RECOMBINANT PINORESINOL/LARICIRESINOL REDUCTASE, RECOMBINANT DIRIGENT PROTEIN, AND METHODS OF USE

Abstract: Dirigent proteins and pinoresinol/lariciresinol reductases have been isolated, together with cDNAs encoding dirigent proteins and pinoresinol/lariciresinol reductases. Accordingly, isolated DNA sequences are provided which code for the expression of dirigent proteins and pinoresinol/laricinesinol reductases. In other aspects, replicable recombinant cloning vehicles are provided which code for dirigent proteinsor pinoresinol/lariciresinol reductases or for a base sequence sufficiently complementary to at least a portion of dirigent protein or pinoresinol/lariciresinol reductase DNA or RNA to enable hybridization therewith. In yet other aspects, modified host cells are provided that have been transformed, transfected, infected and/or injected with a recombinant cloning vehicle and/or DNA sequence encoding dirigent protein or pinoresinol/lariciresinol reductase. Thus, systems and methods are provided for the recombinant expression of dirigent proteins and/or pinoresinol/lariciresinol reductases.

Description: RECOMBINANT PINORESINOL/LARICIRESINOL REDUCTASE, RECOMBINANT DIRIGENT PROTEIN, AND METHODS OF USE Field of the Invention The present invention relates to isolated dirigent proteins and pinoresinol/lariciresinol reductases from Forsythia intermedia, Tsuga heterophylla and Thuja plicata, to nucleic acid sequences which code for dirigent proteins and pinoresinol/lariciresinol reductases from Forsythia intermedia, Tsuga heterophylla and Thuja plicata, and to vectors containing the sequences, host cells containing the sequences and methods of producing recombinant pinoresinol/lariciresinol reductases, recombinant dirigent protein and their mutants. Background of the Invention Lignans are a large, structurally diverse, class of vascular plant metabolites having a wide range of physiological functions and pharmacologically important properties (Ayres, D.C., and Loike, J.D. in Chemistry and Pharmacology of Natural Products. Lignans. Chemical, Biological and Clinical Properties, Cambridge University Press, Cambridge, England (1990); Lewis et al., in Chemistry of the Amazon, Biodiversity Natural Products, and Environmental Issues, 588, (P.R. Seidl, O.R. Gottlieb and M.A.C. Kaplan) 135-167, ACS Symposium Series, Washington D.C. (1995)). Because of their pronounced antibiotic properties (Markkanen, T. et al., Drugs Exptl. Clin. Res. 7:711-718 (1981)), antioxidant properties (Faure, M. et al., Phytochemistry 29:3773-3775 (1990); Osawa, T. et al., Agric. Biol. Chem. 49:3351-3352 (1985)) and antifeedant properties (Harmatha, J., and Nawrot, J., Biochem. Syst. Ecol. 12:95-98 (1984)), a major role of lignans in vascular plants is to help confer resistance against various opportunistic biological pathogens and predators. Lignans have also been proposed as cytokinins (Binns, A.N. et al., Proc. Natl. Acad. Sci. USA 84:980-984 (1987)) and as intermediates in lignification (Rahman, M.M.A. et al., Phytochemistry 29:1861-1866 (1990)), suggesting a critical role in plant growth and development. It is widely held that elaboration of biochemical pathways to lignins/lignans and related substances from phenylalanine (tyrosine) was essential for the successful transition of aquatic plants to their vascular dry-land counterparts (Lewis, N.G., and Davin, L.B., in Isoprenoids and Other Natural Products. Evolution and Function, 562 (W.D. Nes, ed) 202-246, ACS Symposium Series: Washington, DC (1994)), some four hundred and eighty million years ago (Graham, L.E., Origin of Land Plants, John Wiley & Sons, Inc., New York, NY (1993)). Based on existing chemotaxonomic data, lignans are present in "primitive" plants, such as the fern Blechnum orientate (Wada, H. et al., Chem. Pharm. Bull. 40:2099-2101 (1992)) and the hornworts, e.g., Dendroceros japonicus and Megaceros flagellaris (Takeda, R. et al., in Bryophytes. Their Chemistry and Chemical Taxonomy, Vol. 29 (Zinsmeister, H.D. and Mues, R. eds) pp. 201-207, Oxford University Press: New York, NY (1990); Takeda, R. et al., Tetrahedron Lett. 31:4159-4162 (1990)), with the latter recently being classified as originating in the Silurian period (Graham, L.E., J. Plant Res. 109: 241-252 (1996)). Interestingly, evolution of both gymnosperms and angiosperms was accompanied by major changes in the structural complexity and oxidative modifications of the lignans (Lewis, N.G., and Davin, L.B., in Isoprenoids and Other Natural Products. Evolution and Function, 562 (W. D. Nes, ed) 202-246, ACS Symposium Series: Washington, DC (1994); Gottlieb, O.R., and Yoshida, M., in Natural Products of Woody Plants. Chemicals Extraneous to the Lignocellulosic Cell Wall (Rowe, J.W. and Kirk, CH. eds) pp. 439-511, Springer Verlag: Berlin (1989)). Indeed, in some species, such as Western Red Cedar (Thuja plicata), lignans can contribute extensively to heartwood formation/generation by enhancing the resulting heartwood color, quality, fragrance and durability. In addition to their functions in plants, lignans also have important pharmacological roles. For example, podophyllotoxin, as its etoposide and teniposide derivatives, is an example of a plant compound that has been successfully employed as an anticancer agent (Ayres, D.C, and Loike, J.D. in Chemistry and Pharmacology of Natural Products. Lignans. Chemical, Biological and Clinical Properties, Cambridge University Press, Cambridge, England (1990)). Antiviral properties have also been reported for selected lignans. For example, (-)-arctigenin (Schroder, H.C. et al., Z. Naturforsch. 45c, 1215-1221 (1990)), (-)-trachelogenin (Schroder, H.C et al., Z. Naturforsch. 45c, 1215-1221 (1990)) and nordihydroguaiaretic acid (Gnabre, J.N. et al., Proc. Natl. Acad. Sci. USA 92:11239-11243 (1995)) are each effective against HTV due to their pronounced reverse transcriptase inhibitory activities. Some lignans, e.g., matairesinol (Nikaido, T. et al., Chem. Pharm. Bull. 29:3586-3592 (1981)), inhibit cAMP-phosphodiesterase, whereas others enhance cardiovascular activity, e.g., syringaresinol β-D-glucoside (Nishibe, S. et al., Chem. Pharm. Bull. 38:1763-1765 (1990)). There is also a high correlation between the presence, in the diet, of the "mammalian" lignans or "phytoestrogens", enterolactone and enterodiol, formed following digestion of high fiber diets, and reduced incidence rates of breast and prostate cancers (so-called chemoprevention) (Axelson, M., and Setchell, K.D.R., FEBS Lett. 123:337-342 (1981); Adlercreutz et al., J. Steroid Biochem. Molec. Biol. 41:3-8 (1992); Adlercreutz et al., J. Steroid Biochem. Molec. Biol. 52:97-103 (1995)). The "mammalian lignans," in turn, are considered to be derived from lignans such as matairesinol and secoisolariciresinol (Boriello et al., J. Applied Bacteriol., 58:37-43 (1985)). The biosynthetic pathways to the lignans are only now being defined, although there are no prior art reports of the isolation of enzymes or genes involved in the lignan biosynthetic pathway. Based on radiolabeling experiments with crude enzyme extracts from Forsythia intermedia, it was first established that entry into the 8,8'-linked lignans, which represent the most prevalent dilignol linkage known (Davin, L.B., and Lewis, N.G., in Rec. Adv. Phytochemistry , Vol. 26 (Stafford, H.A., and Ibrahim, R.K., eds), pp. 325-375, Plenum Press, New York, NY (1992)), occurs via stereoselective coupling of two achiral coniferyl alcohol molecules, in the form of oxygenated free radicals, to afford the furofuran lignan (+)-pinoresinol (Davin, L.B., Bedgar, D.L., Katayama, T., and Lewis, N.G., Phytochemistry 31:3869-3874 (1992); Pare, P.W. et al., Tetrahedron Lett. 35:4731- 4734 (1994)) (FIGURE 1). Bimolecular phenoxy radical coupling reactions, such as the stereoselective coupling of two achiral coniferyl alcohol molecules to afford the furofuran lignan (+)-pinoresinol, are involved in numerous biological processes. These are presumed to include lignin formation in vascular plants (M. Nose et al., Phytochemistry 39:71 (1995)), lignan formation in vascular plants (N.G. Lewis and L.B. Davin, ACS Symp. Ser. 562:202 (1994); P. W. Pare et al., Tetrahedron Lett. 35:4731 (1994)), suberin formation in vascular plants (M.A. Bernards et al., J. Biol. Chem. 270:7382 (1995)), fruiting body development in fungi (J.D. Bu'Lock et al., J. Chem. Soc. 2085 (1962)), insect cuticle melanization and sclerotization (M. Miessner et al., Helv. Chim. Ada 74:1205 (1991); V.J. Marmaras et al., Arch. Insect Biochem. Physiol. 31:119 (1996)), the formation of aphid pigments (D.W. Cameron and Lord Todd, in Organic Substances of Natural Origin. Oxidative Coupling of Phenols, W.I. Taylor and A.R. Battersby, Eds. (Dekker, New York, 1967), Vol. 1,' p.203), and the formation of algal cell wall polymers (M.A. Ragan, Phytochemistry 23:2029 (1984)). In contrast to the marked regiochemical and/or stereochemical specificities observed in the biosynthesis of the foregoing lignin and lignan substances in vivo, all previously described chemical (J. Iqbal et al., Chem. Rev. 94:519 (1994)) and enzymatic (K. Freudenberg, Science 148:595 (1965)) bimolecular phenoxy radical coupling reactions in vitro have lacked strict regio- and stereospecific control. That is, if chiral centers are introduced during coupling in vitro, the products are racemic, and different regiochemistries can result if more than one potential coupling site is present. Thus, the ability to generate a particular enantiomeric form or a specific coupling product in vitro is not under explicit control. Consequently, it is inferred that a mechanism exists in vivo to control the regiochemistry and stereochemistry of bimolecular phenoxy radical coupling reactions leading to the formation of, for example, lignans. In Forsythia intermedia, and presumably other species, (+)-pinoresinol, the product of the stereospecific coupling of two E-coniferyl alcohol molecules, undergoes sequential reduction to generate (+)-lariciresinol and then (-)-secoisolariciresinol (Katayama, T. et al., Phytochemistry 32:581-591 (1993); Chu, A. et al., J. Biol. Chem. 268:27026-27033 (1993)) (FIGURE 1). While it has hitherto been unclear whether more than one reductase is required to catalyze the sequential steps, the reductions proceed via abstraction of the pro-R hydride of NADPH, resulting in an "inversion" of configuration at both the C-7 and C-7' positions of the products, (+)-lariciresinol and (-)-secoisolariciresinol (Chu, A., et al., J. Biol. Chem. 268:27026-27033 (1993)). (-)-Matairesinol is subsequently formed via dehydrogenation of (-)-secoisolariciresinol, further metabolism of which presumably affords lignans such as the antiviral (-)-trachelogenin in Ipomoea cairica and (-)-podophyllotoxin in Podophyllum peltatum. Thus, the stereospecific formation of (+)-pinoresinol and the subsequent reductive steps giving (+)-lariciresinol and (-)-secoisolariciresinol are pivotal points in lignan metabolism, since they represent entry into the furano, dibenzylbutane, dibenzyl- butyrolactone and aryltetrahydronaphthalene lignan subclasses. Additionally, it should be noted that while lignans are normally optically active, the particular enantiomer present may differ between plant species. For example, (-)-pinoresinol occurs in Xanthoxylum ailanthoides (Ishii et al., Yakugaku Zasshi, 103:279-292 (1983)), and (-)-lariciresinol is present in Daphne tangutica (Lin-Gen, et al., Planta Medica, 45:172-176 (1982)). The optical activity of a particular lignan may have important ramifications regarding biological activity. For example, (-)-trachelogenin inhibits the in vitro replication of HΓV-1, whereas its (+)-enantiomer is much less effective (Schroder et al.,' Naturforsch. 45c:1215-1221(1990)). Summary of the Invention In accordance with the foregoing, in one aspect of the invention it has now been discovered that a 78-kD dirigent protein is involved in conferring stereospecificity in 8,8'- linked lignan formation. This protein has no detectable catalytically active oxidative center and apparently serves only to bind and orient coniferyl alcohol-derived free radicals, which then undergo stereoselective coupling to form (+)-pinoresinol. The formation of free-radicals, in the first instance, requires the oxidative capacity of either a nonspecific oxidase or even a non-enzymatic electron oxidant. In another aspect of the invention, it has been discovered that a single enzyme, designated pinoresinol/lariciresinol reductase, catalyzes the conversion of pinoresinol to lariciresinol and then to secoisolariciresinol. Thus, one aspect of the invention relates to isolated dirigent proteins and to isolated pinoresinol/lariciresinol reductases, such as, for example, those from Forsythia intermedia, Thuja plicata and Tsuga heterophylla. In other aspects of the invention, cDNAs encoding dirigent protein from several plant species have been isolated and sequenced, and the corresponding amino acid sequences have been deduced. Also, cDNAs encoding pinoresinol/lariciresinol reductase from several plant species have been isolated and sequenced, and the corresponding amino acid sequences have been deduced. Thus, the present invention relates to isolated proteins and to isolated DNA sequences which code for the expression of dirigent protein or pinoresinol/lariciresinol reductase. In other aspects, the present invention is directed to replicable vectors comprising a nucleic acid sequence which codes for a pinoresinol/lariciresinol reductase or for a dirigent protein. The present invention is also directed to a base sequence sufficiently complementary to at least a portion of a pinoresinol/lariciresinol reductase DNA or RNA, or to at least a portion of a dirigent protein DNA or RNA, to enable hybridization therewith. The aforesaid complementary base sequences include, but are not limited to: antisense pinoresinol/lariciresinol reductase RNA; antisense dirigent protein RNA; fragments of DNA that are complementary to a pinoresinol/lariciresinol reductase DNA, or to a dirigent protein DNA, and which are therefore useful as polymerase chain reaction primers, or as probes for pinoresinol/lariciresinol reductase genes, dirigent protein genes, or related genes. In yet other aspects of the invention, modified host cells are provided that have been transformed, transfected, infected and/or injected with a replicable vector and/or DNA sequence of the invention. Thus, the present invention provides for the recombinant expression of pinoresinol/lariciresinol reductases and dirigent proteins in plants, animals, microbes and in cell cultures. The inventive concepts described herein may be used to facilitate the production, isolation and purification of significant quantities of recombinant pinoresinol/lariciresinol reductase or dirigent protein, or of their enzyme products, in plants, animals, microbes or cell cultures. Brief Description of the Drawings The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: FIGURE 1 shows the stereospecific conversion of E-coniferyl alcohol to (+)-pinoresinol in Forsythia intermedia. The stereoselectivity of this reaction is controlled by dirigent protein. (+)-Pinoresinol is then sequentially converted to (+)-lariciresinol and (-)-secoisolariciresinol by (+)-pinoresinol/(+)-lariciresinol reductase. (+)-pinoresinol, (+)-lariciresinol and (-)-secoisolariciresinol are the precursors of the furofuran, furano and dibenzylbutane families of lignans, respectively. Detailed Description of the Preferred Embodiment As used herein, the terms "amino acid" and "amino acids" refer to all naturally occurring L- -amino acids or their residues. The amino acids are identified by either the single-letter or three-letter designations: Asp D aspartic acid lie I isoleucine Thr T threonine Leu L leucine Ser S serine Tyr Y tyrosine Glu E glutamic acid Phe F phenylalanine Pro P proline His H histidine Gly G glycine Lys K lysine Ala A alanine Arg R arginine Cys C cysteine Trp W tryptophan Val V valine Gin Q glutamine Met M methionine Asn N asparagine As used herein, the term "nucleotide" means a monomeric unit of DNA or RNA containing a sugar moiety (pentose), a phosphate and a nitrogenous heterocyclic base. The base is linked to the sugar moiety via the glycosidic carbon (1 * carbon of pentose) and that combination of base and sugar is called a nucleoside. The base characterizes the nucleotide with the four bases of DNA being adenine ("A"), guanine ("G"), cytosine ("C") and thymine ("T"). Inosine ("I") is a synthetic base that can be used to substitute for any of the four, naturally-occurring bases (A, C, G or T). The four RNA bases are A,G,C and uracil ("U"). The nucleotide sequences described herein comprise a linear array of nucleotides connected by phosphodiester bonds between the 3' and 5' carbons of adjacent pentoses. The term "percent identity" (%I) means the percentage of amino acids or nucleotides that occupy the same relative position when two amino acid sequences, or two nucleic acid sequences, are aligned side by side. The term "percent similarity" (%S) is a statistical measure of the degree of relatedness of two compared protein sequences. The percent similarity is calculated by a computer program that assigns a numerical value to each compared pair of amino acids based on chemical similarity (e.g., whether the compared amino acids are acidic, basic, hydrophobic, aromatic, etc.) and/or evolutionary distance as measured by the minimum number of base pair changes that would be required to convert a codon encoding one member of a pair of compared amino acids to a codon encoding the other member of the pair. Calculations are made after a best fit alignment of the two sequences has been made empirically by iterative comparison of all possible alignments. (Henikoff, S. and Henikoff, J.G., Proc. Nat'l Acad Sci USA 89:10915-10919 (1992)). "Oligonucleotide" refers to short length single or double stranded sequences of deoxyribonucleotides linked via phosphodiester bonds. The oligonucleotides are chemically synthesized by known methods and purified, for example, on polyacrylamide gels. The term "pinoresinol/lariciresinol reductase" is used herein to mean an enzyme capable of catalyzing two reduction reactions: the reduction of pinoresinol to lariciresinol, and the reduction of lariciresinol to secoisolariciresinol. The products of these reactions, lariciresinol and secoisolariciresinol, can be either the (+)- or (-)-enantiomers. The term "dirigent protein" is used herein to mean a protein capable of guiding a bimolecular phenoxy radical coupling reaction thereby determining the stereochemistry and regiochemistry of the product of the reaction and/or its polymeric derivatives. The term "stringent wash conditions" refers to the conditions used to wash a nucleic acid blot, such as a Southern blot. The following are representative hybridization and stringent wash conditions useful for identifying (by Southern blotting) nucleic acid molecules of the invention that are capable of hybridizing to a nucleic acid molecule selected from the group consisting of SEQ ID NOS:12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105, or to the antisense complement of a nucleic acid molecule selected from the group consisting of SEQ ID NOS: 12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105: hybridization in 6 X SSC, 5 X Denhardt's, 0.5% SDS at 55-58°C for 12 hours, followed by washing in 2 X SSC, 0.5% SDS at 55-58°C for 30 minutes. An optional further wash can be conducted in 1 X SSC, 0.5% SDS at 55-58°C for 30 minutes, followed by an additional, optional wash in 0.5 X SSC, 0.5% SDS at 55-58°C for 30 minutes. The following are representative hybridization and stringent wash conditions useful for identifying (by Southern blotting) nucleic acid molecules of the invention that are capable of hybridizing to a nucleic acid molecule selected from the group consisting of SEQ ID NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117, or to the antisense complement of a nucleic acid molecule selected from the group consisting of SEQ ID NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117: hybridization in 6 X SSC, 5 X Denhardt's, 0.5% SDS at 57-58°C for 12 hours, followed by one wash in 4 X SSC, 0.5% SDS at room temperature (typically 20°C to 30°C) for 5 minutes, followed by one wash in 2 X SSC, 0.5% SDS at 57-58°C for 20 minutes. An optional further wash can be conducted in 1 X SSC, 0.5% SDS at 57-58°C for 30 minutes, followed by an additional, optional wash in 0.5 X SSC, 0.5% SDS at 57-58°C for 30 minutes. The terms "alteration", "amino acid sequence alteration", "variant" and "amino acid sequence variant" refer to dirigent protein or pinoresinol/lariciresinol reductase molecules with some differences in their amino acid sequences as compared to the corresponding native dirigent protein or pinoresinol/lariciresinol reductase. Ordinarily, the variants will possess at least about 70% homology with the corresponding, native dirigent protein or pinoresinol/lariciresinol reductase, and preferably they will be at least about 80% homologous with the corresponding, native dirigent protein or pinoresinol/lariciresinol reductase. The amino acid sequence variants of dirigent protein or pinoresinol/lariciresinol reductase falling within this invention possess substitutions, deletions, and/or insertions at certain positions. Sequence variants of dirigent protein or pinoresinol/lariciresinol reductase may be used to attain desired enhanced or reduced enzymatic activity, modified regiochemistry or stereochemistry, or altered substrate utilization or product distribution. Substitutional dirigent protein variants or pinoresinol/lariciresinol reductase variants are those that have at least one amino acid residue in the corresponding native dirigent protein sequence or pinoresinol/lariciresinol reductase sequence removed and a different amino acid inserted in its place at the same position. The substitutions may be single, where only one amino acid in the molecule has been substituted, or they may be multiple, where two or more amino acids have been substituted in the same molecule. Substantial changes in the activity of the dirigent protein or pinoresinol/lariciresinol reductase molecule may be obtained by substituting an amino acid with a side chain that is significantly different in charge and/or structure from that of the native amino acid. This type of substitution would be expected to affect the structure of the polypeptide backbone and/or the charge or hydrophobicity of the molecule in the area of the substitution. Moderate changes in the activity of the dirigent protein or pinoresinol/lariciresinol reductase molecule would be expected by substituting an amino acid with a side chain that is similar in charge and/or structure to that of the native molecule. This