coli chaperonins, which co-purify with the enzyme and complicate purification, and because the taxadiene synthase preprotein tends to form inclusion bodies when expressed in E. coli. The actual cleavage site for removal of the transit peptide may not be at the predicted cleavage site between residue 136 (Ser) and residue 137 (Pro). A transit peptide of 136 residues appears quite long, and other (monoteφene) synthases have a tandem pair of arginines (Arg-Arg) at about residue 60 (Met). Truncation immediately amino-terminal to the tandem pair of arginines of these synthases has resulted in excellent expression in E. coli. Taxadiene synthase lacks an Arg-Arg element. Also, truncation beyond residues 83-84 leads to lower activity. This invention has been detailed both by example and by direct description. It should be apparent that one having ordinary skill in the relevant art would be able to surmise equivalents to the invention as described in the claims which follow but which would be within the spirit of the foregoing description. Those equivalents are to be included within the scope of this invention.

Claims: WHAT IS CLAIMED IS 1 An isolated polynucleotide comprising at least 15 consecutive nucleotides of a native taxadiene synthase gene 2 An isolated polynucleotide comprising at least 30 consecutive nucleotides of a native taxadiene synthase gene 3 The polynucleotide of claim 1 comprising a sequence that encodes a polypeptide having taxadiene synthase biological activity 4 A cell comprising the polynucleotide of claim 1 5 A plant cell compnsmg the polynucleotide of claim 1 6 A transgenic plant comprising the polynucleotide of claim 1 7 An isolated polynucleotide compnsmg a polypeptide-encoding sequence that encodes a polypeptide with taxadiene synthase biological activity, wherein the polynucleotide-encoding sequence has at least 70% nucleotide sequence similarity with a native Pacific yew taxadiene synthase gene 8 The polynucleotide of claim 7 wherein the polypeptide-encoding sequence has at least 80% nucleotide sequence similarity with the native Pacific yew taxadiene synthase gene 9 The polynucleotide of claim 8 wherein the polypeptide-encoding sequence has at least 90% nucleotide sequence similarity with the native Pacific yew taxadiene synthase gene 10 The polynucleotide of claim 7 wherein the polypeptide-encoding sequence encodes a polypeptide having only conservative amino acid substitutions to the taxadiene synthase polypeptide sequence of FIG 2 except for an amino acid substitution at at least one location selected from the group consisting of cysteine residues 329, 650, 719, and 777, histidine residues 370, 415, 579, and 793, a DDXXD motif, a DXXDD motif, a conserved arginine, and a RWWK element 11 The polynucleotide of claim 7 wherein the polypeptide-encoding sequence encodes a polypeptide having only conservative amino acid substitutions to a native Pacific yew taxadiene synthase polypeptide sequence 12 The polynucleotide of claim 7 wherein the polypeptide-encoding sequence encodes a polypeptide that lacks at least part of a transit peptide sequence of a native Pacific yew taxadiene synthase polypeptide 13 The polynucleotide of claim 7 wherein the polypeptide-encoding sequence encodes a polypeptide that is completely homologous with a native taxadiene synthase polypeptide 14 A cell comprising the polynucleotide of claim 7 15 A plant cell comprising the polynucleotide of claim 7 16 A transgenic plant comprising the polynucleotide of claim 7 17 An isolated polypeptide having taxadiene synthase activity 18 The polypeptide of claim 17 having at least 70% amino acid sequence identity with a native Pacific yew taxadiene synthase polypeptide 19 The polypeptide of claim 18 having at least 80% amino acid sequence identity with the taxadiene synthase polypeptide 20 The polypeptide of claim 19 having at least 90% amino acid sequence identity with the taxadiene synthase polypeptide 21 The polypeptide of claim 17 having only conservative substitutions to the native Pacific yew taxadiene synthase polypeptide sequence except for an ammo acid substitution at one or more locations of the native Pacific yew taxadiene synthase polypeptide sequence that are selected from the group consisting of cysteine residues 329, 650, 719, and 777, histidine residues 370, 415, 579, and 793, a DDXXD motif, a DXXDD motif, a conserved arginine, and a RWWK element 22 The polypeptide of claim 17 having only conservative ammo acid substitutions to the native Pacific yew taxadiene synthase polypeptide sequence 23 The polypeptide of claim 17 that is completely homologous with the native Pacific yew taxadiene svnthase polypeptide sequence 24 The polypeptide of claim 17 lacking part or all of a transit peptide 25 An isolated polypeptide comprising at least 10 consecutive ammo acids of a native Pacific yew taxadiene synthase polypeptide 26 An isolated mature native Pacific yew taxadiene synthase polypeptide 27 An antibody specific for a native Pacific yew taxadiene synthase polypeptide 28 A method of expressing a taxadiene synthase polypeptide in a cell, the method comprising the steps of providing a cell that compnses an expressible polynucleotide that encodes a taxadiene synthase polypeptide according to claim 17, and culturing the cell under conditions suitable for expression of the polypeptide 29 The method of claim 28 wherem the cell is a taxoid-producing cell 30 The method of claim 29 wherein expression of the polynucleotide causes the cell to produce a higher level of a taxoid than an otherwise similar cell that lacks the expressible polynucleotide 31 A method of obtaining a taxadiene synthase gene comprising the steps of contacting a nucleic acid of a taxoid-producing organism with a probe or primer compnsmg a polynucleotide of claim 1 under stringent hybridization conditions, thereby causing the probe or primer to hybridize to a taxadiene synthase gene of the organism, and isolating the taxadiene synthase gene of the organism

Inventors: CROTEAU RODNEY B, WILDUNG MARK R, CROTEAU, RODNEY, B., WILDUNG, MARK, R.
Applicants: UNIV WASHINGTON, WASHINGTON STATE UNIVERSITY RESEARCH FOUNDATION
Application year: 1997
Main classification: A01H5
IPC codes: A01H5, C07K7, C07K14, C07K16, C12N5, C12N9, C12N15, C12P21, C12Q1