type of substitution, referred to as a conservative substitution, would not be expected to substantially alter either the structure of the polypeptide backbone or the charge or hydrophobicity of the molecule in the area of the substitution. Insertional dirigent protein variants or pinoresinol/lariciresinol reductase variants are those with one or more amino acids inserted immediately adjacent to an amino acid at a particular position in the native dirigent protein or pinoresinol/lariciresinol reductase molecule. Immediately adjacent to an amino acid means connected to either the -carboxy or -amino functional group of the amino acid. The insertion may be one or more amino acids. Ordinarily, the insertion will consist of one or two conservative amino acids. Amino acids similar in charge and/or structure to the amino acids adjacent to the site of insertion are defined as conservative. Alternatively, this invention includes insertion of an amino acid with a charge and/or structure that is substantially different from the amino acids adjacent to the site of insertion. Deletional variants are those where one or more amino acids in the native dirigent protein or pinoresinol/lariciresinol reductase molecule have been removed. Ordinarily, deletional variants will have one or two amino acids deleted in a particular region of the dirigent protein or pinoresinol/lariciresinol reductase molecule. The term "antisense" or "antisense RNA" or "antisense nucleic acid" is used herein to mean a nucleic acid molecule that is complementary to all or part of a messenger RNA molecule. Antisense nucleic acid molecules are typically used to inhibit the expression, in vivo, of complementary, expressed messenger RNA molecules. The terms "biological activity", "biologically active", "activity" and "active" when used with reference to a pinoresinol/lariciresinol reductase molecule refer to the ability of the pinoresinol/lariciresinol reductase molecule to reduce pinoresinol and lariciresinol to yield lariciresinol and secoisolariciresinol, respectively, as measured in an enzyme activity assay, such as the assay described in Example 8 below. The terms "biological activity", "biologically active", "activity" and "active" when used with reference to a dirigent protein refer to the ability of the dirigent protein to guide a bimolecular phenoxy radical coupling reaction thereby determining the stereochemistry and regiochemistry of the product of the reaction and of its polymeric derivatives. Amino acid sequence variants of dirigent protein or pinoresinol/lariciresinol reductase may have desirable altered biological activity including, for example, altered reaction kinetics, substrate utilization, product distribution or other characteristics such as regiochemistry and stereochemistry. The terms "DNA sequence encoding", "DNA encoding" and "nucleic acid encoding" refer to the order or sequence of deoxyribonucleotides along a strand of deoxyribonucleic acid. The order of these deoxyribonucleotides determines the order of amino acids along the translated polypeptide chain. The DNA sequence thus codes for the amino acid sequence. The term "replicable vector" refers to a piece of DNA, usually double-stranded, which may have inserted into it a piece of foreign DNA. Foreign DNA is defined as heterologous DNA, which is DNA not naturally found in the host. The vector is used to transport the foreign or heterologous DNA into a suitable host cell. Once in the host cell, the vector can replicate independently of or coincidentally with the host chromosomal DNA, and several copies of the vector and its inserted (foreign) DNA may be generated. The term "replicable vector" includes replicable expression vectors that contain the necessary elements that permit translating the foreign DNA into a polypeptide. Many molecules of the polypeptide encoded by the foreign DNA can thus be rapidly synthesized. Replicable vectors can also include insert DNA that is normally found in the host. The terms "transformed host cell," "transformed" and "transformation" refer to the introduction of DNA into a cell. The cell is termed a "host cell", and it may be a prokaryotic or a eukaryotic cell. Typical prokaryotic host cells include various strains of E. coli. Typical eukaryotic host cells are plant cells, such as maize cells, yeast cells, insect cells or animal cells. The introduced DNA is usually in the form of a vector containing an inserted piece of DNA. The introduced DNA sequence may be from the same species as the host cell or from a different species from the host cell, or it may be a hybrid DNA sequence, containing some foreign DNA and some DNA derived from the host species. In accordance with the present invention, cDNAs encoding dirigent protein and pinoresinol/lariciresinol reductase from several plant species, including Forsythia intermedia, Thuja plicata and Tsuga heterophylla, were isolated, sequenced and expressed in the following manner. With respect to the cDNAs encoding dirigent protein from Forsythia intermedia, an empirically-determined purification protocol was developed to isolate the Forsythia dirigent protein. This procedure yielded at least six isoforms of the dirigent protein. Amino acid sequencing of the amino terminus of each of these isoforms revealed that the sequence of each isoform was identical. Sequencing of the N-terminus of a mixture of these isoforms yielded a 28 amino acid sequence (SΕQ ID NO:l). Tryptic digestion of a mixture of these isoforms yielded six peptide fragments which were purified in sufficient quantity to permit sequencing SΕQ ID NOS: 2-7. A primer designated PSINT1 (SΕQ ID NO: 8) was synthesized based on the sequence of amino acids 9 to 15 of the N-terminal peptide (SΕQ JD NO:l). A primer designated PSI1R (SΕQ ID NO:9) was synthesized based on the sequence of amino acids 3 to 9 of the internal peptide sequence set forth in (SΕQ ID NO:2). A primer designated PSI2R (SΕQ ID NO: 10) was synthesized based on the sequence of amino acids 13 to 20 of the internal peptide sequence set forth in (SΕQ ID NO:2). A primer designated PSI7R (SΕQ ID NO: 11) was synthesized based on the sequence of amino acids 6 to 12 of the internal peptide sequence set forth in (SΕQ ID NO:3). Forsythia total RNA was isolated by means of a protocol adapted from a method specifically designed for woody tissues which contain a large concentration of polyphenols. Poly A+ RNA was isolated and a cDNA library constructed using standard means. A PCR reaction utilizing primers PSINT1 (SΕQ ID NO:8) and one of PSI7R, (SEQ ID NO: 11) PSI2R (SEQ ID NO: 10) or PSI1R (SEQ ID NO:9), together with an aliquot of Forsythia cDNA as substrate, each yielded a single cDNA band of -370 bp, -155 bp and -125 bp, respectively. The -370 bp product of the PSINT1 (SEQ ID NO:8)- PSI7R (SEQ ID NO: 11) reaction was amplified by PCR and utilized as a probe to screen approximatley 600,000 PFU of a Forsythia intermedia cDNA library. Two distinct cDNAs were identified, called pPSDFil (SEQ ID NO: 12) and pPSDFi2 (SEQ ID NO: 14). The cDNA insert encoding dirigent protein was excised from plasmid pPSDFil and cloned into the baculovirus transfer vector pBlueBac4. The resulting construct was used to transform Spodoptera frugiperda from which functional dirigent protein was purified. In another aspect, Forsythia cDNAs were used as probes to isolate two dirigent protein clones from Tsuga heterophylla (SEQ ID NOS: 16, 18), and eight dirigent protein cDNA clones from Thuja plicata (SEQ ID NOS:20, 22, 24, 26, 28, 30, 32, 34). With respect to the cDNAs encoding (+)-pinoresinol/(+)-lariciresinol reductase from Forsythia intermedia, an empirically-determined purification protocol, consisting of eight chromatographic steps, was developed to isolate the Forsythia (+)-pinoresinol/(+)- lariciresinol reductase protein. This procedure yielded two isoforms of (+)- pinoresinol/(+)-lariciresinol reductase which were both capable of catalyzing the reduction of (+)-pinoresinol and (+)-lariciresinol. Sequencing of the N-terminus of each of these isoforms yielded an identical 30 amino acid sequence (SEQ ID NO:36). Tryptic digestion of a mixture of both of these isoforms yielded four peptide fragments which were purified in sufficient quantity to permit sequencing (SEQ ID Nos:37-40). Additionally, cyanogen bromide cleavage of a mixture of both of these isoforms yielded three peptide fragments which were purified in sufficient quantity to permit sequencing (SEQ ID Nos:41-43). A primer designated PLRN5 (SEQ ID NO:44) was synthesized based on the sequence of amino acids 7 to 13 of the N-terminal peptide (SEQ ID NO:36). A primer designated PLR14R (SEQ ID NO:45) was synthesized based on the sequence of amino acids 2 to 8 of the internal peptide sequence set forth in SEQ ID NO: 37. A primer designated PLR15R (SEQ ID NO:46) was synthesized based on the sequence of amino acids 9 to 15 of the internal peptide sequence set forth in SEQ ID NO: 37. The sequence of amino acids 9 to 15 of the internal peptide sequence set forth in SEQ ID NO: 37, upon which the sequence of primer PLR15R (SEQ ID NO:46) was based, also corresponded to the sequence of amino acids 4 to 10 of the cyanogen bromide-generated, internal fragment set forth in SEQ ID NO:41. Forsythia total RNA was isolated by means of a protocol adapted from a method specifically designed for woody tissues which contain a large concentration of polyphenols. Poly A+ RNA was isolated and a cDNA library constructed using standard means. A PCR reaction utilizing primers PLRN5 (SEQ ID NO:44) and either PLR14R (SEQ ID NO:45) or PLR15R (SEQ ID NO:46), together with an aliquot of Forsythia cDNA as substrate, yielded two, amplified bands of 380 bp and 400 bp. One 400 bp cDNA insert was utilized as a probe with which to screen the Forsythia cDNA library. The 400 bp probe corresponded to bases 22 to 423 of SEQ ID NO:47. Six cDNA clones were isolated and sequenced (SEQ ID Nos:47, 49, 51, 53, 55, 57). The clones shared a common coding region, many had a different 5'-untranslated region and the 3'- untranslated region of each terminated at a different point. One of these cDNAs (SEQ ID NO:47), expressed as a β-galactosidase fusion protein in E. coli, catalyzed the same enantiomer-specific reactions as the native plant protein. In another aspect, (+)-pinoresinol/(+)-lariciresinol reductase and (-)-pinoresinol/(-)-lariciresinol reductase from Thuja plicata were isolated by synthesizing Thuja plicata cDNA which was utilized as a template in a PCR reaction in which the primers were a 3' linker-primer (SEQ ID NO:59) and a 5' primer, designated CR6-NT, (SEQ ID NO:60). At least two bands of the expected length (1.2 kb) were generated and cloned into a plasmid vector. One clone, designated plr-Tpl, (SEQ ID NO:61) was completely sequenced and expressed as a β-galactosidase fusion protein in E. coli. plr-Tpl (SEQ ID NO:61) encodes a (-)-pinoresinol/(-)-lariciresinol reductase. The cDNA insert of clone plr-Tpl (SEQ JD NO:61) was used to screen the T. plicata cDNA library and identified an additional, unique clone, designated plr-Tp2, (SEQ ID NO:63). plr-Tp2 (SEQ ID NO:63) has high homology to plr-Tpl (SEQ ID NO:61) but encodes a (+)-pinoresinol/(+)-lariciresinol reductase. The cDNA insert of clone plr-Tpl (SEQ K) NO:61) was used to screen the T. plicata cDNA library and identify an additional two pinoresinol/lariciresinol reductase cDNAs (SEQ ID NOS:65, 67). Two cDNAs encoding pinoresinol/lariciresinol reductases from Tsuga heterophylla (SEQ ID Nos:69, 71) were isolated by screening a Tsuga heterophylla cDNA library with the plr-Tpl cDNA insert (SEQ ID NO:61). Additional pinoresinol/lariciresinol reductase cDNAs and dirigent protein cDNAs were isolated as described in Examples 17-22 herein. The isolation of cDNAs encoding dirigent proteins, (+)-pinoresinol/- (+)-lariciresinol reductase and (-)-pinoresinol/(-)-lariciresinol reductase permits the development of an efficient expression system for these functional enzymes; provides useful tools for examining the developmental regulation of lignan biosynthesis and permits the isolation of other dirigent proteins and pinoresinol/lariciresinol reductases. The isolation of the dirigent protein and pinoresinol/lariciresinol reductase cDNAs also permits the transformation of a wide range of organisms in order to enhance or modify lignan biosynthesis. The proteins and nucleic acids of the present invention can be utilized to predetermine the stereochemistry, regiochemistry, or both, of the products of bimolecular phenoxy coupling reactions, such as the furofuran, furano and dibenzylbutane lignans. By way of non-limiting examples, the proteins and nucleic acids of the present invention can be utilized to: elevate or otherwise alter the levels of health-protecting lignans, such as podophyllotoxin, in plant species, including but not limited to vegetables, grains and fruits, and to food items incorporating material derived from such genetically altered plants; genetically alter plant species to provide an abundant, natural supply of lignans useful for a variety of purposes, for example as neutriceuticals and dietary supplements; to genetically alter living organisms to produce an abundant supply of optically pure lignans having desirable biological properties, for example (-)-arctigenin which possesses antiviral properties. In particular, characterization of the dirigent protein binding site and mechanism of action permits the development of synthetic proteins consisting of an array of dirigent protein binding sites which serve as templates for stereochemically-controlled polymeric assembly. N-terminal transport sequences well known in the art (see, e.g., von Heijne, G. et al., Eur. I. Biochem 180:535-545 (1989); Stryer, Biochemistry W.H. Freeman and Company, New York, NY, p. 769 (1988)) may be employed to direct the dirigent protein or pinoresinol/lariciresinol reductase to a variety of cellular or extracellular locations. Sequence variants of wild-type dirigent protein clones and pinoresinol/- lariciresinol clones that can be produced by deletions, substitutions, mutations and/or insertions are intended to be within the scope of the invention except insofar as limited by the prior art. Dirigent protein or pinoresinol/lariciresinol reductase amino acid sequence variants may be constructed by mutating the DNA sequence that encodes wild-type dirigent protein or wild-type pinoresinol/lariciresinol reductase, such as by using techniques commonly referred to as site-directed mutagenesis. Various polymerase chain reaction (PCR) methods now well known in the field, such as a two primer system like the Transformer Site-Directed Mutagenesis kit from Clontech, may be employed for this purpose. Following denaturation of the target plasmid in this system, two primers are simultaneously annealed to the plasmid; one of these primers contains the desired site- directed mutation, the other contains a mutation at another point in the plasmid resulting in elimination of a restriction site. Second strand synthesis is then carried out, tightly linking these two mutations, and the resulting plasmids are transformed into a mutS strain of E. coli. Plasmid DNA is isolated from the transformed bacteria, restricted with the relevant restriction enzyme (thereby linearizing the unmutated plasmids), and then retransformed into E. coli. This system allows for generation of mutations directly in an expression plasmid, without the necessity of subcloning or generation of single-stranded phagemids. The tight linkage of the two mutations and the subsequent linearization of unmutated plasmids results in high mutation efficiency and allows minimal screening. Following synthesis of the initial restriction site primer, this method requires the use of only one new primer type per mutation site. Rather than prepare each positional mutant separately, a set of "designed degenerate" oligonucleotide primers can be synthesized in order to introduce all of the desired mutations at a given site simultaneously. Transformants can be screened by sequencing the plasmid DNA through the mutagenized region to identify and sort mutant clones. Each mutant DNA can then be restricted and analyzed by electrophoresis on Mutation Detection Enhancement gel (J.T. Baker) to confirm that no other alterations in the sequence have occurred (by band shift comparison to the unmutagenized control). The verified mutant duplexes can be cloned into a replicable expression vector, if not already cloned into a vector of this type, and the resulting expression construct used to transform E. coli, such as strain E. coli BL21(DE3)pLysS, for high level production of the mutant protein, and subsequent purification thereof. The method of FAB-MS mapping can be employed to rapidly check the fidelity of mutant expression. This technique provides for sequencing segments throughout the whole protein and provides the necessary confidence in the sequence assignment. In a mapping experiment of this type, protein is digested with a protease (the choice will depend on the specific region to be modified since this segment is of prime interest and the remaining map should be identical to the map of unmutagenized protein). The set of cleavage fragments is fractionated by microbore HPLC (reversed phase or ion exchange, again depending on the specific region to be modified) to provide several peptides in each fraction, and the molecular weights of the peptides are determined by FAB-MS. The masses are then compared to the molecular weights of peptides expected from the digestion of the predicted sequence, and the correctness of the sequence quickly ascertained. Since this mutagenesis approach to protein modification is directed, sequencing of the altered peptide should not be necessary if the MS agrees with prediction. If necessary to verify a changed residue, CAD-tandem MS/MS can be employed to sequence the peptides of the mixture in question, or the target peptide purified for subtractive Edman degradation or carboxypeptidase Y digestion depending on the location of the modification. In the design of a particular site directed mutant, it is generally desirable to first make a non-conservative substitution (e.g., Ala for Cys, His or Glu) and determine if activity is greatly impaired as a consequence. The properties of the mutagenized protein are then examined with particular attention to the kinetic parameters of K m and k cat as sensitive indicators of altered function, from which changes in binding and/or catalysis per se may be deduced by comparison to the native enzyme. If the residue is by this means demonstrated to be important by activity impairment, or knockout, then conservative substitutions can be made, such as Asp for Glu to alter side chain length, Ser for Cys, or Arg for His. For hydrophobic segments, it is largely size that will be altered, although aromatics can also be substituted for alkyl side chains. Changes in the normal product distribution can indicate which step(s) of the reaction sequence have been altered by the mutation. Other site directed mutagenesis techniques may also be employed with the nucleotide sequences of the invention. For example, restriction endonuclease digestion of DNA followed by ligation may be used to generate dirigent protein or pinoresinol/lariciresinol reductase deletion variants, as described in Section 15.3 of Sambrook et al. (Molecular Cloning: A Laboratory Manual, 2nd Ed., Cold Spring Harbor Laboratory Press, New York, NY (1989)). A similar strategy may be used to construct insertion variants, as described in Section 15.3 of Sambrook et al., supra. Oligonucleotide-directed mutagenesis may also be employed for preparing substitution variants of this invention. It may also be used to conveniently prepare the deletion and insertion variants of this invention. This technique is well known in the art as described by Adelman et al. (DNA 2:183 (1983)). Generally, oligonucleotides of at least 25 nucleotides in length are used to insert, delete or substitute two or more nucleotides in the dirigent protein gene or pinoresinol/lariciresinol reductase gene. An optimal oligonucleotide will have 12 to 15 perfectly matched nucleotides on either side of the nucleotides coding for the mutation. To mutagenize the wild-type dirigent protein or wild-type pinoresinol/lariciresinol reductase, the oligonucleotide is annealed to the single-stranded DNA template molecule under suitable hybridization conditions. A DNA polymerizing enzyme, usually the Klenow fragment of E. coli DNA polymerase I, is then added. This enzyme uses the oligonucleotide as a primer to complete the synthesis of the mutation-bearing strand of DNA. Thus, a heteroduplex molecule is formed such that one strand of DNA encodes the wild-type dirigent protein or pinoresinol/lariciresinol reductase inserted in the vector, and the second strand of DNA encodes the mutated form of dirigent protein or pinoresinol/lariciresinol reductase inserted into the same vector. This heteroduplex molecule is then transformed into a suitable host cell. Mutants with more than one amino acid substituted may be generated in one of several ways. If the amino acids are located close together in the polypeptide chain, they may be mutated simultaneously using one oligonucleotide that codes for all of the desired amino acid substitutions. Ii however, the amino acids are located some distance from each other (separated by more than ten amino acids, for example) it is more difficult to generate a single oligonucleotide that encodes all of the desired changes. Instead, one of two alternative methods may be employed. In the first method, a separate oligonucleotide is generated for each amino acid to be substituted. The oligonucleotides are then annealed to the single-stranded template DNA simultaneously, and the second strand of DNA that is synthesized from the template will encode all of the desired amino acid substitutions. An alternative method involves two or more rounds of mutagenesis to produce the desired mutant. The first round is as described for the single mutants: wild-type dirigent protein or pinoresinol/lariciresinol reductase DNA is used for the template, an oligonucleotide encoding the first desired amino acid substitution(s) is annealed to this template, and the heteroduplex DNA molecule is then generated. The second round of mutagenesis utilizes the mutated DNA produced in the first round of mutagenesis as the template. Thus, this template already contains one or more mutations. The oligonucleotide encoding the additional desired amino acid substitution(s) is then annealed to this template, and the resulting strand of DNA now encodes mutations from both the first and second rounds of mutagenesis. This resultant DNA can be used as a template in a third round of mutagenesis, and so on. Eukaryotic expression systems may be utilized for dirigent protein or pinoresinol/lariciresinol reductase production since they are capable of carrying out any required posttranslational modifications and of directing the enzyme to the proper membrane location. A representative eukaryotic expression system for this purpose uses the recombinant baculovirus, Autographa californica nuclear polyhedrosis virus (AcNPV; M.D. Summers and G.E. Smith, A Manual of Methods for Baculovirus Vectors and Insect Cell Culture Procedures (1986); Luckow et al., Bio-technology 6:47-55 (1987)) for expression of the dirigent protein or pinoresinol/lariciresinol reductases of the invention. Infection of insect cells (such as cells of the species Spodoptera frugiperda) with the recombinant baculoviruses allows for the production of large amounts of the dirigent protein or pinoresinol/lariciresinol reductase protein. In addition, the baculovirus system has other important advantages for the production of recombinant dirigent protein or pinoresinol/lariciresinol reductase. For example, baculoviruses do not infect humans and can therefore be safely handled in large quantities. In the baculovirus system, a DNA construct is prepared including a DNA segment encoding dirigent protein or pinoresinol/lariciresinol reductase and a vector. The vector may comprise the polyhedron gene promoter region of a baculovirus, the baculovirus flanking sequences necessary for proper cross-over during recombination (the flanking sequences comprise about 200-300 base pairs adjacent to the promoter sequence) and a bacterial origin of replication which permits the construct to replicate in bacteria. The vector is constructed so that (i) the DNA segment is placed adjacent (or operably-linked or "downstream" or "under the control of") to the polyhedron gene promoter and (ii) the promoter/pinoresinol/lariciresinol reductase, or promoter/dirigent protein, combination is flanked on both sides by 200-300 base pairs of baculovirus DNA (the flanking sequences). To produce a dirigent protein DNA construct, or a pinoresinol/lariciresinol reductase DNA construct, a cDNA clone encoding a full length dirigent protein or pinoresinol/lariciresinol reductase is obtained using methods such as those described herein. The DNA construct is contacted in a host cell with baculovirus DNA of an appropriate baculovirus (that is, of the same species of baculovirus as the promoter encoded in the construct) under conditions such that recombination is effected. The resulting recombinant baculoviruses encode the full dirigent protein or pinoresinol/lariciresinol reductase. For example, an insect host cell can be cotransfected or transfected separately with the DNA construct and a functional baculovirus. Resulting recombinant baculoviruses can then be isolated and used to infect cells to effect production of dirigent protein or pinoresinol/lariciresinol reductase. Host insect cells include, for example, Spodoptera frugiperda cells. Insect host cells infected with a recombinant baculovirus of the present invention are then cultured under conditions allowing expression of the baculovirus-encoded dirigent protein or pinoresinol/lariciresinol reductase. Recombinant protein thus produced is then extracted from the cells using methods known in the art. Other eukaryotic microbes such as yeasts may also be used to practice this invention. The baker's yeast Saccharomyces cerevisiae, is a commonly used yeast, although several other strains are available. The plasmid YRp7 (Stinchcomb et al., Nature 282:39 (1979); Kingsman et al., Gene 7:141 (1979); Tschemper et al., Gene 10:157 (1980)) is commonly used as an expression vector in Saccharomyces. This plasmid contains the trpl gene that provides a selection marker for a mutant strain of yeast lacking the ability to grow in tryptophan, such as strains ATCC No. 44,076 and PEP4-1 (Jones, Genetics 85:12 (1977)). The presence of the trpl lesion as a characteristic of the yeast host cell genome then provides an effective environment for detecting transformation by growth in the absence of tryptophan. Yeast host cells are generally transformed using the polyethylene glycol method, as described by Hinnen (Proc. Natl. Acad. Sci. USA 75:1929 (1978)). Additional yeast transformation protocols are set forth in Gietz et al., N.A.R. 20(17): 1425 (1992); Reeves et al., FEMS 99:193-197 (1992). Suitable promoting sequences in yeast vectors include the promoters for 3-phosphoglycerate kinase (Hitzeman et al., /. Biol. Chem. 255:2073 (1980)) or other glycolytic enzymes (Hess et al., 1. Adv. Enzyme Reg. 7:149 (1968); Holland et al., Biochemistry 17:4900 (1978)), such as enolase, glyceraldehyde-3-phosphate dehydro- genase, hexokinase, pyruvate decarboxylase, phosphofructokinase, glucose-6-phosphate isomerase, 3-phosphoglycerate mutase, pyruvate kinase, triosephosphate isomerase, phosphoglucose isomerase, and glucokinase. In the construction of suitable expression plasmids, the termination sequences associated with these genes are also ligated into the expression vector 3' of the sequence desired to be expressed to provide polyadenylation of the mRNA and termination. Other promoters that have the additional advantage of transcription controlled by growth conditions are the promoter region for alcohol dehydrogenase 2, isocytochrome C, acid phosphatase, degradative enzymes associated with nitrogen metabolism, and the aforementioned glyceraldehyde-3-phosphate dehydrogenase, and enzymes responsible for maltose and galactose utilization. Any plasmid vector containing yeast-compatible promoter, origin of replication and termination sequences is suitable. Cell cultures derived from multicellular organisms, such as plants, may be used as hosts to practice this invention. Transgenic plants can be obtained, for example, by transferring plasmids that encode pinoresinol/lariciresinol reductase, and/or dirigent protein, and a selectable marker gene, e.g., the kan gene encoding resistance to kanamycin, into Agrobacterium tumifaciens containing a helper Ti plasmid as described in Hoeckema et al., Nature 303:179-181 (1983) and culturing the Agrobacterium cells with leaf slices of the plant to be transformed as described by An et al., Plant Physiology 81:301-305 (1986). Transformation of cultured plant host cells is normally accomplished through Agrobacterium tumifaciens, as described above. Cultures of mammalian host cells and other host cells that do not have rigid cell membrane barriers are usually transformed using the calcium phosphate method as originally described by Graham and Van der Eb (Virology 52:546 (1978)) and modified as described in Sections 16.32-16.37 of Sambrook et al., supra. However, other methods for introducing DNA into cells such as Polybrene (Kawai and Nishizawa, Mol. Cell. Bio I. 4:1172 (1984)), protoplast fusion (Schaffner, Proc. Natl. Acad. Sci. USA 77:2163 (1980)), electroporation (Neumann et al., EMBO J. 1:841 (1982)), and direct microinjection into nuclei (Capecchi, Cell 22:479 (1980)) may also be used. Additionally, animal transformation strategies are reviewed in Monastersky G.M. and Robl, J.M., Strategies in Transgenic Animal Science, ASM Press, Washington, D.C. (1995). Transformed plant calli may be selected through the selectable marker by growing the cells on a medium containing, e.g., kanamycin, and appropriate amounts of phytohormone such as naphthalene acetic acid and benzyladenine for callus and shoot induction. The plant cells may then be regenerated and the resulting plants transferred to soil using techniques well known to those skilled in the art. In addition, a gene regulating pinoresinol/lariciresinol reductase production, or dirigent protein production, can be incorporated into the plant along with a necessary promoter which is inducible. In the practice of this embodiment of the invention, a promoter that only responds to a specific external or internal stimulus is fused to the target cDNA. Thus, the gene will not be transcribed except in response to the specific stimulus. As long as the gene is not being transcribed, its gene product is not produced. An illustrative example of a responsive promoter system that can be used in the practice of this invention is the glutathione-S-transferase (GST) system in maize. GSTs are a family of enzymes that can detoxify a number of hydrophobic electrophilic compounds that often are used as pre-emergent herbicides (Weigand et al., Plant Molecular Biology 7:235-243 (1986)). Studies have shown that the GSTs are directly involved in causing this enhanced herbicide tolerance. This action is primarily mediated through a specific 1.1 kb mRNA transcription product. In short, maize has a naturally occurring quiescent gene already present that can respond to external stimuli and that can be induced to produce a gene product. This gene has previously been identified and cloned. Thus, in one embodiment of this invention, the promoter is removed from the GST responsive gene and attached to a pinoresinol/lariciresinol reductase gene, or a dirigent protein gene, that previously has had its native promoter removed. This engineered gene is the combination of a promoter that responds to an external chemical stimulus and a gene responsible for successful production of pinoresinol/lariciresinol reductase or dirigent protein. In addition to the methods described above, several methods are known in the art for transferring cloned DNA into a wide variety of plant species, including gymnosperms, angiosperms, monocots and dicots (see, e.g., Glick and Thompson, eds., Methods in Plant Molecular Biology, CRC Press, Boca Raton, Florida (1993)). Representative examples include electroporation-facilitated DNA uptake by protoplasts (Rhodes et al., Science 240(4849):204-207 (1988)); treatment of protoplasts with polyethylene glycol (Lyznik et al., Plant Molecular Biology 13:151-161 (1989)); and bombardment of cells with DNA laden microprojectiles (Klein et al., Plant Physiol. 91:440-444 (1989) and Boynton et al, Science 240(4858):1534-1538 (1988)). Numerous methods now exist, for example, for the transformation of cereal crops (see, e.g., McKinnon, G.E. and Henry, R.J., J. Cereal Science, 22(3):203-210 (1995); Mendel, R.R. and Teeri, T.H., Plant and Microbial Biotechnology Research Series, 3:81-98, Cambridge University Press (1995); McElroy, D. and Brettell, R.I.S., Trends in Biotechnology, 12(2):62-68 (1994); Christou et al., Trends in Biotechnology, 10(7):239-246 (1992); Christou, P. and Ford, T.L., Annals of Botany, 75(5): 449-454 (1995); Park et al., Plant Molecular Biology, 32(6): 1135-1148 (1996); Altpeter et al., Plant Cell Reports, 16:12-17 (1996)). Additionally, plant transformation strategies and techniques are reviewed in Birch, R.G., Ann Rev Plant Phys Plant Mol Biol 48:297 (1997); Forester et al., Exp. Agric. 33:15-33 (1997). Minor variations make these technologies applicable to a broad range of plant species. Each of these techniques has advantages and disadvantages. In each of the techniques, DNA from a plasmid is genetically engineered such that it contains not only the gene of interest, but also selectable and screenable marker genes. A selectable marker gene is used to select only those cells that have integrated copies of the plasmid (the construction is such that the gene of interest and the selectable and screenable genes are transferred as a unit). The screenable gene provides another check for the successful culturing of only those cells carrying the genes of interest. A commonly used selectable marker gene is neomycin phosphotransferase II (NPT U). This gene conveys resistance to kanamycin, a compound that can be added directly to the growth media on which the cells grow. Plant cells are normally susceptible to kanamycin and, as a result, die. The presence of the NPT U gene overcomes the effects of the kanamycin and each cell with this gene remains viable. Another selectable marker gene which can be employed in the practice of this invention is the gene which confers resistance to the herbicide glufosinate (Basta). A screenable gene commonly used is the β-glucuronidase gene (GUS). The presence of this gene is characterized using a histochemical reaction in which a sample of putatively transformed cells is treated with a GUS assay solution. After an appropriate incubation, the cells containing the GUS gene turn blue. Preferably, the plasmid will contain both selectable and screenable marker genes. The plasmid containing one or more of these genes is introduced into either plant protoplasts or callus cells by any of the previously mentioned techniques. If the marker gene is a selectable gene, only those cells that have incorporated the DNA package survive under selection with the appropriate phytotoxic agent. Once the appropriate cells are identified and propagated, plants are regenerated. Progeny from the transformed plants must be tested to insure that the DNA package has been successfully integrated into the plant genome. Mammalian host cells may also be used in the practice of the invention. Examples of suitable mammalian cell lines include monkey kidney CVI line transformed by SV40 (COS-7, ATCC CRL 1651); human embryonic kidney line 293S (Graham et al., J. Gen. Virol. 36:59 (1977)); baby hamster kidney cells (BHK, ATCC CCL 10); Chinese hamster ovary cells (Urlab and Chasin, Proc. Natl. Acad. Sci USA 77:4216 (1980)); mouse sertoli cells (TM4, Mather, Biol. Reprod. 23:243 (1980)); monkey kidney cells (CVI-76, ATCC CCL 70); African green monkey kidney cells (VERO-76, ATCC CRL-1587); human cervical carcinoma cells (HELA, ATCC CCL 2); canine kidney cells (MDCK, ATCC CCL 34); buffalo rat liver cells (BRL 3A, ATCC CRL 1442); human lung cells (W138, ATCC CCL 75); human liver cells (Hep G2, HB 8065); mouse mammary tumor cells (MMT 060562, ATCC CCL 51); rat hepatoma cells (HTC, MI.54, Baumann et al., 1. Cell Biol. 85:1 (1980)); and TRI cells (Mather et al., Annals NY. Acad. Sci. 383:44 (1982)). Expression vectors for these cells ordinarily include (if necessary) DNA sequences for an origin of replication, a promoter located in front of the gene to be expressed, a ribosome binding site, an RNA splice site, a polyadenylation site, and a transcription terminator site. Promoters used in mammalian expression vectors are often of viral origin. These viral promoters are commonly derived from polyoma virus, Adenovirus 2, and most frequently Simian Virus 40 (SV40). The SV40 virus contains two promoters that are termed the early and late promoters. These promoters are particularly useful because they are both easily obtained from the virus as one DNA fragment that also contains the viral origin of replication (Fiers et al., Nature 273:113 (1978)). Smaller or larger SV40 DNA fragments may also be used, provided they contain the approximately 250-bp sequence extending from the HindHI site toward the Bgll site located in the viral origin of replication. Alternatively, promoters that are naturally associated with the foreign gene (homologous promoters) may be used provided that they are compatible with the host cell line selected for transformation. An origin of replication may be obtained from an exogenous source, such as SV40 or other virus (e.g., Polyoma, Adeno, VSV, BPV) and inserted into the cloning vector. Alternatively, the origin of replication may be provided by the host cell chromosomal replication mechanism. If the vector containing the foreign gene is integrated into the host cell chromosome, the latter is often sufficient. The use of a secondary DNA coding sequence can enhance production levels of pinoresinol/lariciresinol reductase or dirigent protein in transformed cell lines. The secondary coding sequence typically comprises the enzyme dihydrofolate reductase (DHFR). The wild-type form of DHFR is normally inhibited by the chemical methotrexate (MTX). The level of DHFR expression in a cell will vary depending on the amount of MTX added to the cultured host cells. An additional feature of DHFR that makes it particularly useful as a secondary sequence is that it can be used as a selection marker to identify transformed cells. Two forms of DHFR are available for use as secondary sequences, wild-type DHFR and MTX-resistant DHFR. The type of DHFR used in a particular host cell depends on whether the host cell is DHFR deficient (such that it either produces very low levels of DHFR endogenously, or it does not produce functional DHFR at all). DHFR-deficient cell lines such as the CHO cell line described by Urlaub and Chasin, supra, are transformed with wild-type DHFR coding sequences. After transformation, these DHFR-deficient cell lines express functional DHFR and are capable of growing in a culture medium lacking the nutrients hypoxanthine, glycine and thymidine. Nontransformed cells will not survive in this medium. The MTX-resistant form of DHFR can be used as a means of selecting for transformed host cells in those host cells that endogenously produce normal amounts of functional DHFR that is MTX sensitive. The CHO-K1 cell line (ATCC No. CL 61) possesses these characteristics, and is thus a useful cell line for this purpose. The addition of MTX to the cell culture medium will permit only those cells transformed with the DNA encoding the MTX-resistant DHFR to grow. The nontransformed cells will be unable to survive in this medium. Prokaryotes may also be used as host cells for the initial cloning steps of this invention. They are particularly useful for rapid production of large amounts of DNA, for production of single-stranded DNA templates used for site-directed mutagenesis, for screening many mutants simultaneously, and for DNA sequencing of the mutants generated. Suitable prokaryotic host cells include E. coli K12 strain 294 (ATCC No. 31,446), E. coli strain W3110 (ATCC No. 27,325) E. coli X1776 (ATCC No. 31,537), and E. coli B; however many other strains of E. coli, such as HB101, JM101, NM522, NM538, NM539, and many other species and genera of prokaryotes including bacilli such as Bacillus subtilis, other enterobacteriaceae such as Salmonella typhimurium or Serratia marcesans, and various Pseudomonas species may all be used as hosts. Prokaryotic host cells or other host cells with rigid cell walls are preferably transformed using the calcium chloride method as described in section 1.82 of Sambrook et al., supra. Alternatively, electroporation may be used for transformation of these cells. Prokaryote transformation techniques are set forth in Dower, W. J., in Genetic Engineering, Principles and Methods, 12:275-296, Plenum Publishing Corp. (1990); Hanahan et al., Meth. Enxymol, 204:63 (1991). As a representative example, cDNA sequences encoding dirigent protein or pinoresinol/lariciresinol reductase may be transferred to the (His)6»Tag pET vector commercially available (from Novagen) for overexpression in E. coli as heterologous host. This pET expression plasmid has several advantages in high level heterologous expression systems. The desired cDNA insert is ligated in frame to plasmid vector sequences encoding six histidines followed by a highly specific protease recognition site (thrombin) that are joined to the amino terminus codon of the target protein. The histidine "block" of the expressed fusion protein promotes very tight binding to immobilized metal ions and permits rapid purification of the recombinant protein by immobilized metal ion affinity chromatography. The histidine leader sequence is then cleaved at the specific proteolysis site by treatment of the purified protein with thrombin, and the dirigent protein or pinoresinol/lariciresinol reductase eluted. This overexpression-purification system has high capacity, excellent resolving power and is fast, and the chance of a contaminating E. coli protein exhibiting similar binding behavior (before and after thrombin proteolysis) is extremely small. As will be apparent to those skilled in the art, any plasmid vectors containing replicon and control sequences that are derived from species compatible with the host cell may also be used in the practice of the invention. The vector usually has a replication site, marker genes that provide phenotypic selection in transformed cells, one or more promoters, and a polylinker region containing several restriction sites for insertion of foreign DNA. Plasmids typically used for transformation of E. coli include pBR322, pUC18, pUC19, pUCI18, pUC119, and Bluescript M13, all of which are described in Sections 1.12-1.20 of Sambrook et al., supra. However, many other suitable vectors are available as well. These vectors contain genes coding for ampicillin and/or tetracycline resistance which enables cells transformed with these vectors to grow in the presence of these antibiotics. The promoters most commonly used in prokaryotic vectors include the β-lactamase (penicillinase) and lactose promoter systems (Chang et al. Nature 375:615 (1978); Itakura et al., Science 198:1056 (1977); Goeddel et al., Nature 281:544 (1979)) and a tryptophan (trp) promoter system (Goeddel et al., Nucl. Acids Res. 8:4057 (1980); ΕPO Appl. Publ. No. 36,776), and the alkaline phosphatase systems. While these are the most commonly used, other microbial promoters have been utilized, and details concerning their nucleotide sequences have been published, enabling a skilled worker to ligate them functionally into plasmid vectors (see Siebenlist et al., Cell 20:269 (1980)). Many eukaryotic proteins normally secreted from the cell contain an endogenous secretion signal sequence as part of the amino acid sequence. Thus, proteins normally found in the cytoplasm can be targeted for secretion by linking a signal sequence to the protein. This is readily accomplished by ligating DNA encoding a signal sequence to the 5' end of the DNA encoding the protein and then expressing this fusion protein in an appropriate host cell. The DNA encoding the signal sequence may be obtained as a restriction fragment from any gene encoding a protein with a signal sequence. Thus, prokaryotic, yeast, and eukaryotic signal sequences may be used herein, depending on the type of host cell utilized to practice the invention. The DNA and amino acid sequence encoding the signal sequence portion of several eukaryotic genes including, for example, human growth hormone, proinsulin, and proalbumin are known (see Stryer, Biochemistry W.H. Freeman and Company, New York, NY, p. 769 (1988)), and can be used as signal sequences in appropriate eukaryotic host cells. Yeast signal sequences, as for example acid phosphatase (Arima et al., Nucleic Acids Res. 11:1657 (1983)), alpha-factor, alkaline phosphatase and invertase may be used to direct secretion from yeast host cells. Prokaryotic signal sequences from genes encoding, for example, LamB or OmpF (Wong et al., Gene 68:193 (1988)), MalΕ, PhoA, or beta-lactamase, as well as other genes, may be used to target proteins from prokaryotic cells into the culture medium. Trafficking sequences from plants, animals and microbes can be employed in the practice of the invention to direct the gene product to the cytoplasm, endoplasmic reticulum, mitochondria or other cellular components, or to target the protein for export to the medium. These considerations apply to the overexpression of pinoresinol/lariciresinol reductase or dirigent protein, and to direction of expression within cells or intact organisms to permit gene product function in any desired location. The construction of suitable vectors containing DNA encoding replication sequences, regulatory sequences, phenotypic selection genes and the dirigent protein DNA or pinoresinol/lariciresinol reductase DNA of interest are prepared using standard recombinant DNA procedures. Isolated plasmids and DNA fragments are cleaved, tailored, and ligated together in a specific order to generate the desired vectors, as is well known in the art (see, for example, Sambrook et al., supra). As discussed above, pinoresinol/lariciresinol reductase variants, or dirigent protein variants, are preferably produced by means of mutation(s) that are generated using the method of site-specific mutagenesis. This method requires the synthesis and use of specific oligonucleotides that encode both the sequence of the desired mutation and a sufficient number of adjacent nucleotides to allow the oligonucleotide to stably hybridize to the DNA template. A dirigent protein gene and/or pinoresinol/lariciresinol reductase gene, or an antisense nucleic acid fragment complementary to all or part of a dirigent protein gene or pinoresinol/lariciresinol reductase gene, may be introduced, as appropriate, into any plant species for a variety of purposes including, but not limited to: altering or improving the color, texture, durability and pest-resistance of wood tissue, especially heartwood tissue; reducing the formation of lignans and/or lignins in plant species, such as corn, which are useful as animal fodder, thereby enhancing the availability of the cellulose fraction of the plant material to the digestive system of animals ingesting the plant material; reducing the lignan/lignin content of plant species utilized in pulp and paper production, thereby making pulp and paper production easier and cheaper; improving the defensive capability of a plant against predators and pathogens by enhancing the production of defensive lignans or lignins; the alteration of other ecological interactions mediated by lignans or lignins; producing elevated levels of optically-pure lignan enantiomers as medicines or food additives; introducing, enhancing or inhibiting the production of dirigent proteins or pinoresinol/lariciresinol reductases, or the production of pinoresinol or lariciresinol and their derivatives. A dirigent protein and/or pinoresinol/lariciresinol reductase gene may be introduced into any organism for a variety of purposes including, but not limited to: introducing, enhancing or inhibiting the production of dirigent protein and/or pinoresinol/lariciresinol reductase, or the production of pinoresinol or lariciresinol and their derivatives. The foregoing may be more fully understood in connection with the following representative examples, in which "Plasmids" are designated by a lower case p followed by an alphanumeric designation. The starting plasmids used in this invention are either commercially available, publicly available on an unrestricted basis, or can be constructed from such available plasmids using published procedures. In addition, other equivalent plasmids are known in the art and will be apparent to the ordinary artisan. "Digestion", "cutting" or "cleaving" of DNA refers to catalytic cleavage of the DNA with an enzyme that acts only at particular locations in the DNA. These enzymes are called restriction endonucleases, and the site along the DNA sequence where each enzyme cleaves is called a restriction site. The restriction enzymes used in this invention are commercially available and are used according to the instructions supplied by the manufacturers. (See also Sections 1.60-1.61 and Sections 3.38-3.39 of Sambrook et al., supra.) "Recovery" or "isolation" of a given fragment of DNA from a restriction digest means separation of the resulting DNA fragment on a polyacrylamide or an agarose gel by electrophoresis, identification of the fragment of interest by comparison of its mobility versus that of marker DNA fragments of known molecular weight, removal of the gel section containing the desired fragment, and separation of the gel from DNA. This procedure is known generally. For example, see Lawn et al. (Nucleic Acids Res. 9:6103-6114 (1982)), and Goeddel et al. (Nucleic Acids Res., supra). The following examples merely illustrate the best mode now contemplated for practicing the invention, but should not be construed to limit the invention. All literature citations herein are expressly incorporated by reference. EXAMPLE 1 Purification of Dirigent Protein from Forsythia intermedia Plant Materials. Forsythia intermedia plants were either obtained from Bailey's Nursery (var. Lynwood Gold, St., Paul, MN), and maintained in Washington State University greenhouse facilities, or were gifts from the local community. Initial Extraction and Ammonium Sulphate Precipitation. Solubilization of bound proteins was carried out at 4°C Frozen Forsythia intermedia stems (2 kg) were pulverized in a Waring Blendor (Model CB6) in the presence of liquid nitrogen. The resulting powder was homogenized with 0.1 M KH2PO4-K2HPO4 buffer (pH 7.0, 4 liters) containing 5 mM dithiothreitol, and filtered through four layers of cheesecloth. The insoluble residue was consecutively extracted, with continuous agitation at 250 rpm, as follows: with chilled (-20°C) re-distilled acetone (4 liters, 3 x 30 min); 0.1 M KH2PO4-K2HPO4 buffer (pH 6.5) containing 0.1% β-mercaptoethanol (solution A, 8 liters, 30 min); solution A containing 1% Triton X100 (8 liters, 4 hours) and finally solution A (8 liters, 16 hours). Between each extraction, the residue was filtered through one layer of Miracloth (Calbiochem). Solubilization of the (+)-pinoresinol forming system was achieved by mechanically stirring the residue in solution A containing 1 M NaCI (8 liters, 4 hours). The homogenate was decanted and the resulting solution consecutively filtered through Miracloth (Calbiochem) and glass fiber (G6, Fisher Sci.). The filtrate was concentrated in an Amicon cell (Model 2000, YM 30 membrane) to a final volume of -800 ml, and subjected to ( H 4 )2SO4 fractionation. Proteins precipitating between 40 and 80% saturation were recovered by centrifugation (15,000 , 30 min) and the (NH4) 2 SO 4 pellet stored at -20°C until required. Mono S Column Chromatography. Purification of 78-kD dirigent protein and partial purification of oxidase. The ammonium sulfate pellet (obtained from 2 kg of F intermedia stems) was reconstituted in 40 mM MES [2-(N-Morpholino)ethanesulfonic acid] buffer, adjusted to pH 5.0 with 6 M NaOH (solution B, 30 ml), the slurry being centrifuged (3,600g, 5 min), and the supernatant dialyzed overnight against solution B (4 liters). The dialyzed extract was filtered (0.22 μm) and the sample (35 to 40 mg proteins) was applied to a MonoS HR5/5 (50 mm by 5 mm) column equilibrated in solution B at 4°C After eluting (flow rate 5 ml min" 1 cm -2 ) with solution B (13 ml), proteins were desorbed with Na 2 SO4 in solution B, using a linear gradient from 0 to 100 mM in 8 ml and holding at this concentration for 32 ml, then implementing a series of step gradients at 133 mM for 50 ml, 166 mM for 50 ml, 200 mM for 40 ml, 233 mM for 40 ml and finally 333 mM Na2SO for 40 ml. Fractions capable of forming (+)-pinoresinol from E-coniferyl alcohol were eluted with 333 mM Na 2 SO4, combined and stored (-80°C) until needed. POROS SP-M Matrix Column Chromatography (First Column). Fractions from 15 individual elutions from the MonoS HR5/5 column (33mM Na 2 SO ) were combined (18.5 mg proteins, 180 ml) and dialyzed overnight against solution C. The dialyzed enzyme solution (190 ml) was filtered (0.22 μm) and an aliquot (47 ml) was applied to the POROS SP-M column. All separations on a POROS SP-M matrix (100 mm by 4.6 mm), previously equilibrated in 25 mM MΕS-HΕPΕS-sodium acetate buffer (pH 5.0, solution C), were performed at a flow rate of 60 ml min -1 cnr 2 and at room temperature. After elution with solution C (12 ml), the proteins were desorbed with a linear Na2SO4 gradient (0 to 0.7 M in 66.5 ml) in solution C, whereupon the concentration established was held for an additional 16.6 ml. Under these conditions, separation of four fractions (I, π, HI and IV) was achieved at -40, 47, 55 and 61 mS, respectively. This purification step was repeated three times with the remaining dialyzed enzymatic extract, and fractions I, TJ, HI, and TV from each experiment were separately combined. When protease inhibitors [that is, phenylmethanesulfonyl fluoride (0.1 mmol ml" 1 ), ΕDTA (0.5 nmol ml" 1 ), pepstatin A (1 μgmH), and antipain (1 μg ml" 1 )] were .added during the solubilization and all subsequent purification stages, no differences were observed in the elution profiles of fractions I, π, HI, and IV. POROS SP-M Matrix Column Chromatography (Second Column). Fraction I from the first POROS SP-M Matrix column chromatography step (2.62 mg proteins, 40 ml, -24.6 mS) was diluted in filtered, cold distilled water until the conductivity reached -8 mS (final volume = 150 ml). The diluted protein solution was then applied onto a POROS SP-M column (100 mm by 4.6 mm). After elution with solution C (12 ml), fraction I was desorbed using a linear Na2SO4 gradient from 0 to 0.25 M in 20 ml, whereupon the concentration established was held for another 25 ml. This was followed by another linear Na 2 SO4 gradient from 0.25 to 0.7 M in 26 ml which was then held at 0.7 M for an additional 16.6 ml. Fractions eluted at -30 mS (the ionic strength of the eluent was measured with a flow-through detector) were combined (15 ml, 1.3 mg), diluted with water and rechromatographed. The resulting protein (eluted at -30 mS with the gradient described above) was stored (-80°C) until needed. Gel filtration. An aliquot from fraction I (595.5 μg proteins, 3 ml, eluted at -30 mS), was concentrated to 0.6 ml (Centricon 10, Amicon) and loaded onto a S200 (73.2 cm by 1.6 cm, Pharmacia-LKB) gel chromatographic column equilibrated in 0.1 M MES- HEPES-sodium acetate buffer (pH 5.0) containing 50 mM Na 2 SO 4 at 4°C An apparently homogenous 78-kD dirigent protein (242 μg) was eluted (flow rate 0.25 ml 1 9 min " cm " ) as a single component at 133 ml (Vo = 105 ml). Molecular weights were estimated by comparison of their elution profiles with the standard proteins, β-amylase (200,000), alcohol dehydrogenase (150,000), bovine serum albumin (66,000), ovalbumin (45,000), carbonic anhydrase (29,000) and cytochrome c (12,400). EXAMPLE 2 Characterization of the Purified Dirigent Protein Molecular Weight and Isoelectric Point Determination. Polyacrylamide gel electrophoresis (PAGE) was performed in Laemmli's buffer system with gradient (4 to 15% acrylamide, Bio-Rad) gels under denaturing and reducing conditions. Proteins were visualized by silver staining. Gel filtration (S200) chromatography of fraction I gave a protein of native molecular weight -78 kD, whereas SDS-polyacrylamide gel electrophoresis showed a single band at -27 kD, suggesting that the native protein exists as a trimer. Isoelectric focusing of the native protein on a polyacrylamide gel (pH 3 to 10 gradient) revealed the presence of six bands. After isoelectric focusing, each of these bands was electroblotted onto a polyvinylidene fluoride (PVDF) membrane and subjected to amino terminal sequencing, which established that all had similar sequences indicating a series of isoforms. The ultraviolet-visible spectrum of the protein had only a characteristic protein absorbance at 280 nm with a barely perceptible shoulder at -330 nm. Inductively coupled plasma (ICP) analysis gave no indication of any metal being present in the protein. Thus, the 78-kD dirigent protein lacks any detectable catalytically active oxidative center. Assay of the Ability of the Purified Dirigent Protein to Form (+)Pinoresinol from E-Coniferyl alcohol. The four fractions (I to IV) from the first POROS SP-M chromatographic step (Example 1) were individually rechromatographed, with each fraction subsequently assayed for (+)-pinoresinol-forming activity with E-[9- 3 H] coniferyl alcohol as substrate for one hour. Fraction I (containing dirigent protein) had very little 01/49833 -Zo- (+)-pinoresinol-forming activity (<5% of total activity loaded onto the POROS SP-M column), whereas fraction JH catalyzed nonspecific oxidative coupling to give (±)-dehydrodiconiferyl alcohols, (±)-pinoresinols, and (±)-erythro/threo guaiacylglycerol 8-0-4'-coniferyl alcohol ethers. Thus, Fraction HI appeared to contain an endogenous plant oxygenating protein. Although the putative oxidase preparation (Fraction JU) was not purified to electrophoretic homogeneity, the electron paramagnetic resonance (EPR) spectrum of this protein preparation resembled that of a typical plant laccase, i.e., a class of naturally- occurring plant oxygenase proteins. We then studied the fate of E-[9- 3 H]coniferyl alcohol (2 μ ol ml" 1 , 14.7 kBq) in the presence of, respectively, the oxidase (fraction JU), the 78-kD dirigent protein (Fraction I), and both fraction JU and the 78-kD protein together. With the fraction HE preparation alone, only nonspecific bimolecular radical coupling occurs to give (±)-dehydrodiconiferyl alcohols, (±)-pinoresinols and (±)-erythro/threo guaiacylglycerol 8-O-4'-coniferyl alcohol ethers. With the 78-kD protein by itself, however, a small amount of (+)-pinoresinol formation (<5% over 10 hours) was observed, this being presumed to result from residual traces of oxidizing capacity in the preparation. When both fraction JU and the 78-kD protein were combined, full catalytic activity and regio- and stereo-specificity in the product was reestablished, whereby essentially only (+)-pinoresinol was formed. Additionally, with fraction JU alone, and when fraction JU was combined with the 78-kD protein, the rates of substrate depletion and dimeric product formation were nearly identical. Moreover, essentially no turnover of the dimeric lignan products occurred in either case in the presence of the oxidase, over the time-period (8 hours) examined: subsequent dimer oxidation does not occur when E-coniferyl alcohol, the preferred substrate, is still present in the assay mixture. The 78-kD protein therefore appears to determine the specificity of the bimolecular phenoxy radical coupling reaction. Gel filtration studies were also carried out with mixtures of the dirigent and fraction HI proteins, in order to establish if any detectable protein-protein interaction might account for the stereoselectivity. But no evidence in support of complex formation (i.e., to higher molecular size entities) was observed. EXAMPLE 3 Effect of the 78-KD Dirigent Protein on Plant Laccase-Catalyzed Monolignol Coupling E-coniferyl alcohol coupling assay. E-[9- 3 H] Coniferyl alcohol (4 μmol ml" 1 , 29.3 kBq) was incubated with a 120-kD laccase (previously purified from Forsythia intermedia stem tissue) over a 24-hour period, in the presence and absence of the dirigent protein, as follows. Each assay consisted of E-[9- 3 H]coniferyl alcohol (4 μmol ml" 1 , 29.3 kBq, 7.3 MBq mole liter 1 ; or 2 μmol ml- 1 , 14.7 kBq with fraction IH), the 78-kD dirigent protein, an oxidase or oxidant, or both [final concentrations: 770 pmol ml" 1 dirigent protein; 10.7 pmol protein ml" 1 Forsythia laccase; 12 μg protein ml" 1 fraction HI; 0.5 μmol ml" 1 FMN; 0.5 μmol ml" 1 FAD; 1 and 10 μmol ml" 1 ammonium peroxydisulfate] in buffer (0.1 M MES-HEPES-sodium acetate, pH 5.0) to a total volume of 250 μl. The enzymatic reaction was initiated by addition of E-[9- 3 H]coniferyl alcohol. Controls were performed in the presence of buffer alone. After one hour incubation at 30 °C while shaking, the assay mixture was extracted with ethyl acetate (ΕtOAc, 500 μl) containing (±)-pinoresinols (7.5 μg), (±)-dehydrodiconiferyl alcohols (3.5 μg) and erythro/threo (±)-guaiacylglycerol 8-0-4'- coniferyl alcohol ethers (7.5 μg) as radiochemical carriers and ferulic acid (15.0 μg) as an internal standard. After centrifugation (13,800g, 5 min), the ΕtOAc soluble components were removed and the extraction procedure repeated with ΕtOAc (500 μl). The ΕtOAc soluble components from each assay were combined, the solutions evaporated to dryness in vacuo, redissolved in methanol-water solution (1:1; 100 μl) with an aliquot (50 μl) thereof subjected to reversed-phase column chromatography (Waters, Nova-Pak C^g, 150 mm by 3.8 mm). The elution conditions were as follows: acetonitrile/3 % acetic acid in H 2 O (5:95) from 0 to 5 min, then linear gradients to ratios of 10:90 between 5 and 20 min, then to 20:80 between 20 and 45 min and finally to 50:50 between 45 and 60 min, at a flow rate of 8.8 ml min -1 cm -2 . Fractions corresponding to E-coniferyl alcohol, erythro/threo (±)-guaiacylglycerol 8-O-4' -coniferyl alcohol ethers, (±)-dehydrodiconiferyl alcohols and (±)-pinoresinols were individually collected, aliquots removed for liquid scintillation counting, and the remainder freeze-dried. Pinoresinol-containing fractions were redissolved in methanol (100 μl) and subjected to chiral column chromatography (Daicel, Chiralcel OD, 50 mm by 4.6 mm) with a solution of hexanes and ethanol (1:1) as the mobile phase (flow rate 3 ml min -1 cm -2 ), whereas dehydrodiconiferyl alcohol fractions were subjected to Chiralcel OF (250 mm by 4.6 mm) column chromatography eluted with a solution of hexanes and isopropanol (9:1) as the mobile phase (flow rate 2.4 ml min" 1 cm" 2 ), the radioactivity of the eluent being measured with a flow-through detector (Radiomatic, Model A120). Results of E-coniferyl alcohol coupling assay. Incubation with laccase alone gave only racemic dimeric products, with (±)-dehydrodiconiferyl alcohols predominating. In the presence of the dirigent protein, however, the process was now primarily stereoselective, affording (+)-pinoresinol, rather than being nonspecific as observed when only laccase was present. The rates of both E-coniferyl alcohol (substrate) depletion and the formation of the dimeric lignans were similar with and without the dirigent protein. A substantial difference was noted in the subsequent turnover of the lignan products observed after E-coniferyl alcohol depletion. With the laccase alone no turnover occurred, but when both proteins were present the disappearance of the products was significant. In order to understand the difference, assays were conducted where bovine serum albumin (BSA) and ovalbumin were individually added to the laccase-containing solutions at levels matching the weight concentrations of the dirigent protein. In this way, it was established that the differences in product turnover were simply due to stabilization of laccase activity at the higher protein concentrations, although interestingly the dirigent protein, BSA and ovalbumin afforded somewhat different degrees of protection. The findings were quite comparable when a fungal laccase (from Trametes versicolor) was used in place of the plant laccase. When the oxidizing capacity (i.e., laccase concentration) was lowered five-fold, only (+)-pinoresinol formation was observed. Thus, complete stereoselectivity is preserved when the oxidative capacity does not exceed a point where the dirigent protein is saturated. Stereoselective E-coniferyl alcohol coupling. Assays were also conducted with E-[9- 2 H2, OC 2 H3J " coniferyl alcohol and the dirigent protein in the presence of laccase as follows. E-[9- 2 H 2 , OC 2 H3]coniferyl alcohol (2 μmol ml" 1 ) was incubated in the presence of dirigent protein (770 pmol ml" 1 ), the purified plant laccase (4.1 pmol ml" 1 ) and buffer (0.1 M MΕS-HΕPΕS-sodium acetate, pH 5.0) in a total volume of 250 μl. After one hour incubation, the reaction mixture was extracted with ΕtOAc, but with the addition of an internal standard and radiochemical carriers omitted. After reversed-phase column chromatography, the enzymatically formed pinoresinol was collected, freeze-dried, redissolved in methanol (100 μl) and subjected to chiral column chromatography (Daicel, Chiralcel OD, 50 mm by 4.6 mm) with detection at 280 nm and analysis by mass spectral fragmentation in the ΕI mode (Waters, Integrity System). Liquid chromatography-mass spectrometry (LC-MS) analysis of the resulting (+)-pinoresinol (>99% enantiomeric excess) gave a molecular ion with a mass to charge ratio (m/z) 368, thus establishing the presence of 10 2 H atoms and verifying that together the laccase and dirigent protein catalyzed stereoselective coupling of E-[9- 2 H2, OC 2 H3J " coniferyl alcohol. Other auxiliary one-electron oxidants can also facilitate stereoselective coupling with the dirigent protein. Ammonium peroxydisulfate readily undergoes homolytic cleavage (A. Usaitis, R. Makuska, Polymer 35:4896 (1994)) and is routinely used as an one-electron oxidant in acrylamide polymerization. Ammonium peroxydisulfate was first incubated with E-[9- 3 H]coniferyl alcohol (4 μmol ml -1 , 29.3 kBq) for 6 hours using the E-coniferyl alcohol coupling assay procedure described above. Nonspecific bimolecular radical coupling was observed, to afford predominantly (±)-dehydrodiconiferyl alcohols as well as the other racemic lignans (Table 1). However, when the dirigent protein was added, the stereoselectivity of coupling was dramatically altered to give primarily (+)-pinoresinol at both concentrations of oxidant, together with small amounts of racemic lignans. This established that even an inorganic oxidant, such as ammonium peroxydisulfate, could promote (+)-pinoresinol synthesis in the presence of the dirigent protein, even if it was not oxidatively as selective toward the monolignol as was the fraction HI oxidase or laccase. Table 1. Effect of dirigent protein on product distribution from E-coniferyl oxidized by ammonium peroxydisulfate (6 hour assay). £-Coniferyl alcohol (±)-Guaiacyl-glycerol (±)-Dehydro- in dimer 8-0-4'-coniferyl diconiferyl Dirigent protein equivalents alcohol ethers alcohols (±)-Pin Oxidant (770 pmol ml "1 ) depleted (nmol ml" 1 ) (nmol ml" 1 ) s (nm (nmol ml" 1 ) Ammonium absent 200 ±4 10 ±1 35 + 2 1 peroxydisulfate (1 μmol ml" 1 ) present 250 ± 55 6±0 13 ±1 Ammonium absent 860 ± 30 90 ±4 250 ± 10 13 peroxydisulfate (10 μmol ml "1 ) present 1030 ±25 30 ±1 90 ±3 Dirigent present 61 ±20 5±1 8±1 protein Effect of Other Oxygenating Agents on the Stereospecific Conversion of E- Coniferyl Alcohol to (+)-pinoresinol. The effects of incubating E-coniferyl alcohol (4 μmol ml" 1 , 29.3 kBq) with flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) were investigated since, in addition to their roles as enzyme cofactors, they can also oxidize various organic substrates (T.C Bruice, Ace. Chem. Res. 13:256 (1980)). E-[9- 3 H]coniferyl alcohol was respectively incubated with FMN and FAD for 48 hours. To obtain the FMN, snake (Naja naja atra, Formosan cobra) venom was added to a solution of FAD (5 μmol ml" 1 in H2O) and, after 30 min incubation at 30°C, the enzymatically formed FMN was separated from the protein mixture by filtration through a Centricon 10 (Amicon) microconcentrator. In every instance, E-coniferyl alcohol oxidation was more rapid in the presence of FMN than FAD. Although these differences between the FMN and FAD catalyzed rates of E-coniferyl alcohol oxidation were not anticipated, a consistent pattern was sustained: racemic lignan products were obtained, with the (±)-dehydrodiconiferyl alcohols predominating as before. When the time courses were repeated in the presence of the dirigent protein, a dramatic change in stereoselectivity was observed, where essentially only (+)-pinoresinol formation occurred. Again, the rates of E-coniferyl alcohol depletion, when adjusted for the traces of residual oxidizing capacity (<5% over 10 hours) in the dirigent protein preparation, were dependent only upon [FMN] and [FAD], as were the total amounts of dimers formed. When full depletion of E-coniferyl alcohol occurs, the corresponding lignan dimers can begin to undergo oxidative changes as a function of time; specifically, FMN is able subsequently to oxidize pinoresinol, in open solution, after the E-coniferyl alcohol has been fully depleted. Investigation of Substrate-Specific Stereoselectivity. The coupling stereoselectivity was substrate specific. Neither E-p-[9- 3 H]coumaryl (4 μmol ml" 1 , 44.5 kBq) or E-[8- 14 C]sinapyl alcohols (4 μmol ml" 1 , 8.3 kBq), which differ from E-coniferyl alcohol only by a methoxyl group substituent on the aromatic ring, yielded stereoselective products when incubated for 6 hours with FMN and ammonium peroxydisulfate respectively, in the presence and absence of the dirigent protein. Incubations were carried out as described above with the following modifications: E-/?-[9- 3 H]coumaryl (4 μmol ml" 1 , 44.5 kBq) or E-[8- 14 C]sinapyl alcohols (4 μmol ml" 1 , 8.3 kBq) were used as substrates and, after 6 hour incubation at 30°C, the reaction mixture was extracted with ΕtOAc but without addition of radiochemical carriers. E- Sinapyl alcohol readily underwent coupling to afford syringaresinol, but chiral HPLC analysis revealed that the resulting products were, in every instance, racemic (Table 2). Interestingly, by itself, the 78-kD dirigent protein preparation catalyzed a low level of dimer formation, as previously noted, but only gave rise to racemic (±)-syringaresinol formation, which is presumably a consequence of the residual traces of contaminating oxidizing capacity present in the protein preparation. In an analogous manner, no stereoselective coupling was observed with E-p- coumaryl alcohol as substrate. That is, only E-coniferyl alcohol undergoes stereoselective coupling in the presence of the dirigent protein. Given the marked substrate specificity of the dirigent protein for E-coniferyl alcohol, it will be of considerable interest to determine, in the future, how it differs from that affording (+)-syringaresinol in Eucommia ulmoides (T. Deyama, Chem. Pharm. Bull. 31, 2993 (1983)). Table 2. Effect of dirigent protein on coupling of E-sinapyl alcohol (6 hour assay). E-Sinapyl alcohol in dimer equivalents Racemic Dirigent protein depleted (±)-syringaresinols (770 pmol ml" 1 ) (nmol ml" 1 ) (nmol ml" 1 ) FMN absent 570 ± 100 290 ± 40 (0.5 μmol ml" 1 ) present 610 ± 110 340 ± 40 Ammonium absent 1400 ± 120 1020 ± 40 peroxydisulfate (10 μmol ml" 1 ) present 1520 ± 10 1060 ± 30 Dirigent protein present 110 ± 10 50 ± 10 Although the inventors do not intend to be bound by any particular mechanism for stereoselective coupling, three distinct possibilities can be envisaged. The most likely is that the oxidase or oxidant generates free-radical species from E-coniferyl alcohol, and that the latter are the true substrates that bind to the dirigent protein prior to coupling. The other two possibilities would require that E-coniferyl alcohol molecules are bound and oriented on the dirigent protein, thereby ensuring that only (+)-pinoresinol formation occurs upon subsequent oxidative coupling: this could occur either if both substrate phenolic hydroxyl groups were exposed so that they could readily be oxidized by an oxidase or oxidant, or if an electron transfer mechanism were operative between the oxidase or oxidant and an electron acceptor site or sites on the dirigent protein. Among the three alternative mechanisms, three lines of evidence suggest "capture" of phenoxy radical intermediates by the dirigent protein. First, the rates of both substrate depletion and product formation are largely unaffected by the presence of the dirigent protein. If capture of the free-radical intermediates is the operative mechanism, the dirigent protein would only affect the specificity of coupling when single-electron oxidation of coniferyl alcohol is rate-determining. Second, an electron transfer mechanism is currently ruled out, since we observed no new ultraviolet-visible chromophores in either the presence or absence of an auxiliary oxidase or oxidant, under oxidizing conditions. Third, preliminary kinetic data (as disclosed in Example 4) support the concept of free-radical capture based on the formal values of Michaelis constant (K m ) and maximum velocity (V max ) characterizing the conversion of E-coniferyl alcohol into (+)-pinoresinol, with the dirigent protein alone and in the presence of the various oxidases or oxidants. EXAMPLE 4 Kinetic Characterization of the Conversion of E-Coniferyl Alcohol to (+ -pinoresinol in the Presence of Dirigent Protein and an Oxygenating Agent. Assays were carried out as described in Example 3 by incubating a series of E-[9- 3 H] coniferyl alcohol concentrations (between 8.00 and 0.13 μmol ml -1 , 7.3 MBq mole liter 1 ) with dirigent protein (770 pmol ml" 1 ) alone and in presence of Forsythia laccase (2.1 pmol ml" 1 ), fraction HI (12 μg protein ml" 1 ), or FMN (0.5 μmol ml -1 ). Assays with dirigent protein, in presence or absence of FMN, were incubated at 30°C for 1 hour, whereas assays with Forsythia laccase or fraction HI in presence or absence of dirigent protein were incubated at 30 °C for 15 min. If free-radical capture by the dirigent protein is the operative mechanism, the Michaelis-Menten parameters obtained will only represent formal rather than true values, because the highest free-energy intermediate state during the conversion of E-coniferyl alcohol into (+)-pinoresinol is still unknown and the relation between the concentration of substrate and that of the corresponding intermediate free-radical in open solution has not been delineated. Bearing these qualifications in mind, we estimated formal K m and V max values for the dirigent protein preparation. As noted earlier, it was capable of engendering formation of low levels of both (+)-pinoresinol from E-coniferyl alcohol, and racemic (±)-syringaresinols from E-sinapyl alcohol, because of traces of contaminating oxidizing capacity. With this preparation (Table 3), a formal K m of 10 ± 6 mM and V max of 0.02 ± 0.02 mol s" 1 mol" 1 were obtained. However, with addition of fraction HI, laccase, and FMN, the formal K m values (mM) were reduced to 1.6 ± 0.3, 0.100 ± 0.003, and 0.10 ± 0.01, respectively, whereas the V max values were far less affected at these concentrations of auxiliary oxidase/oxidant. Formal K m and V max values were calculated for the laccase and fraction TH oxidase with respect to E-coniferyl alcohol conversion into the three racemic lignans. However, no direct comparisons can be made to the 78-kD protein, since the formal K m values involve only the corresponding oxidases. For completeness, the K m (mM) and V max (mol s" 1 mok 1 enzyme) were as follows: with respect to the laccase, 0.200 ± 0.001 and 3.9 ± 0.2 for (±)-erythro/threo guaiacylglycerol 8-O-4' -coniferyl alcohol ethers, 0.3000 ± 0.0003 and 13.1 ± 0.6 for (±)-dehydrodiconiferyl alcohols, and 0.300 ± 0.002 and 7.54 ± 0.50 for (±)-pinoresinols; with respect to the fraction TH oxidase (estimated to have a native molecular weight of 80 kDa), 2.2 ± 0.3 and 0.20 ± 0.03 for (±)-erythro/threo guaiacylglycerol 8-0-4'- coniferyl alcohol ethers, 2.2 ± 0.2 and 0.7 ± 0.1 for (±)-dehydrodiconiferyl alcohols, and 3.7 ± 0.7 and 0.6 ± 0.1 for (±)-pinoresinols. These preliminary kinetic parameters are in harmony with the finding that dirigent protein does not substantially affect the rate of E-coniferyl alcohol depletion in the presence of fraction TH, laccase and FMN. Both sets of results are together in accord with the working hypothesis that the dirigent protein functions by capturing free-radical intermediates which then undergo stereoselective coupling. Table 3. Effect of various oxidants on formal K m and V max values for the dirigent protein (770 pmol ml -1 ) during (+)-pinoresinol formation from E-coniferyl alcohol. v v max (mol s _1 mol -1 Oxidase/Oxidant Formal K m (mM) dirigent protein) Dirigent protein 10 ± 6 0.02 ± 0.02 Fraction TH (12 μg protein ml" 1 ) 1.6 ± 0.3 0.10 ± 0.03 Laccase (2.07 pmol ml" 1 ) 0.100 ± 0.003 0.0600 ± 0.0002 FMN (0.5 μmol ml" 1 ) 0.10 ± 0.01 0.024 ± 0.001 EXAMPLE 5 Cloning of the Dirigent Protein cDNA From Forsythia intermedia Plant Materials - Forsythia intermedia plants were either obtained from Bailey's Nursery (var. Lynwood Gold, St., Paul, MN), and maintained in Washington State University greenhouse facilities, or were gifts from the local community. Materials - All solvents and chemicals used were reagent or HPLC grade. Taq thermostable DNA polymerase was obtained from Promega, whereas restriction enzymes were from Gibco BRL (H eHJ), Boehringer Mannheim (S w3a) and Promega (Taqϊ). pT7Blue T-vector and competent NovaBlue cells were purchased from Novagen and radiolabeled nucleotide ([or 32 P]dCTP) was from DuPont NEN. Oligonucleotide primers for polymerase chain reaction (PCR) and sequencing were synthesized by Gibco BRL Life Technologies. GENECLEAN H® kits (BIO 101 Inc.) were used for purification of PCR fragments, with the gel-purified DNA concentrations determined by comparison to a low DNA mass ladder (Gibco BRL) in 1.5% agarose gels. Instrumentation - UV (including RNA and DNA determinations at OD 2 6o) spectra were recorded on a Lambda 6 UV/VIS spectrophotometer. A Temptronic H thermocycler (Thermolyne) was used for all PCR amplifications. Purification of DNA for sequencing employed a QIAwell Plus plasmid purification system (QIAGEN) followed by PEG precipitation (Sambrook, J., Fritsch, E. F., and Maniatis, T. (1994) Molecular Cloning: A Laboratory Manual, 3 volumes, 3rd Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, NY), with DNA sequences determined using an Applied Biosystems Model 373A automated sequencer. Amino acid sequences were obtained using an Applied Biosystems protein sequencer with on-line HPLC detection, according to the manufacturer's instructions. Dirigent Protein Amino Acid Sequencing - The dirigent protein N-terminal amino acid sequence (SEQ ID NO:l) was obtained from the purified protein using an Applied Biosystems protein sequencer with on-line HPLC detection. For trypsin digestion, the purified enzyme (150 pmol) was suspended in 0.1 M Tris-HCl (50 μl, pH 8.5, Boehringer Mannheim, sequencing grade), with urea added to give a final concentration of 8 M in 77.5 μl. The mixture was incubated for 15 min at 50°C, following which 100 mM iodoacetamide (2.5 μl) was added, with the whole kept at room temperature for 15 min. Trypsin (1 μg in 20 μl) was then added, with the mixture digested for 24h at 37°C, following which TFA (4 μl) was added to stop the enzymatic reaction. The resulting mixture was subjected to reversed phase HPLC analysis (C-8 column, Applied Biosytems), this being eluted with a linear gradient over 2 h from 0 to 100% acetonitrile (in 0.1% TFA) at a flow rate of 0.2 ml/min with detection at 280 nm. Fractions containing individual oligopeptide peaks were collected manually and directly submitted to amino acid sequencing (SEQ ID Nos:2-7). Forsythia intermedia stem cDNA Library Synthesis - Total RNA (-300 μg/g fresh weight) was obtained (Dong, Z.D., and Dunstan, D.I. (1996) Plant Cell Reports 15:516-521) from young green stems of greenhouse-grown Forsythia intermedia plants (var. Lynwood Gold). A Forsythia intermedia stem cDNA library was constructed using 5 μg of purified poly A+ mRNA (Oligotex-dT™ Suspension, QIAGEN) with the ZAP- cDNA® synthesis kit, the Uni-ZAP™ XR vector and the Gigapack® H Gold packaging extract (Stratagene), with a titer of 1.2 x 10 6 PFU for the primary library. A portion (30 ml) of the amplified library (1.2 x 10 10 PFU/rnl; 158 ml total) (Sambrook, J. et al., supra) was used to obtain pure cDNA library DNA (Ausubel, F.M., Brent, R., Kingston, R.E., Moore, D.D., Seidnam, J.G., Smith, J.A., and Struhl, K. (1991) Current Protocols in Molecular Biology, 2 volumes, Greene Publishing Associates and Wiley-Interscience, John Wiley & Sons, NY) for PCR. Dirigent Protein DNA Probe Synthesis - The N-terminal and internal peptide amino acid sequences were used to construct the degenerate oligonucleotide primers. Purified E. intermedia cDNA library DNA (5 ng) was used as the template in 100 μl PCR reactions (10 mM Tris-HCl [pH 9.0], 50 mM KC1, 0.1% Triton X-100, 2.5 mM MgCl 2 , 0.2 mM each dNTP and 2.5 units Taq DNA polymerase) with primer PSJ Tl (SΕQ ID NO:8) (100 pmol) and either primer PSI7R (SΕQ ID NO: 11) (20 pmol), primer PSI2R (SΕQ ID NO:10) (20 pmol) or primer PSI1R (SΕQ ID NO:9) (20 pmol). PCR amplification was carried out in a thermocycler as follows: 35 cycles of 1 min at 94°C, 2 min at 50°C and 3 min at 72°C; with 5 min at 72°C and an indefinite hold at 4°C after the final cycle. Single-primer, template-only and primer-only reactions were performed as controls. PCR products were resolved in 1.5% agarose gels, where a single band (-370-, -155- or ~125-bp, respectively) was observed for each reaction. To determine the nucleotide sequence of the amplified bands, five 100 μl PCR reactions were performed as above with PSINT1 (SΕQ H) NO:8) +PSI7R (SEQ ID NO: 11), PSINTl (SEQ ID NO:8) +PSI2R (SEQ ID NO: 10) and PSINT1 (SEQ TD NO:8) +PSI1R (SEQ TD NO:9) primer pairs. The 5 reactions from each primer pair were concentrated (Microcon 30, Amicon Inc.) and washed with TE buffer (10 mM Tris-HCl, pH 8.0, 1 mM EDTA; 2 x 200 μl), with the PCR products subsequently recovered in TE buffer (2 x 50 μl). These were resolved in preparative 1.5% agarose gels. , Each gel- purified PCR product (-0.2 pmol) was then ligated into the pT7Blue T-vector and transformed into competent NovaBlue cells, according to Novagen's instructions. Insert sizes were determined using the rapid boiling lysis and PCR technique (with R20mer and U19mer primers) according to the manufacturer's instructions. Restriction analyses were performed to determine whether all inserts from the reactions utilizing each of the foregoing primer pairs were the same, as follows: to 20 μl each of a 100 μl PCR reaction (insert of interest amplified with R20mer(SEQ ID NO:74) and U19mer(SEQ TD NO:75) primers) were added 4 units HaeTE, 1.5 units Sau3a or 5 units Tαqϊ restriction enzyme. Restriction digestions were allowed to proceed for 60 min at 37°C for HαeTR and Sαu3A and at 65°C for Tαql reactions. Restriction products were resolved in 1.5% agarose gels giving one restriction group for each insert tested. Five recombinant plasmids from PSINTl (SEQ ID NO: 8) +PSI7R (SEQ ID NO: 11) (called pT7PSIl-pT7PSI5) and 2 recombinant plasmids from PSINTl (SEQ TD NO: 8) +PSI2R (SEQ TD NO: 10) (called pT7PSI6 and pT7PSI7) PCR products were selected for DNA sequencing; all contained the same open reading frame (ORF) (SEQ TD NO:69). The dirigent protein probe was next constructed as follows: five 100 μl PCR reactions were performed as above with 10 ng pT7PSIl DNA (SEQ ID NO:69) with primers PSINTl (SEQ H NO:8) and PSI7R (SEQ TD NO: 11). Gel-purified pT7PSIl insert (50 ng) was used with Pharmacia's T7 QuickPrime® kit and [α- 32 P]dCTP, according to kit instructions, to produce a radiolabeled probe (in 0.1 ml), which was purified over BioSpin 6 columns (Bio-Rad) and added to carrier DNA (0.5 mg ml sheared salmon sperm DNA [Sigma], 0.9 ml). Library Screening - 600,000 PFU of F. intermedia amplified cDNA library were plated for primary screening, according to Stratagene's instructions. Plaques were blotted onto Magna Nylon membrane circles (Micron Separations Inc.), which were then allowed to air dry. The membranes were placed between two layers of Whatman® 3MM Chr paper. cDNA library phage DNA was fixed to the membranes and denatured in one step by autoclaving for 2 min at 100°C with fast exhaust. The membranes were washed for 30 min at 37°C in 6X standard saline citrate (SSC) and 0.1% SDS and prehybridized for 5 h with gentle shaking at 57-58°C in preheated 6X SSC, 0.5% SDS and 5X Denhardt's reagent (hybridization solution, 300 ml) in a crystallization dish (190 x 75 mm). The [ 32 P]radiolabeled probe was denatured (boiling, 10 min), quickly cooled (ice, 15 min) and added to a preheated fresh hybridization solution (60 ml, 58°C) in a crystallization dish (150 x 75 mm). The prehybridized membranes were next added to this dish, which was then covered with plastic wrap. Hybridization was performed for 18 h at 57-58°C with gentle shaking. The membranes were washed in 4X SSC and 0.5% SDS for 5 min at room temperature, transferred to 2X SSC and 0.5% SDS (at room temperature) and incubated at 57-58°C for 20 min with gentle shaking, wrapped with plastic wrap to prevent drying and finally exposed to Kodak X-OMAT AR film for 24 h at -80°C with intensifying screens. Twenty positive plaques were purified through two more rounds of screening with hybridization conditions as above. In vivo Excision and Sequencing of Dirigent Protein cDNA-containing Phagemids - Purified cDNA clones were rescued from the phage following Stratagene's in vivo excision protocol. Both strands of several different cDNAs that coded for dirigent protein were completely sequenced using overlapping sequencing primers. Two distinct cDNAs were identified, called pPSD_Fil(SEQ ID NO:12) and pPSD_Fi2(SEQ TD NO: 14). Sequence Analysis - DNA and amino acid sequence analyses were performed using the Unix-based GCG Wisconsin Package (Program Manual for the Wisconsin Package, Version 8, September 1994, Genetics Computer Group, 575 Science Drive, Madison, Wisconsin, USA 53711; Rice, P. (1996) Program Manual for the EGCG Package, Peter Rice, The Sanger Centre, Hinxton Hall, Cambridge, CB10 lRq, England) and the ExPASy World Wide Web molecular biology server (Geneva University Hospital and University of Geneva, Geneva, Switzerland). EXAMPLE 6 Expression of Functional Dirigent Protein in Spodoptera frugiperda Attempts to express functional dirigent protein in Escherichia coli failed. Consequently, we expressed the dirigent protein in Spodoptera frugiperda utilizing a baculovirus expression system. The full-length 1.2 kb cDNA clone for the dirigent protein (PSD) in E. intermedia, containing both the 5' and 3' untranslated regions, was excised from the pBlueScript (Stratagene) derived plasmid pPSD_Fil (SΕQ ID NO: 12) using the restriction endonucleases BamH I and Xho I. This 1.2 kb fragment was directionally subcloned into these same restriction sites in the multiple cloning site of the baculovirus transfer vector pBlueBac4 (Invitrogen, San Diego, CA). This produced the 6.0 kb construct pBB4/PSD which generates a non-fusion dirigent protein with translation being initiated at the dirigent protein cDNA start codon. This construct was then co-transfected with linearized Bac-N-Blue DNA (Invitrogen) into Spodoptera frugiperda Sf9 cells by the technique of cationic liposome mediated transfection to produce, by means of homologous recombination, the recombinant Autographa californica nuclear polyhedrosis viral (AcMNPV) DNA Bac-N-Blue dirigent protein (BB/PSD) which was purified from plaques according to procedures described by Invitrogen. The final recombinant AcMNPV-BB/PSD contains the PSD gene under the polyhedrin promoter control and the essential sequence needed for replication of the recombinant virus. To verify that the dirigent protein was successfully expressed in the insect cell culture, log phase Sf9 cells infected with the AcMNPV-PSD recombinant viral high titer stock were used to obtain heterologous protein production. Maximal dirigent protein yield occurred by 48-70 hours post-infection. As determined by SDS-PAGΕ and (+)-pinoresinol forming activity, the protein was found secreted into the medium and showed a molecular mass and activity which corresponded to the indigenous protein originally isolated from Forsythia intermedia. EXAMPLE 7 Isolation of Dirigent Protein Clones from Thuja plicata and Tsuga heterophylla The coding region of a Forsythia dirigent protein cDNA, psd-Fil (SEQ TD NO: 12), was used to screen cDNA libraries from Thuja plicata and Tsuga heterophylla. The conditions and methods were as disclosed in Example 5, except that hybridization was carried out at 45-50°C Two dirigent protein cDNAs were isolated from Tsuga heterophylla (SEQ ID Nos:16, 18), and eight dirigent protein cDNAs were isolated from Thuja plicata (SEQ ID Nos:20, 22, 24, 26, 28, 30, 32, 34). EXAMPLE 8 Purification of Pinoresinol/lariciresinol Reductases from Forsythia Intermedia Plant Materials. Forsythia intermedia plants were either obtained from Bailey's Nursery (var. Lynwood Gold, St., Paul, MN), and maintained in Washington State University greenhouse facilities, or were gifts from the local community. Materials. All solvents and chemicals used were reagent or HPLC grade. Unlabeled (±)-pinoresinols and (±)-lariciresinols were synthesized as described (Katayama, T. et al., Phytochemistry 32:581-591 (1993)). [4R-3H]NADPH was obtained as previously reported (Chu, A. et al., /. Biol. Chem. 268:27026-27033 (1993)) by modification of the procedure of Moran et al. (Moran, R.G. et al., Anal. Biochem. 138:196-204 (1984)), and [4R-2H]NADPH was prepared according to Anderson and Lin (Anderson, J.A., and Lin B.K., Phytochemistry 32:811-812 (1993)). Yeast glucose-6- phosphate dehydrogenase (Type IX,22.32. mmol h '1 mg "1 ) and yeast hexokinase (Type F300, 15.12 mmol "1 mg "1 ) were purchased from Sigma and dihydrofolate reductase (Lactobacillus casei, 33.48 mmol h "1 mg "1 ) was obtained from Biopure Co. Affi-Gel Blue Gel (100-200 mesh) and Bio-Gel HT Hydroxyapatite were purchased from Bio-Rad, whereas Phenyl Sepharose CL-4B, MonoQ HR 5/5, MonoP HR 5/20, Superose 6, Superose 12, Superdex 75, PD-10 columns, molecular weight standards and Polybuffer 74 were obtained from Pharmacia LKB Biotechnology, Inc. Adenosine 2',5'-diphosphate Sepharose and Reactive Yellow 3 Agarose were from Sigma Chemical Co. Instrumentation. 1H Nuclear magnetic resonance spectra (300 and 500 MHz) were recorded on Briiker AMX300 and Varian VXR500S spectrometers, respectively, using CDC1 3 as solvent with chemical shifts (δ ppm) reported downfield from tetramethylsilane (internal standard). UV (including RNA and DNA determinations at OD 26 o) and mass spectra were obtained on Lambda 6 UV/VIS and VG 7070E (ionizing voltage 70 eV) spectrophotometers, respectively. High performance liquid chromatography was carried out using either reversed-phase (Waters, Nova-pak C18, 150 x 3.9 mm inner diameter) or chiral (Daicel, Chiralcel OD or Chiralcel OC, 240 x 4.6 mm inner diameter) columns, with detection at 280 nm (Chu, A. et al., J. Biol. Chem. 268:27026-27033 (1993)). Radioactive samples were analyzed in Ecolume (ICN) and measured using a liquid scintillation counter (Packard, Tricarb 2000 CA). Amino acid sequences were obtained using an Applied Biosystems protein sequencer with on-line HPLC detection, according to the manufacturer's instructions. Enzyme Assays. Pinoresinol and lariciresinol reductase activities were assayed by monitoring the formation of (+)-[ H] lariciresinol and (-)-[ H] secoisolariciresinol (Chu, A. et al., J. Biol. Chem. 268:27026-27033 (1993)). Briefly, each assay for pinoresinol reductase activity consisted of (±)-pinoresinols (5 mM in MeOH, 20 μl), the enzyme preparation at the corresponding stage of purity (100 μl), and buffer (20 mM Tris-HCl, pH 8.0, 110 μl). The enzymatic reaction was initiated by addition of [4R- 3 H]NADPH (10 mM, 6.79 kBq/mmol in 20 μl of double- distilled H2O). After 30 min incubation at 30°C with shaking, the assay mixture was extracted with EtOAc (500 μl) containing (±)-lariciresinols (20 μg) and (±)-secoisolariciresinols (20 μg) as radiochemical carriers. After centrifugation (13,800 x g, 5 min), the EtOAc solubles were removed and the extraction procedure was repeated. For each assay, the EtOAc solubles were combined with an aliquot (100 μl) removed for determination of its radioactivity using liquid scintillation counting. The remainder of the combined EtOAc solubles was evaporated to dryness in vacuo, reconstituted in MeOH/3% acetic acid in H2O (30:70, 100 μl) and subjected to reversed phase and chiral column HPLC. Controls were performed using either denatured enzyme (boiled for 10 min) or in the absence of (±)-pinoresinols as substrate. Lariciresinol reductase activity was assayed by monitoring the formation of (-)-[ H]secoisolariciresinol. These assays were carried out exactly as described above, except that (±)-lariciresinols (5 mM in MeOH, 20 μl) were used as substrates, with (±)-secoisolariciresinols (20 μg) added as radiochemical carriers. General Procedures for Enzyme Purification. Protein purification procedures were carried out at 4°C with chromatographic eluents monitored at 280 nm, unless otherwise stated. Protein concentrations were determined by the method of Bradford (Bradford, M.M., Anal. Biochem. 72:248-254 (1976)) using γ-globulin as standard. Polyacrylamide gel electrophoresis used gradient (4-15%, Bio-Rad) gels under denaturing and reducing conditions, these being performed in Laemmli's buffer system (Laemmli, U.K., Nature 227:680-685 (1970)). Proteins were visualized by silver staining (Morrissey, J.H., Anal Biochem. 117:307-310 (1981)). Preparation of crude extracts. F. intermedia stems (20 kg) were harvested, cut into 3-6 cm sections, and stored at -20°C until needed. Batches of stems (2 kg) were frozen in liquid nitrogen and pulverized in a Waring Blendor. The resulting powder was homogenized with potassium phosphate buffer (0.1 mM, pH 7.0, 4 L), containing 5 mM dithiothreitol. The homogenate was filtered through four layers of cheesecloth into a beaker containing 10% (w/v) polyvinylpolypyrolidone. The filtrate was centrifuged (12,000 x g, 15 min). The resulting supernatant was fractionated with with proteins precipitating between 40 and 60% saturation recovered by centrifugation (10,000 x g, 1 h). The pellet was next reconstituted in a minimum amount of Tris-HCl buffer (20 mM, pH 8.0), containing 5 mM dithiothreitol (buffer A) and desalted using prepacked PD-10 columns (Sephadex G-25 medium) equilibrated with buffer A. Affinity (Affi Blue Gel) Chromatography. The crude enzyme preparation (191 mg in buffer A, 5 nmol h "1 mg "1 ) was applied to an Affi Blue Gel column (2.6 x 70 cm) equilibrated in buffer A. After washing the column with 200 ml of buffer A, pinoresinol/lariciresinol reductase was eluted with a linear NaCI gradient (1.5-5 M in 300 ml) in buffer A at a flow rate of 1 ml min "1 . Active fractions were stored (-80°C) until needed. Hydrophobic Interaction Chromatography (Phenyl Sepharose). After thawing, ten preparations resulting from the Affi Blue chromatography step (150 mg, 51 nmol h "1 mg "1 ) were combined and applied to a Phenyl Sepharose column (1 x 10 cm) equilibrated in buffer A, containing 5 M NaCI. The column was washed with two bed volumes of the same buffer. Pinoresinol/lariciresinol reductase was eluted using a linear gradient of decreasing concentration of NaCI (5-0 M in 40 ml) in buffer A at a flow rate of 1 ml min "1 . Fractions catalyzing pinoresinol/lariciresinol reduction were combined and pooled. Hydroxyapatite I Chromatography. Active protein (31 mg, 91 nmol h "1 mg "1 ) from the phenyl sepharose purification step was applied to an hydroxyapatite column (1.6 x 70 cm) equilibrated in 10 mM potassium phosphate buffer, pH 7.0, containing 5 mM dithiothreitol (buffer B). Pinoresinol/lariciresinol reductase was eluted with a linear gradient of potassium phosphate buffer, pH 7.0 (0.01-0.4 M in 200 ml) at a flow rate of 1 ml min "1 . Active fractions were combined. The buffer was then exchanged with buffer A using PD-10 prepacked columns. Affinity (2',5'-ADP Sepharose) Chromatography. The enzyme solution resulting from the hydroxyapatite purification step (6.5 mg, 463 nmol h "1 mg "1 ) was next loaded on a 2',5'-ADP Sepharose (1 x 10 cm) column, previously equilibrated in buffer A containing 2.5 mM EDTA (buffer A') and then washed with 25 ml of buffer A'. Pinoresinol/lariciresinol reductase was eluted with a step gradient of NADP+ (0.3 mM in 10 ml) in buffer A' at a flow rate of 0.5 ml min "1 . [NAD+ (up to 3 mM) did not elute pinoresinol/lariciresinol reductase activity.] Because of the interference of the absorbance of the NADP+, it was not possible to directly monitor the eluent at 280 nm. Protein concentrations for each fraction were determined spectrophotometrically according to Bradford (Bradford, M.M., Anal. Biochem. 72:248-254 (1976)). Hydroxyapatite II Chromatography. Fractions from the 2',5'-ADP Sepharose column that exhibited pinoresinol/lariciresinol reductase activity (0.85 mg, 1051 nmol h "1 mg "1 ) were combined and directly applied to a second hydroxyapatite column (1 x 3 cm), equilibrated in buffer B, with the enzyme eluted with a linear gradient of potassium phosphate buffer, pH 7.0 (0.01-0.4 M in 45 ml) at a flow rate of 1 ml min "1 . Affinity (Affi Yellow) Chromatography - Active fractions (160 μg, 7960 nmol h "1 mg "1 ) from the second hydroxyapatite column purification step were next applied to a Reactive Yellow 3 Agarose column (1 x 3 cm), equilibrated in buffer A. Pinoresinol/- lariciresinol reductase was eluted with a linear NaCI gradient (0-2.5 M in 100 ml) at a flow rate of 1 ml min "1 . Fast Protein Liquid Chromatography (Superose 12 Chromatography) - Combined fractions from the Affi Yellow purification step having the highest activity (50 μg, 10,940 nmol h "1 mg "1 ) were pooled and concentrated to 1 ml, using a Centricon 10 microconcentrator (Amicon, Inc.). The enzyme solution was then applied in portions of 200 μl to a fast protein liquid chromatography column (Superose 12, HR 10/30). Gel filtration was performed in a buffer containing 20 mM Tris-HCl, pH 8.0, 150 mM NaCI and 5 mM dithiothreitol at a flow rate of 0.4 ml min "1 . Pinoresinol/lariciresinol reductase was eluted with 12.8 ml of the mobile phase. The active fractions which coincided with the UV profile (absorbance at 280 nm) were pooled (20 μg, 15,300 nmol h "1 mg "1 ) and desalted (PD-10 prepacked columns). The foregoing purification protocol resulted in a 3060-fold purification of (+)-pinoresinol/(+)-lariciresinol reductase. As for many of the enzymes involved in phenylpropanoid metabolism, the protein was in very low abundance, i.e. 20 kg F. intermedia stems yielded only -20 μg of the purified (+)-pinoresinol/(+)-lariciresinol reductase. EXAMPLE 9 Characterization of Purified Pinoresinol/lariciresinol Reductases from Forsythia Intermedia Isoelectric Focussing and pi Determination. In all stages of the purification protocol, (+)-pinoresinol/(+)-lariciresinol reductase activities coeluted. Given this observation, it was essential to unambiguously ascertain whether more than one form of the protein existed, i.e., whether one form of the protein catalyzed the reduction of pinoresinol, and another form of the protein catalyzed the reduction of lariciresinol. To this end, the isoelectric point of pinoresinol/lariciresinol reductase was estimated by chromatofocussing on a MonoP HR 5/20 FPLC column. Active fractions from the Superose 12 gel filtration column (Example 1) were pooled and the buffer exchanged with 25 mM Bis-Tris, pH 7.1, using prepacked PD-10 columns, equilibrated in the same buffer. The preparation so obtained was loaded on the chromatofocussing column and a pH gradient between 7.1 and 3.9 was formed, using Polybuffer 74 as eluent at a flow rate of 0.5 ml min "1 . Aliquots (200 μl) of each fraction were assayed for pinoresinol/lariciresinol reductase activities. The remainder of the fractions was used to determine the pH gradient. Molecular Weight Determination. Application of the MonoP HR 5/20 FPLC column preparation of pinoresinol/lariciresinol reductase to SDS-gradient gel electrophoresis (4-15% polyacrylamide) revealed the presence of two protein bands of similar apparent molecular weight, whose separation was achieved via anion-exchange chromatography on a MonoQ HR 5/5 FPLC matrix. Pooled fractions from the Sepharose 12 purification step (Example 1) were applied to a MonoQ HR 5/5 column (Pharmacia), equilibrated in buffer A. The column was washed with 10 ml of buffer A and pinoresinol/lariciresinol reductase activity eluted using a linear NaCI gradient (0-500 mM in 50 ml) in buffer A at a flow rate of 0.5 ml min "1 . Aliquots (30 μl) of the collected fractions were analyzed by SDS polyacrylamide gel electrophoresis, using a gradient (4-15% acrylamide) gel. Proteins were visualized by silver staining. Active fractions 34 through 37 (27,760 nmol h "1 mg "1 ) and 38 through 41 (30,790 nmol h "1 mg '1 ) were pooled separately and immediately used for characterization. The two protein bands thus resolved under denaturing conditions had apparent molecular masses of -36 and -35 kDa, respectively. Each of the two reductase forms had a pl~5.7. Native molecular weights of each reductase isoform were estimated via comparison of their elution behavior on Superose 12, Superose 6 and Superdex 75 gel filtration FPLC columns with the elution behavior of calibrated molecular weight standards. Gel filtration was carried out as set forth in Example 8. For each reductase, an apparent native molecular weight of 59,000 was calculated based on its elution volume, in contrast to that of -36,000 and -35,000 by SDS-polyacrylamide gel electrophoresis. While the discrepancy between molecular weights from gel filtration and SDS-PAGE remains unknown, it can tentatively be proposed that although the native protein likely exists as a dimer, it could also be a monomer of asymmetric shape, thereby altering its effective Stokes radius (Cantor, C.R., and Shimmel, P.R., Biophysical Chemistry, Part H, W.H. Freeman and Company, San Francisco, CA (1980); Stellwagen, E., Methods in Enzymology 182:317-328 (1990)), as reported for human thioredoxin reductase (Oblong, J.E., et al., Biochemistry 32:7271-7277 (1993)) and yeast metalloendopeptidase (Hrycyna, C.A., and Clarke, S., Biochemistry 32:11293-11301 (1993)). pH and Temperature Optima. To determine the pH-optimum of pinoresinol/lariciresinol reductase, the enzyme preparation from the gel Superose 12 filtration step (Example 8) was assayed utilizing standard assay conditions (Example 8), except that the buffer was replaced with 50 mM Bis-Tris Propane buffer in the pH range of 6.3 to 9.4. The pH optimum was found to be pH 7.4. The temperature optimum of pinoresinol/lariciresinol reductase was examined in the range between 4°C and 80°C under standard assay conditions (Example 8) utilizing the enzyme preparation from the gel filtration step (Example 8). At optimum pH, the temperature optimum for the reductase activity was established to be ~30°C. Kinetic Parameters. Velocity studies were carried out to ascertain whether the two reductase isoforms catalyzed distinct reductions, i.e., that of the conversion of (+)-pinoresinol to (+)-lariciresinol, and (+)-lariciresinol to (-)-secoisolariciresinol, respectively, or whether either displayed a preference for (+)-pinoresinol or (+)-lariciresinol as substrates. The initial velocity studies were carried out individually utilizing the two isoforms of the enzyme, and individually employing both (+)-pinoresinol and (+)-lariciresinol as substrates. Initial velocity studies were performed in triplicate experiments, using 50 mM Bis-Tris Propane buffer, pH 7.4 containing 5 mM dithiothreitol, pure enzyme (after MonoQ anion-exchange chromatography), ten different substrate concentrations (between 8.8 and 160 μM) at a constant NADPH concentration (80 μM). Incubations were carried out at 30 °C for 10 min (within the linear kinetic range). Kinetic parameters were determined from Lineweaver-Burk plots. Importantly, the kinetic parameters were essentially the same for both the 35 kDa and the 36 kDa forms of the enzyme (i.e., Km for pinoresinol: 27±1.5μm for the 35 kDa form of the enzyme, and 23±1.3μM for the 36 kDa form of the enzyme; Km for lariciresinol: 121±5.0μM for the 35 kDa form of the enzyme and 123±6.0μM for the 36 kDa form of the enzyme). In an analogous manner, apparent maximum velocities (expressed as μmol h "1 mg "1 of protein) were also essentially identical (i.e., Vmax for pinoresinol: 16.2±0.4 for the 35 kDa form of the enzyme and 17.3±0.5 for the 36 kDa form of the enzyme; for lariciresinol: 25.2±0.7 for the 35 kDa form of the enzyme and 29.9±0.7 for the 36 kDa form of the enzyme). Thus, all available evidence suggests that (+)-pinoresinol/(+)-lariciresinol reductase exists as two isoforms, with each capable of catalyzing the reduction of both substrates. How this reduction is carried out, i.e., whether both reductions are done in tandem, in either quinone or furano ring form, awaits further study using a more abundant protein source. Enzymatic Formation of (+)-[7'R- H] Lariciresinol. Since the two (+)-pinoresinol/(+)-lariciresinol reductase isoforms exhibited essentially identical catalytic characteristics, the Sepharose 12 enzyme preparation (Example 8), containing both isoforms, was used to examine the stereospecificity of the hydride transfer. The strategy adopted utilized selective deuterium labeling using NADP 2 H as cofactor for the reduction of (+)-pinoresinol, with the enzymatic product, (+)-lariciresinol, being analyzed by 1H NMR and mass spectroscopy. Thus, a solution of (±)-pinoresinols (5.2 mM in MeOH, 4 ml) was added to Tris-HCl buffer (20 mM, pH 8.0, containing 5 mM dithiothreitol, 22 ml) and stereospecifically deutero-labeled [4R- 2 H]NADPH (20 mM in H2O, 4 ml) prepared via the method of Anderson and Lin (Anderson, J.A., and Lin B.K., Phytochemistry 32:811-812 (1993)), with the whole added to the enzyme preparation (20 ml). After incubation at 30°C for lh with shaking, the assay mixture was extracted with EtOAc (2 x 50 ml). The EtOAc soluble fraction was combined, washed with saturated NaCI (50 ml), dried (Na2SO_ι), and evaporated to dryness in vacuo. The resulting extract was reconstituted in a minimum amount of EtOAc, applied to a silica gel column (0.5 x 7 cm), and eluted with EtOAc/hexanes (1:2). Fractions containing the enzymatic product were combined and evaporated to dryness. The enzymatic product was established to be (+)-[7'R- 2 H]lariciresinol, as evidenced by the disappearance of the 7'-proR proton at δ 2.51 ppm due to its replacement by deuterium and by its molecular ion at (m/z) 361 (M++1) corresponding to the presence of one deuterium atom at C-7. 1H NMR (300 MHz) (CDC1 3 ): 2.39 (m,1H, C8H), 2.71 (m,1H, C8'H), 2.88 (δ,1H, J7'S,8'=5.0 Hz, C7'HS), 3.73 (δδ,1H, J8',9'b=7.0 Hz, J9'a,9'b=8.5 Hz, C9'HB), 3.76 (δδ,1H, J8,9S=6.5 Hz, J9R,9S=8.5 Hz, C9HS), 3.86 (s,1H, OCH 3 ), 3.88 (s, 3 H, OCH 3 ), 3.92 (δδ,1H, J8,9R=6.0 Hz, J9R,9S=9.5 Hz, C9HR), 4.04 (δδ,1H, J8',9'a=7.0 Hz, J9'a9'b=8.5 Hz, C9'Ha), 4.77 (δ,1H, J7,8=6.6 Hz, C7H), 6.68 - 6.70 (m, 2 H, ArH), 6.75 - 6.85 (m,4H, ArH); MS m/z (%) : 361 (M++1, 71.2), 360 (M+, 31.1), 237 (11.1), 153 (41.5), 152 (20.2), 151 (67.0), 138 (100), 137 (71.1). Thus, hydride transfer from (+)-pinoresinol to (+)-lariciresinol had occuned in a manner whereby only the 7'-proR hydrogen position of (+)-lariciresinol was deuterated. An analogous result was observed for the conversion of (+)-lariciresinol into (-)-secoisolariciresinol, thereby establishing that the overall hydride transfer was completely stereospecific. EXAMPLE 10 Amino Acid Sequence Analysis of Purified Pinoresinol/Lariciresinol Reductase from Forsythia intermedia Pinoresinol/Lariciresinol Reductase Amino Acid Sequencing. The (+)-pinoresinol/(+)-lariciresinol reductase N-terminal amino acid sequence was obtained from each of the purified proteins, and a mixture of both, using an Applied Biosystems protein sequencer with on-line HPLC detection. The N-terminal sequence was the same for both isoforms (SEQ ID NO:36). For trypsin digestion, 150 pmol of the enzyme purified from the Sepharose 12 column (Example 8) was suspended in 0.1 M Tris-HCl (50 μl, pH 8.5), with urea added to give a final concentration of 8 M in 77.5 μl. The mixture was incubated for 15 min at 50°C, then 100 mM iodoacetamide (2.5 μl) was added, with the whole kept at room temperature for 15 min. Trypsin (1 μg in 20 μl) was then added, with the mixture digested for 24 h at 37°C, after which TFA (4 μl) was added to stop the enzymatic reaction. The resulting mixture was subjected to reversed phase HPLC analysis (C-8 column, Applied Biosytems), this being eluted with a linear gradient over 2 h from 0 to 100% acetonitrile (in 0.1% TFA) at a flow rate of 0.2 ml/min with detection at 280 nm. Fractions containing individual oligopeptide peaks were collected manually and directly submitted to amino acid sequencing. Four tryptic fragments were resolved in sufficient quantity to permit amino acid sequence determination. (SEQ ID Nos:37-40). Cyanogen bromide digestion was performed by incubation of 150 pmol of the reductase purified from the Sepharose 12 column (Example 8) with 0.5 M cyanogen bromide in 70% formic acid for 40 h at 37°C, following which the cyanogen bromide and formic acid were removed by centrifugation under reduced pressure (SpeedVac). The resulting oligopeptide fragments were separated by HPLC and three were resolved in sufficient quantity to permit sequencing (SEQ ID Nos:41-43). EXAMPLE 11 Cloning of Pinoresinol/Lariciresinol Reductase from Forsythia intermedia Plant Materials. Forsythia intermedia plants were either obtained from Bailey's Nursery (var. Lynwood Gold, St., Paul, MN), and maintained in Washington State University greenhouse facilities, or were gifts from the local community. Materials. All solvents and chemicals used were reagent or HPLC grade. UV RNA and DNA determinations at OD 26 o were obtained on a Lambda 6 UV/VIS spectrophotometer. A Temptronic H thermocycler (Thermolyne) was used for all PCR amplifications. Taq thermostable DNA polymerase was obtained from Promega, whereas restriction enzymes were from Gibco BRL (HaeJH), Boehringer Mannheim (Sau3a) and Promega (Taql). pT7Blue T-vector and competent NovaBlue cells were purchased from Novagen and radiolabeled nucleotides ([α- 32 P]dCTP and [γ- 32 P]ATP) were from DuPont NEN. Oligonucleotide primers for polymerase chain reaction (PCR) and sequencing were synthesized by Gibco BRL Life Technologies. GENECLEAN H® kits (BIO 101 Inc.) were used for purification of PCR fragments, with the gel-purified DNA concentrations determined by comparison to a low DNA mass ladder (Gibco BRL) in 1.5% agarose gels. Forsythia RNA Isolation. Initial attempts to isolate functional F. intermedia RNA from fast-growing, green stem tissue were unsuccessful, due to difficulties encountered via facile oxidation by its plant phenolic constituents. This problem was, however, successfully overcome by utilization of an RNA isolation procedure, specifically designed for woody plant tissue, which uses low pH and reducing conditions in the extraction buffer to prevent oxidation (Dong, Z.D., and Dunstan, D.I., Plant Cell Reports 15: 516-521(1996)). Forsythia intermedia stem cDNA Library Synthesis. Total RNA (-300 μg/g fresh weight) was obtained from young green stems of greenhouse-grown Forsythia intermedia plants (var. Lynwood Gold) (Dong, Z.D., and Dunstan, D.I., Plant Cell Reports 15:516-521 (1996)). A Forsythia intermedia stem cDNA library was constructed using 5 μg of purified poly A+ mRNA (Oligotex-dT™ Suspension, QIAGEN) with the ZAP- cDNA® synthesis kit, the Uni-ZAP™ XR vector and the Gigapack® H Gold packaging extract (Stratagene), with a titer of 1.2xl0 6 PFU for the primary library. A portion (30 ml) of the amplified library (1.2xl0 10 PFU/ml; 158 ml total) was used to obtain pure cDNA library DNA for PCR (Sambrook, J. et al., Molecular Cloning: A Laboratory Manual, 3 volumes, 3rd Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, NY (1994); Ausubel, F.M. et al., Current Protocols in Molecular Biology, 2 volumes, Greene Publishing Associates and Wiley-Interscience, John Wiley & Sons, NY (1991)). Pinoresinol/Lariciresinol Reductase DNA Probe Synthesis - The N-terminal and internal peptide amino acid sequences were used to construct the degenerate oligonucleotide primers. Specifically, the primer PLRN5 (SEQ ID NO:44) was based on the sequence of amino acids 7 to 13 of the N-terminal peptide (SEQ ID NO:36). The primer PLR14R (SEQ TD NO:45) was based on the sequence of amino acids 2 to 8 of the internal peptide sequence set forth in (SEQ ID NO:37). The primer PLR15R (SEQ ID NO:46) was based on the sequence of amino acids 9 to 15 of the internal peptide sequence set forth in (SEQ ID NO:37). The sequence of amino acids 9 to 15 of the internal peptide sequence set forth in SEQ ID NO:37, upon which the sequence of primer PLR15R (SEQ ID NO:46) was based, also corresponded to the sequence of amino acids 4 to 10 of the cyanogen bromide-generated, internal fragment set forth in SEQ ID NO:41. Purified F. intermedia cDNA library DNA (5 ng) was used as the template in 100 μl PCR reactions (10 mM Tris-HCl [pH 9.0], 50 mM KC1, 0.1% Triton X-100, 2.5 mM MgCl 2 , 0.2 mM each dNTP and 2.5 units Taq DNA polymerase) with primer PLRN5 (SEQ ID NO:44) (100 pmol) and either primer PLRI5R (SEQ JD NO:46) (20 pmol) or primer PLRI4R (SEQ ID NO:45) (20 pmol). PCR amplification was carried out in a thermocycler as follows: 35 cycles of 1 min at 94°C, 2 min at 50°C and 3 min at 72°C; with 5 min at 72°C and an indefinite hold at 4°C after the final cycle. Single-primer, template-only and primer-only reactions were performed as controls. PCR products were resolved in 1.5% agarose gels. The combination of primers PLRN5 (SEQ ID NO:44) and PLRI4R (SEQ ID NO:45) yielded a single band of 380-bp corresponding to bases 22 to 393 of SEQ JD NO:47. The combination of primers PLRN5 (SEQ ID NO:44) and PLRI5R (SEQ JD NO:46) yielded a single band of 400-bp corresponding to bases 22 to 423 of SEQ ID NO:47. To determine the nucleotide sequence of the two amplified bands, five, 100 μl PCR reactions were performed as above with each of the following combinations of template and primers: 380 bp amplified product plus primers PLRN5 (SEQ TD NO:44) and PLRI4R (SEQ ID NO:45); 400 bp amplified product plus primers PLRN5 (SEQ ID NO:44) and PLRI5R (SEQ TD NO:46). The 5 reactions from each combination of primers and template were concentrated (Microcon 30, Amicon Inc.) and washed with TE buffer (10 mM Tris-HCl, pH 8.0, 1 mM EDTA; 2 x 200 μl), with the PCR products subsequently recovered in TE buffer (2 x 50 μl). These were resolved in preparative 1.5% agarose gels. Each gel-purified PCR product (-0.2 pmol) was then ligated into the pT7Blue T-vector and transformed into competent NovaBlue cells, according to Novagen's instructions. Insert sizes were determined using the rapid boiling lysis and PCR technique (utilizing R20mer (SEQ ID NO:74) and U19mer (SEQ TD NO:75) primers according to the manufacturer's (Novagen's) instructions. Restriction analysis was performed to determine whether all inserts for each combination of primers and template were the same. Restriction analysis was carried out as follows: each of the inserts was amplified by PCR utilizing the R20 (SEQ ID NO:74) and U19 (SEQ TD NO:75) primers. To 20 μl each of a 100 μl PCR reaction were added 4 units HaelH, 1.5 units Sau3a or 5 units Taql restriction enzyme. Restriction digestions were allowed to proceed for 60 min at 37°C for HaelH and Sau3A and at 65°C for Taql reactions. Restriction products were resolved in 1.5% agarose gels giving one restriction group for all inserts tested. Five of the resulting, recombinant plasmids were selected for DNA sequencing. The inserts from three of the recombinant plasmids (called pT7PLRl-pT7PLR3) were generated by a combination of primers PLRN5 (SEQ ID NO:44) and PLRI5R (SEQ TD NO:46) with the 400 bp PCR product as substrate. The inserts from the remaining two recombinant plasmids (called pT7PLR4 and pT7PLR5) were generated from a combination of primers PLRN5 (SEQ JD NO:44) and PLRI4R (SEQ ID NO:45) and the 380 bp PCR product as substrate. All of the five, sequenced PCR products contained the same open reading frame. The (+)-pinoresinol/(+)-lariciresinol reductase probe was constructed as follows: five, 100 μl PCR reactions were performed as described above with 10 ng pT7PLR3 DNA with primers PLRN5 (SEQ JD NO:44) and PLRI5R (SEQ HO NO:46). Gel-purified pT7PLR3 cDNA insert (50 ng) was used with Pharmacia's T7QuickPrime® kit and [α- 32 P]dCTP, according to kit instructions, to produce a radiolabeled probe (in 0.1 ml), which was purified over BioSpin 6 columns (Bio-Rad) and added to carrier DNA (0.9 ml of 0.5 mg/ml sheared salmon sperm DNA obtained from Sigma). Library Screening. 600,000 PFU of E. intermedia amplified cDNA library were plated for primary screening, according to Stratagene's instructions. Plaques were blotted onto Magna Nylon membrane circles (Micron Separations Inc.), which were then allowed to air dry. The membranes were placed between two layers of Whatman® 3MM Chr paper. cDNA library phage DNA was fixed to the membranes and denatured in one step by autoclaving for 2 min at 100°C with fast exhaust. The membranes were washed for 30 min at 37°C in 6X standard saline citrate (SSC) and 0.1% SDS and prehybridized for 5 h with gentle shaking at 57-58°C in preheated 6X SSC, 0.5% SDS and 5X Denhardt's reagent (hybridization solution, 300 ml) in a crystallization dish (190x75 mm). The [ 32 P]radiolabeled probe was denatured (boiling, 10 min), quickly cooled (ice, 15 min) and added to a preheated fresh hybridization solution (60 ml, 58°C) in a crystallization dish (150x75 mm). The prehybridized membranes were next added to this dish, which was then covered with plastic wrap. Hybridization was performed for 18 h at 57-58°C with gentle shaking. The membranes were washed in 4X SSC and 0.5% SDS for 5 min at room temperature, transfened to 2X SSC and 0.5% SDS (at room temperature) and incubated at 57-58 °C for 20 min with gentle shaking, wrapped with plastic wrap to prevent drying and finally exposed to Kodak X-OMAT AR film for 24 h at -80°C with intensifying screens. This screening procedure resulted in more than 350 positive plaques, with twenty (of different signal intensities) being subjected to two additional rounds of screening. After final purification, six of the twenty cDNAs were subcloned by in vivo excision into pBluescript. These six cDNAs were called plr-Fil to ρlr-Fi6 (SEQ ID Nos:47, 49, 51, 53, 55, 57). In vivo Excision and Sequencing of plr-Fil -plr-Fi6 Phagemids. The six purified cDNA clones were rescued from the phage following Stratagene's in vivo excision protocol. Both strands of the six different cDNAs (plr-Fil to plr-Fi6) that coded for (+)-pinoresinol/ (+)-lariciresinol reductase were completely sequenced using overlapping sequencing primers. Purification of DNA for sequencing employed a QIAwell Plus plasmid purification system (QIAGEN) followed by PEG precipitation (Sambrook, J., Molecular Cloning: A Laboratory Manual, 3 volumes, 3rd Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, NY (1994)), with DNA sequences determined using an Applied Biosystems Model 373 A automated sequencer. DNA and amino acid sequence analyses were performed using the Unix-based GCG Wisconsin Package (Program Manual for the Wisconsin Package, Version 8, September 1994, Genetics Computer Group, 575 Science Drive, Madison, Wisconsin, USA 53711; Rice, P., Program Manual for the EGCG Package, Peter Rice, The Sanger Centre, Hinxton Hall, Cambridge, CB10 lRq, England (1996)) and the ExPASy World Wide Web molecular biology server (Geneva University Hospital and University of Geneva, Geneva, Switzerland). All six cDNAs had the same coding but different 5 '-untranslated regions. On the other hand, analysis of the 3 '-untranslated region of each of the six cDNAs established that all were truncated versions of the longest cDNA's 3'-region. Preliminary RNA gel blot analysis with total RNA from greenhouse-grown plant stem tips confirmed a single transcript with a length of approximately 1.2 kb. RNA gel blot analysis. For RNA gel blot analysis, total RNA (30 μg per lane) from F. intermedia stem tips was separated by size by denaturing agarose gel electrophoresis. The RNA was transferred to charged nylon membranes (GeneScreen Plus®, Dupont NEN), cross-linked to the membrane (Stratalinker from Stratagene), prehybridized, hybridized with the same probe used to screen the cDNA library during cDNA cloning and washed according to the manufacturer's instructions for aqueous hybridization conditions. The membrane was then exposed to Kodak X-OMAT film for 48 hr at -80°C with intensifying screens. EXAMPLE 12 Expression of (+)-Pinoresinol/(+)-Lariciresinol Reductase cDNA plr-Fil (SEQ ID NO:47) in E.coli Expression in Escherichia coli. In order to confirm that the putative (+)-pinoresinol/(+)-lariciresinol reductase cDNAs encoded functional (+)-pinoresinol/(+)- lariciresinol reductase, the cDNAs putatively encoding (+)-pinoresinol/(+)-lariciresinol reductase were heterologously expressed in E. coli. Heterologous expression was also necessary in order to obtain sufficient protein to enable the systematic study of the precise biochemical mechanism of (+)-pinoresinol/(+)-lariciresinol reductase at a future date. Examination of the six putative (+)-pinoresinol/(+)-lariciresinol reductase clones revealed that one, plr-Fil (SEQ TD NO:47), was in frame with the -complementation particle of β-galactosidase in pBluescript. This was fortuitous, since it potentially provided a facile means to express the fully functional fusion protein, and hence to provide proof that the cloned sequence was correct. Purified plasmid DNA from plr-Fil (SEQ ID NO:47) was transformed into NovaBlue cells according to Novagen's instructions. Transformed cells (5 ml cultures) were grown at 37°C with shaking (225 rpm) to mid log phase (OD 60 o=0.5) in LB medium (Sambrook, J., Molecular Cloning: A Laboratory Manual, 3 volumes, 3rd Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, NY (1994)) supplemented with 12.5 μg ml- 1 tetracycline and 50 μg ml "1 ampicillin. TPTG (isopropyl β-D-thioglucopyranoside) was then added to a final concentration of 10 mM, and the cells were allowed to grow for 2 h. Cells were collected by centrifugation and resuspended in 500 μl (per 5 ml culture tube) buffer (20 mM Tris-HCl, pH 8.0, 5 mM dithiothreitol). Lysozyme (5 μl of O.l mg ml "1 , Research Organics, Inc.) was next added and following incubation for 10 min, the cells were lysed by sonication (3 x 15 s). After centrifugation at 14,000 x g at 4°C for 10 min, the supernatant was removed and assayed for (+)-pinoresinol/(+)-lariciresinol reductase activity (210 μl supernatant per assay) as described in Example 8. Catalytic activity was established by incubating cell-free extracts for 2 h at 30°C with (±)-pinoresinols (0.4 mM) and [4R- 3 H]NADPH (0.8 mM) under standard conditions. Following incubation, unlabeled (±)-lariciresinols and (±)-secoiso- lariciresinols were added as radiochemical carriers, with each lignan isolated by reversed- phase HPLC. Controls included assays of a pinoresinol/lariciresinol reductase cDNA which contains an out-of-frame cDNA insert, with all assay components, as well as plr- Fil (SEQ TD NO:47) and an out-of-frame pinoresinol/lariciresinol reductase cDNA with no substrate except [4R- H]NADPH. Separation of products and chiral identification were performed by HPLC as previously described (Chu, A., et al., 1. Biol. Chem. 268:27026-27033 (1993)). Subsequent chiral HPLC analysis revealed that both (+)-lariciresinol and (-)-secoisolariciresinol, but not the corresponding antipodes, were radiolabeled (total activity: 54 nmol h " mg " ). By contrast, no catalytic activity was detected either in the absence of (±)-pinoresinols, or when control cells were used which contained a plasmid in which the cDNA insert was not in-frame with the β-galactosidase gene. Thus, the heterologously expressed (+)-pinoresinol/(+)-lariciresinol reductase and the plant protein function in precisely the same enantiospecific manner. EXAMPLE 13 Sequence and Homology Analysis of the cDNA Insert of Clone plr-Fil (SEQ TD NO:47) Encoding (+)-pinoresinol/(+)-lariciresinol reductase Sequence Analysis. The full length sequence of the cloned (+)-pinoresinol/ (+)-lariciresinol reductase plr-Fil (SEQ ID NO:47) contained all of the peptide sequences determined by Edman degradation of digest fragments. The single ORF predicts a polypeptide of 312 amino acids (SEQ ID NO:48) with a calculated molecular mass of 34.9 kDa, in close agreement with the value (-35 or -36 kDa) estimated previously by SDS-PAGE for the two isoforms of (+)-pinoresinol/(+)-lariciresinol reductase. An equal number of acidic and basic residues are also present, with a theoretical isoelectric point (pi) of 7.09, in contrast to that experimentally obtained by chromatofocussing (pi -5.7). The amino acid composition reveals seven methionine residues. Interestingly, the N-terminus of the plant-purified enzyme lacks the initial methionine, this being the most common post-translational protein modification known. Consequently, the first methionine in the cDNA can be considered to be the site of translational initiation. The sequence analysis also reveals a possible N-glycosylation site at residue 215 (although no secretory targeting signal is present), and seven possible protein phosphorylation sites at residues 50 and 228 (protein kinase C-type), residues 228, 250, 302 and 303 (casein kinase H-type ) and residue 301 (tyrosine kinase type). Regions of the pinoresinol/lariciresinol polypeptide chain (SEQ ID NO:48) were also identified that contained conserved sequences associated with NADPH binding (Jornvall, H., in Dehydrogenases Requiring Nicotinamide Coenzymes (Jeffery, J., ed) pp. 126-148, Birkhauser Verlag, Basel (1980); Branden, C, and Tooze, J., Introduction to Protein Structure, pp. 141-159, Garland Publishing, Inc., New York and London (1991); Wierenga, R.K. et al., J. Mol. Biol. 187:101-108 (1986)). There is a limited number of invariant amino acids in the sequences of different reductases which are viewed as indicative of NADPH binding sites. These include three conserved glycine residues with the sequence G-X-G-X-X-G (SEQ TD NO:76), where X is any residue, and six conserved hydrophobic residues. The glycine-rich region is considered to play a central role in positioning the NADPH in its correct conformation. In this regard, a comparison of the N-terminal region of (+)-pinoresinol/(+)-lariciresinol reductase with that of the conserved, NADPH-binding regions of Drosophila melanogaster alcohol dehydrogenase (Branden, C, and Tooze, J., Introduction to Protein Structure, pp. 141-159, Garland Publishing, Inc., New York and London (1991)), Pinus taeda cinnamyl alcohol dehydrogenase (MacKay J.J. et al., Mol. Gen. Genet. 247:537-545 (1995)), dogfish muscle lactate dehydrogenase (Branden, C, and Tooze, J., Introduction to Protein Structure, pp. 141-159, Garland Publishing, Inc., New York and London (1991)) and human erythrocyte glutathione reductase (Branden, C, and Tooze, J., Introduction to Protein Structure, pp. 141-159, Garland Publishing, Inc., New York and London (1991)), revealed some interesting parallels. The invariant glycine residues are aligned in every case, as are four of the six hydrophobic residues required for the correct packaging in the formation of the domain. Hence, the NADPH-binding site of (+)-pinoresinol/(+)-lariciresinol reductase isoforms is localized close to the N-terminus. Homology Analysis: Comparison to Isoflavone Reductase. A BLAST search (Altschul, S.F, et al., J. Mol. Biol. 215:403-410 (1990)) was conducted with the translated amino acid sequence of (+)-pinoresinol/(+)-lariciresinol reductase (SEQ JD NO:48) against the non-redundant peptide database at the National Center for Biotechnology Information. Significant homology was noted for (+)-pinoresinol/(+)-lariciresinol reductase with various isoflavone reductases from the legumes, Cicer arietinum (Tiemann, K., et al., Eur. J. Biochem. 200:751-757 (1991)) (63.5% similarity, 44.4% identity), Medicago saliva (Paiva, N.L., et al., Plant Mol. Biol. 17:653-667 (1991)) (62.6% similarity, 42.0% identity) and Pisum sativum (Paiva, N.L., et al., Arch. Biochem. Biophys. 312:501-510 (1994)) (61.6% similarity, 41.3% identity). This observation is of considerable interest since isoflavonoids are formed via a related branch of phenylpropanoid-acetate pathway metabolism. Specifically, isoflavone reductases catalyze the reduction of oc,β-unsaturated ketones during isoflavonoid formation. For example, the Medicago sativa L. isoflavone reductase catalyzes the stereospecific conversion of 2'-hydroxyformononetin to (3R)-vestitone in the biosynthesis of the phytoalexin, (-)-medicarpin (Paiva, N.L. et al., Plant Mol. Biol. 17:653-667 (1991)). This sequence similarity may be significant given that both lignans and isoflavonoids are offshoots of general phenylpropanoid metabolism, with comparable plant defense functions and pharmacological roles, e.g., as "phytoestrogens". Consequently, since both reductases catalyze very similar reactions, it is tempting to speculate that the isoflavone reductases may have evolved from (+)-pinoresinol/(+)-lariciresinol reductase. This is considered likely since the lignans are present in the pteridophytes, hornworts, gymnosperms and angiosperms; hence their pathways apparently evolved prior to the isoflavonoids (Gang et al., hi Phytochemicals for Pest Control, Hedin et al., eds, ACS Symposium Series, Washington D.C, 658:58-59 (1997)). Comparable homology was also observed with putative isoflavone reductase "homologs" from Arabidopsis thaliana (Babiychuk, E., et al., Direct Submission (25-MAY-1995) to the EMBIJGenBank/DDBJ databases (1995)) (65.9% similarity, 50.8% identity), Nicotiana tabacum (Hibi, N., et al., Plant Cell 6:723-735 (1994)) (64.6% similarity, 47.2% identity), Solanum tuberosum (van Eldik, G.J., et al., (1995) Direct submission (06-OCT-1995) to the EMBL GenBank/DDBJ databases) (65.5% similarity, 47.7% identity) Zea mays (Petrucco, S., et al., Plant Cell 8:69-80 (1996)) (61.6% similarity, 44.9% identity) and especially Lupinus albus (Attuci, S., et al., Personal communication and direction submission (06/6/96) to the EMBL/Genbank/DDBJ databases (1996)) (85.9% similarity, 66.2% identity). By contrast, homology with other NADPH-dependent reductases was significantly lower: for example, dihydroflavonol reductases from Petunia hybrida (Beld, M. et al., Plant Mol. Biol. 13:491-502 (1989)) (43.2% similarity, 21.5% identity) and Hordeum vulgare (Kristiansen, K.N., and Rohde, W., Mol. Gen. Genet. 230:49-59 (1991)) (46.2% similarity, 21.1% identity), chalcone reductase from Medicago sativa (Ballance, G.M. and Dixon, R.A., Plant Physiol. 107:1027-1028 (1995)) (39.5% similarity, 15.8% identity), chalcone reductase "homolog" from Sesbania rostrata (Goormachtig, S., et al., (1995) Direct Submission (13-MAR-1995) to the EMBL/GenBank/DDBJ databases) (47.6% similarity, 24.1% identity), cholesterol dehydrogenase from Nocardia sp. (Horinouchi, S., et al., Appl. Environ. Microbiol. 57:1386-1393 (1991)) (46.6% similarity, 21.0% identity) and 3-β-hydroxy-5-ene steroid dehydrogenase from Rattus norvegicus (Zhao, H.-F., et al., lournal Endocrinology 127:3237-3239 (1990)) (43.5% similarity, 20.6% identity). Thus, sequence analysis establishes significant homology between (+)-pinoresinol/(+)-lariciresinol reductase, isoflavone reductases and putative isoflavone reductase "homologs" which do not possess isoflavone reductase activity. EXAMPLE 14 cDNA Cloning of Thuja plicata (-)-Pinoresinol/(- -Lariciresinol Reductases Plant Materials. Western red cedar plants (Tliuja plicata) were maintained in Washington State University greenhouse facilities. Materials. All solvents and chemicals used were reagent or HPLC grade. Taq thermostable DNA polymerase and restriction enzymes (Sad and Xbal) were obtained from Promega. pT7Blue T-vector and competent NovaBlue cells were purchased from Novagen and radiolabeled nucleotide ([ - 32 P]dCTP) was purchased from DuPont NEN. Oligonucleotide primers for polymerase chain reaction (PCR) and sequencing ® were synthesized by Gibco BRL Life Technologies. GENECLEAN H kits (BIO 101 Inc.) were used for purification of PCR fragments, with the gel-purified DNA concentrations determined by comparison to a low DNA mass ladder (Gibco BRL) in 1.3% agarose gels. Instrumentation. UV (including RNA and DNA determinations at OD 2 6θ) spectra were recorded on a Lambda 6 UV/VIS spectrophotometer. A Temptronic H thermocycler (Thermolyne) was used for all PCR amplifications. Purification of plasmid DNA for sequencing employed a QIAwell Plus plasmid purification system (Qiagen) followed by PEG precipitation (Sambrook, J., et al., Molecular Cloning: A Laboratory Manual, 3 volumes, 3rd Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, NY (1994)) or Wizard® Plus SV Minipreps DNA Purification System (Promega), with DNA sequences determined using an Applied Biosystems Model 373 A automated sequencer. Thuja plicata cDNA Library Synthesis. Total RNA (6.7 μg/g fresh weight) was obtained from young green leaves (including stems) of greenhouse-grown western red cedar plants (Thuja plicata) according to the method of Lewinsohn et al (Lewinsohn, E., et al., Plant Mol. Biol. Rep. 12:20-25 (1994)). A T.plicata cDNA library was constructed using 3 μg of purified poly(A)+ mRNA (Oligotex-dT™ Suspension, Qiagen) with the ZAP-cDNA® synthesis kit, the Uni ZAP™ XR vector, and the Gigapack® H Gold packaging extract (Stratagene), with a titer of 1.2 X 10 5 pfu for the primary library. The amplified library (7.1 X 10 8 pfu /ml; 28 ml total) was used for screening (Sambrook, J., et al., Molecular Cloning: A Laboratory Manual, 3 volumes, 3rd Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, NY (1994)). T. plicata (-)-Pinoresinol/(-)-Lariciresinol Reductase cDNA Synthesis. T. plicata (-)-pinoresinol/(-)-lariciresinol reductase cDNA was obtained from mRNA by a reverse transcription-polynierase chain reaction (RT-PCR) strategy (Sambrook, J., et al., Molecular Cloning: A Laboratory Manual, 3 volumes, 3rd Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, NY (1994)). First-strand cDNA was synthesized from the purified mRNA previously used for the synthesis of the T. plicata cDNA library, described above. Purified mRNA (150 ng) was mixed with linker-primer (1.4 μg) from ZAP-cDNA® synthesis kit (Stratagene), heated to 70°C for 10 min, and quickly chilled on ice. The mixture of denatured mRNA template and linker-primer was then mixed with First Strand Buffer (Life Technologies), 10 mM DTT, 0.5 mM each dNTP, and 200 units of Super Script™H (Life Technologies) in a final volume of 20 μl. The reaction was carried out at 42°C for 50 min and then stopped by heating (70°C, 15 min). E. coli RNase H (1.5 units, 1 μl) was added to the solution and incubated at 37°C for 20 min. The first-strand reaction (2 μl) was next used as the template in 100-μl PCR reactions (10 mM Tris-HCl, pH 9.0, 50 mM KC1, 0.1 % Triton X-100, 1.5 mM MgCl 2 , 0.2 mM each dNTP, and 5 units of Taq DNA polymerase) with primer CR6-NT (5'GCACATAAGAGTATGGATAAG3')(SEQ TD NO:60) (10 pmol) and primer Xhol- Poly(dT) (5'GTCTCGAGTTTTTTTTTTTTTTTTTT3')(SEQ TD NO:59) (10 pmol). PCR amplification was carried out in a thermocycler as described in (Dinkova-Kostova, A.T., et al., J. Biol. Chem. 271:29473-29482 (1996)) except for the annealing temperature at 52°C PCR products were resolved in 1.3 % agarose gels, where at least two bands possessing the expected length (about 1,200-bp) were observed. The bands were extracted from the gel. The gel-purified PCR products (56 ng) were then ligated into the pT7Blue T-vector (50 ng) and transformed into competent NovaBlue cells, according to Novagen's instructions. The size and orientation of the inserted cDNAs were determined using the rapid boiling lysis and PCR technique, following the manufacturer's (Novagen's) instructions, with the following primer combinations: R20-mer(SEQ TD NO:74) with U19-mer (SEQ ID NO:75); R20-mer (SEQ ID NO:74) with CR6-NT (SEQ ID NO:60); U19-mer (SEQ ID NO:75) with CR6-NT (SEQ ED NO:60). The CR6-NT primer end of the inserted DNAs was located next to the U19-mer primer site of the T-vector. The T-vectors containing the inserted cDNAs were purified with Wizard® Plus SV Minipreps DNA Purification System. Five inserted cDNAs were completely sequenced using overlapping sequencing primers and were shown to be identical except that polyadenylation sites were different. Therefore, the longest cDNA, designated plr-Tpl, (SEQ ID NO:61) was used for detection of enzyme activity using the pBluescript expression system. Sequence Analysis - DNA and amino acid sequence analyses were performed using the Unix-based GCG Wisconsin Package (Program Manual for the Wisconsin Package, Version 8, September 1994, Genetics Computer Group, 575 Science Drive, Madison, Wisconsin, USA 53711 (1996); Rice, P., Program Manual for the EGCG Package, Peter Rice, The Sanger Centre, Hinxton Hall, Cambridge, CB10 lRq, England) and the ExPASy World Wide Web molecular biology server (Geneva University Hospital and University of Geneva, Geneva, Switzerland). EXAMPLE 15 cDNA Cloning and Expression of Thuja plicata (+)-Pinoresinol/ (+)-Lariciresinol Reductase T. plicata (+)-Pinoresinol/(+)-Lariciresinol Reductase cDNA cloning. After plr- Tpl (SEQ ID NO:61) was cloned and sequenced, the full-length clone was used to screen the T. plicata cDNA library as described in Example 11, except that the entire plr-Tpl cDNA insert (SEQ ID NO:61) was used as a probe. Several positive clones were sequenced, revealing one new, unique cDNA which was called plr-Tp2 (SEQ ID NO:63). This cDNA encodes a reductase (SEQ TD NO:64) with high sequence similarity to plr- Tpl (SEQ ID NO:62) (-81% similarity at the amino acid level), but with substrate specificity properties identical to the original Forsythia intermedia reductase, as described below. Enzyme Assays. Pinoresinol and lariciresinol reductase activities were assayed by monitoring the formation of [ 3 H] lariciresinol and [ 3 H] secoisolariciresinol as set forth in Example 8, with the following modifications. Briefly, each assay for pinoresinol reductase activity consisted of (±)-pinoresinols (5 mM in MeOH, 20 μl) and the enzyme preparation (i.e., total protein extract from E. coli, 210 μl). The enzymatic reaction was initiated by addition of [4R- 3 H]NADPH (10 mM, 6.79 kBq/mmol in distilled H 2 O, 20 μl). After 3 hour incubation at 30°C with shaking, the assay mixture was extracted with EtOAc (500 μl) containing (±)-lariciresinols (20 μg) and (±)-secoisolariciresinols (20 μg) as radiochemical carriers. After centrifugation (13,800 x g, 5 min), the EtOAc solubles were removed and the extraction procedure was repeated. For each assay, the EtOAc solubles were combined with an aliquot (100 μl) removed for determination of its radioactivity using liquid scintillation counting. The remainder of the combined EtOAc solubles was evaporated to dryness in vacuo, reconstituted in MeOIWJ^O (30:70, 100 μl) and subjected to reversed phase and chiral column HPLC. Lariciresinol reductase activity was assayed by monitoring the formation of (+)-[ 3 H]secoisolariciresinol. These assays were carried out exactly as described above, except that (±)-lariciresinols (5 mM in MeOH, 20 μl) were used as substrates, with (±)-secoisolariciresinols (20 μg) added as radiochemical carriers. Expression of plr-Tpl (SEQ ID NO:61) in E. coli - In order for the open reading frame (ORF) of plr-Tpl (SEQ ID NO:61) to be in frame with the β-galactosidase gene α- complementation particle in pBluescript SK(-), plr-Tpl (SEQ ID NO:61) was excised out of pT7Blue T-vector with Sad and Xbal, gel-purified, and then ligated into the expression vector digested with these same enzymes. This plasmid, pPCR-Tpl, was transformed into NovaBlue cells according to Novagen's instructions. The transformed cells (5-ml cultures) were grown at 37°C in LB medium (Sambrook, J., et al., Molecular Cloning: A Laboratory Manual, 3 volumes, 3rd Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, NY (1994)) supplemented with 50 μg ml "1 carbenicillin with shaking (225 rpm) to mid log phase (A 60 o = 0.5-0.7). The cells were next collected by centrifugation (1000 x g, 10 min) and resuspended in fresh LB medium supplemented with 10 mM TPTG (isopropyl β-D-thioglucopyranoside) and 50 μg ml "1 carbenicillin to an absorbance of 0.6 (at 600 nm). The cells, allowed to grow overnight, were collected by centrifugation and resuspended in 500-700 μl of (per 5 ml culture tube) of buffer (50 mM Tris-HCl, pH 7.5, 2 mM EDTA, and 5 mM DTT). Next, the cells were lysed by sonication (5 x 45 s) and after centrifugation (17500 x g, 4°C, 10 min) the supernatant was removed and assayed for (-)-pinoresinol/(-)-lariciresinol reductase activity as described above. Controls included assays of pBluescript (SK(-)) without insert DNA (as negative control) or with pPLR-Fil (SEQ ID NO:47) (cDNA of authentic F. intermedia (+)-pinoresinol (+)-lariciresinol reductase in frame) as stereospecific control, as well as pPLR-Tpl (SEQ ID NO:61) with no substrate except (4R)- 3 HNADPH. The results showed that both (-)-lariciresinol and (+)-secoisolariciresinol were radiolabeled and that no incorporation of radioactivity was found in (-)-secoisolariciresinol. However, accumulation of radiolabel into (+)-lariciresinol was also observed, although at a much slower rate than that observed for (-)-lariciresinol. These results indicate that plr-Tpl (SEQ ID NO: 62) can use both (-)-pinoresinol and (+)-pinoresinol as substrates, with the former being converted via (-)-lariciresinol completely to (+)-secoisolariciresinol, and the latter being converted much more slowly to (+)-lariciresinol, but not further to (-)-secoisolariciresinol. Expression o plr-Tp2 (SEQ ID NO:63) in E. coli. The plr-Tp2 cDNA (SEQ TD NO:63) was found to be in frame with the β-galactosidase gene α-complementation particle in pBluescript SK(-). When evaluated for activity and substrate specificity, as described above, plr-Tp2 (SEQ TD NO:64) was found to possess the same substrate specificity and product formation as the original Forsythia intermedia reductase (Dinkova-Kostova, A.T., et al., J. Biol. Chem. 271:29473-29482 (1996)) except that a small amount of (-)-lariciresinol was also detected. This is interesting, because plr-Tp2 (SEQ TD NO:64) has a higher sequence similarity to plr-Tpl (SEQ ID NO:62) than it does to the Forsythia reductase. All the above observations were confirmed using deuterolabeled substrates (±)-[9,9'- 2 H2, OC 2 H3]pinoresinols with isolation of the conesponding lignans; each was then subjected to chiral column chromatography and HPLC-mass spectral analysis to confirm these findings. EXAMPLE 16 Cloning of Additional Pinoresinol/Lariciresinol Reductases from Tlxuja plicata and Tsuga heterophylla Two additional pinoresinol/lariciresinol reductases were cloned from a Thuja plicata young stem cDNA library as described in Example 15 for the cloning of plr-Tp2 (SEQ ID NO:63). The two additional pinoresinol/lariciresinol reductases were designated plr-Tp3 (SEQ TD NO:65) and plr-Tp4 (SEQ ID NO:67). Two additional pinoresinol/lariciresinol reductases were cloned from a Tsuga heterophylla young stem cDNA library as described in Example 15 for the cloning of plr- Tp2 (SEQ TD NO:63). The two additional pinoresinol/lariciresinol reductases from Tsuga heterophylla were designated plr-Tp3 (SEQ TD NO:69) and plr-Tp4 (SEQ ID NO:71). EXAMPLE 17 Cloning Additional Dirigent Protein cDNAs from Thuja plicata An additional cDNA, called Tp9 (SEQ TD NO:77), encoding a dirigent protein (SEQ TD NO:78) was cloned from mRNA extracted from Western red cedar (Thuja plicata) in the following manner. First strand cDNA was synthesized from Western red cedar cambium RNA using primer CS1-895N (5 - AGAGTGGAGATTGTTGTCAAGAGTA-3')(SEQ ID NO:79) derived from highly conserved sequence motifs found in other Western red cedar dirigent protein isoforms. SuperScript™H RNase H Reverse Transcriptase (Life Technologies, Rockville, MD) was used to synthesize first strand cDNA following the manufacturer's protocol. Then the 3' end of the first strand cDNA was tailed with dATP using terminal transferase (Boehringer Mannheim, Indianapolis, IN). The tailed cDNA was amplified via Expand™ High Fidelity PCR system (Boehringer Mannheim, Indianapolis, IN) using the following primers: first round of PCR utilized the oligo dT-anchor primer (5'- GACCACGCGTATCGATGTCGACTTTTTTTTTTTTTTTTV-3')(SEQ ID NO:80)(wherein "V" at the 3'-end of the oligo-dT anchor primer represents A, C or G) as the 5'-sense primer, and primer CS1-895N (SEQ ID NO:79) as the 3'-antisense primer; second round of PCR utilized PCR anchor primer (5 - GACCACGCGTATCGATGTCGAC-3' )(SEQ ID NO:81) as the 5'-sense primer, and primer CS1-874N (5'-AGTAATAGGATCATCAAACAC-3')(SEQ ID NO:82) as the 3'- antisense primer. The PCR conditions were as follows: first cycle, 94°C for 1 minute; second thru twenty sixth cycles, 94°C for 30 seconds, then 56°C for 45 seconds, then 72°C for two minutes; the twenty seventh cycle, 72°C for seven minutes. The resulting PCR product (that corresponded to nucleic acid residues 1-253 of SEQ ID NO:77) was gel purified and cloned using a TA cloning kit (Invitrogen, Carlsbad, CA), and sequenced to identify the dirigent protein gene. To complete the 3-region of the PCR product (that coπesponded to nucleic acid residues 1-253 of SEQ TD NO:77), the oligo dT-anchor primer (SEQ ID NO:80) was used to generate first strand cDNA. The full length Tp9 cDNA (SEQ TD NO:77) was cloned using the following PCR primers: the first round of PCR utilized primer RT-CS-C1(- )50s (5'-CCAACTTCTTTCTCTACTTCAGAA-3')(SEQ ID NO: 83) as the 5 * -sense primer, and the oligo dT anchor primer (SEQ TD NO: 80) as the 3'-antisense primer; the second round of PCR utilized primer RT-CS-Cl(-)31s (5'- CAGAACCCTGTTTTCTGATTTATT-3 * )(SEQ ID NO:84) as the 5'-sense primer, and the PCR anchor primer (SEQ TD NO: 81) as the 3 '-antisense primer; and the third round of PCR utilized primer RT-CS-Cl(-)13s (5'-TTTATTTTTGCACAATGGCAATCT- 3')(SEQ ID NO:85) as the 5'-sense primer, and the PCR anchor primer (SEQ ID NO:81) as the 3'-antisense primer. The reaction conditions for each round of PCR were as follows: first cycle, 94°C for 1 minute; second thru twenty sixth cycles, 94°C for 30 seconds, then 56°C for 45 seconds, then 72°C for two minutes; the twenty seventh cycle, 72°C for seven minutes. The full length Tp9 cDNA sequence (SEQ ID NO:77) was confirmed by direct PCR amplification of the Tp9 gene using Expand™ High Fidelity PCR system. EXAMPLE 18 Completing the 5'-Ends of Dirigent Protein Clones Tp3 (SEQ ID NO:24) and Tp4 (SEQ ID NO:26) from Thuia ylicata The nucleotide sequences of the Thuja plicata dirigent protein clones Tp3 (SEQ ID NO:24) and Tp4 (SEQ TD NO:26) were incomplete at their 5' ends. Genomic DNA analysis indicated that Western red cedar dirigent protein genes do not contain introns. Consequently, it was possible to complete the partial cDNA sequences of Tp3 (SEQ ID NO:24) and Tp4 (SEQ TD NO:26) by utilizing the Advantage Genomic PCR Kit (Clonetech, Palo Alto, CA) following the manufacturer's instructions. The cDNA clone (SEQ ID NO:86) encoding a full-length, Tp3 dirigent protein (SEQ ID NO: 87) was cloned by PCR using Thuja plicata genomic DNA as template and the following primers: the first round of PCR utilized primer API (5 - GTAATACGACTCACTATAGGGC-3')(SEQ ID NO:88) as the 5'-sense primer, and primer TpS4-213n (5'-AGATTAGCTCCTTGAGGGGCTGAAACAA-3')(SEQ ID NO:89) as the 3'-antisense primer; the second round of PCR utilized primer AP2 (5 - ACTATAGGGCACGCGTGGT-3')(SEQ ID NO:90) as the 5'-sense primer, and primer TpS4-199n (5'-AGGGGCTGAAACAAGTGCAGATGTTGCA-3')(SEQ ID NO:91) as the 3'-antisense primer; the third round of PCR utilized primer AP2 (SEQ ID NO:90) as the 5'-sense primer, and primer TpS4-188n (5'- CAAGTGTGCAGATGTTGCATTCTCTGCAT-3'XSEQ ID NO:92) as the 3'-antisense primer. The cDNA clone (SEQ ID NO:93) encoding a full-length, Tp4 dirigent protein (SEQ ID NO:94) was cloned by PCR using Thuja plicata genomic DNA as template and the following primers: the first round of PCR utilized primer API (SEQ ID NO: 88) as the 5'-sense primer, and primer CS10-826N (5'- CAGTCATAATGGTGAGATTGGCTCCCT-3')(SEQ JD NO:95) as the 3'-antisense primer; the second round of PCR utilized primer AP2 (SEQ TD NO: 90) as the 5 '-sense primer, and primer CS10-814N (5'-TGAGATTGGCTCCCTCAGGGGCTGCAA- 3')(SEQ ID NO:96) as the 3'-antisense primer; the third round of PCR utilized primer AP2 (SEQ ID NO:90) as the 5'-sense primer, and primer CS10-795N (5'- GGCTGCAACAAGTGCAGATGTTGCATT-3')(SEQ ID NO:97) as the 3'-antisense primer. The PCR reaction conditions for cloning the full-length, Tp3 and Tp4 dirigent protein clones having the nucleic acid sequences set forth in SEQ ID NO:86 and SEQ TD NO: 93 were identical and were as follows: the first round of PCR included primary PCR consisting of a first cycle at 94°C for 1 minute and second thru eighth cycles at 94°C for 25 seconds, then 72°C for 12 minutes, followed by secondary PCR consisting of a first cycle at 94°C for 1 minute, second thru thirty three cycles at 94°C for 20 seconds, then 72°C for 6 minutes, and finally a thirty fourth cycle at 68°C for 12 minutes; the second and third rounds of PCR each consisted of a first cycle at 94°C for one minute, then second thru twenty sixth cycles at 94°C for 30 seconds, then 60°C for 45 seconds, then 72°C for two minutes, then a twenty seventh cycle at 72°C for seven minutes. The ability to complete the full-length Tp3 (SEQ ID NO:86) and Tp4 (SEQ TD NO: 93) sequences from genomic DNA overcomes the difficulty in completing a full-length sequence from a cDNA pool. Incomplete synthesis of the first strand cDNA of target cDNAs often occurs due to instability of RNA or secondary structure of RNA, and results in the incomplete synthesis of the first strand cDNA. EXAMPLE 19 Cloning a Dirigent Protein cDNA from Eucommia ulmoides A cDNA (SEQ TD NO:98) encoding a dirigent protein (SEQ ID NO:99) was isolated from Eucommia ulmoides utilizing the strategy used for the cloning of the Tp9 cDNA (SEQ ID NO:77) of Western red cedar. First strand cDNA was synthesized with SuperScript™H RNase H Reverse Transcriptase from E. ulmoides leaf RNA using the oligo dT anchor primer (SEQ ID NO: 80). From the first strand cDNA, a 300 bp cDNA fragment (corresponding to nucleotides 152 thru 452 of SEQ ID NO:98) was amplified using N-terminal primer 5'-GARTTGGTGTTCTATTTCCACGACATMC-3' (SEQ TD NO:100)(wherein "R" represents A or G and "M" represents A or C) and C-terminal primer 5'-CAAAGTGGCAACCCCTGTCGCCATG-3'(SEQ ID NO: 101), derived from a highly conserved sequence among the other dirigent protein isoforms. The PCR reaction conditions were as follows: first cycle, 94°C for lmin, second thru twenty sixth cycles, 94°C for 30 seconds, then 50°C for 45 seconds, then 72°C for 2 min, and a twenty seventh cycle at72°C for 7min. The PCR product was gel purified and cloned using a TA cloning kit. Then this fragment was used to design gene specific primers to clone a complete sequence. In order to clone the missing 5' region, first strand cDNA was synthesized with SuperScript™H RNase H " Reverse Transcriptase using the gene specific, 3'-antisense Sp2 primer (5'- CCCCCGTTCCTCCAACCACCGG-3')(SEQ JD NO:102), and the 3' end of the first strand cDNA was tailed with dATP using terminal transferase. The tailed cDNA was amplified via Expand TM High Fidelity PCR system using the oligo dT-anchor primer (SEQ JD NO:80) and the Sp2 primer (SEQ ID NO: 102). Then the second round of PCR was conducted using the PCR-anchor primer (SEQ ID NO: 81) and the Sp2 primer (SEQ ED NO: 102). The PCR product was gel purified, and cloned using the TA cloning kit. The resulting, partial cDNA corresponded to nucleic acid residues 1 thru 413 of SEQ ID NO:98. The PCR reaction conditions were the same for the first and second rounds of PCR and were as follows: first cycle, 94°C for lmin, second thru twenty sixth cycles, 94°C for 30 seconds, then 50°C for 45 seconds, then 72°C for 2 min, and a twenty seventh cycle at72°C for 7min. To obtain the missing 3 'sequence, the oligo dT-anchor primer (SEQ ID NO: 80) was used to generate first strand cDNA. The full length Eucommia cDNA (SEQ ED NO:98) was amplified through two consecutive rounds of PCR using a set of nested primers: 5 '-sense primers SpNl (5'-GGCCCATGCGGTTAAGCATATTCTCC- 3')(SEQ JD NO: 103) and SpN2 (5'-CCTCTATAAAAACATAATTCTTTTCCCCC- 30(SEQ ED NO: 104), and the primers having the nucleic acid sequences set forth in SEQ ID NO:80 and SEQ TD NO:81 as 3'-antisense primers. The reaction conditions for both the first and the second rounds of PCR were as follows: first cycle, 94°C for 1 minute, second thru twenty sixth cycles, 94°C for 30 seconds, then 60°C for 45 seconds, then 72°C for 2 minutes, a twenty seventh cycle at72°C for 7minutes. The PCR product was gel purified, and cloned using a TA cloning kit. The identity of the full length cDNA (SEQ ID NO:98) was confirmed by sequencing and by direct PCR amplification of the conesponding Eucommia dirigent protein gene. EXAMPLE 20 Cloning a Dirigent Protein cDNA from Schisandra chinensis A cDNA (SEQ ID NO: 105), encoding a dirigent protein (SEQ ID NO: 106) was cloned from mRNA extracted from Schisandra chinensis in the following manner. A cDNA library was made from Schisandra chinensis leaf and stem tissues. The cDNA library was probed for a homologous dirigent protein clone using the 32 P-CTP-labelled F. intermedia dirigent protein gene (Psd-Fil) (SEQ ID NO: 12) as a probe. Three rounds of screening were utilized. For each screen, plaques were lifted onto nylon transfer membrane for 3 minutes, the transfeπed DNA was then fixed to the membrane by autoclaving for 3 minutes at 100°C, and debris were washed off the membranes with 2X SSC for 30 minutes at 37°C Prehybridization solution consisted of 6X SSC, 5X Denhardt's reagent, and 0.5% SDS. 50 μl probe (SEQ ED NO:12) was made with T7 Quick-Prime Kit. 100 μl 5mg/ml salmon sperm DNA and 850 μl deionized, distilled water were then added to the probe. Hybridization solution consisted of 6X SSC, 5X Denhardt's reagent, 0.5% SDS, and probe (SEQ ID NO:12). For the primary screen, nylon membrane filters bearing plaques were prehybridized for seven and a half hours at 48-49°C The filters were then hybridized overnight (approximately 17 hours) at 49°C in the presence of 1.13 x 10 7 cpm radiolabelled probe (SEQ ED NO: 12). After hybridization, the filters were washed with 4X SSC, 0.5% SDS, for 10 min, then with 2X SSC, 0.5% SDS, for 9 min at room temperature (from about 20°C to about 24°C), then with 2X SSC, 0.5% SDS, for 50 minutes, warming from room temperature (from about 20°C to about 24°C) to 49°C Finally the filters were rinsed with 6X SSC briefly before being exposed to film for one and a half days. For the secondary screen, nylon membrane filters bearing plaques were prehybridized for 8 hours at 49°C The filters were then hybridized overnight (approximately 16 hours) at 49°C in the presence of 3.9 x 10 6 cpm radiolabelled probe (SEQ JD NO: 12). After hybridization, the filters were washed with 4X SSC, 0.5% SDS, at room temperature for 5-20 minutes, then rinsed briefly in 6X SSC before being exposed to film overnight. For the tertiary screen, nylon membrane filters bearing plaques were prehybridized overnight at 45°C The filters were then hybridized overnight (approximately 18 hours) at 45°C in the presence of 1.2 x 10 6 cpm radiolabelled probe (SEQ ID NO: 12). After hybridization, the filters were washed with 4X SSC, 0.5% SDS, for 10 minutes, then with 2X SSC, 0.5% SDS, for 2 minutes at room temperature (from about 20°C to about 24°C). Finally the filters were rinsed with 6X SSC briefly before being exposed to film for 4 days. After three rounds of screening, one positive plaque was isolated. The cDNA from this plaque was subsequently excised and cloned into SOLR cells (Stratagene Cloning Systems, 11011 North Toπey Pines Road, La Jolla, CA 92037). Sequence analysis showed it to be a full-length clone, and homology analysis established it to be 51-72% identical and 63-80% similar to other known dirigent proteins at the amino acid level. EXAMPLE 21 Cloning a Pinoresinol/Lariciresinol Reductase cDNA from Linum usitatissimum A cDNA (SEQ JD NO: 107), encoding a pinoresinol/lariciresinol reductase protein (SEQ JD NO: 108), was cloned from mRNA extracted from Linum usitatissimum (Flax) in the following manner. Total RNA was obtained from two week old whorls (each containing 10 developing flax seeds) of green house grown L. usitatissimum plants. Poly(A) + mRNA was purified using the Promega PolyATract® mRNA Isolation System. A L. usitatissimum seed cDNA library was constructed using 5μg of purified mRNA with the Stratagene ZAP-cDNA® Synthesis Kit and the ZAP-cDNA® Gigapack® HI Gold cloning kit, with a titer of 2.8 x 10 5 pfu (plaque forming units) for the primary library. Approximately 35 ml of liquid phage lysate from the amplified library (2.8 x 10^ pfu/ml) was used to obtain pure cDNA library DNA for PCR screening. The following degenerate primers were designed based on N-terminal, C-terminal and internal regions of similarity found in the Forsythia intermedia and Thuja plicata (Western red cedar) pinoresinol/lariciresinol reductase clones: PLR4 forward primer (5 - CCπCIGAGTTCGGJATGGATCCI-3')(SEQ TD NO: 109), and PLR6 reverse primer (5'- IGTATATTTIACTTCIGGGTA-3')(SEQ ED NO: 110). Pure L. usitatissimum cDNA library DNA (-5 ng) was used as the template in 45 μl PCR reactions with various primer combinations. PCR amplification was carried out in an Amplitron® H thermocycler as follows: 5 cycles of 1 minute at 94°C, then 1 minute at 37°C, then 3 minutes at 72°C, followed by 25 cycles of 25 seconds at 94°C, then 1 minute at 48°C, and 2 minutes at 72°C; with 10 minutes at 72°C and an indefinite hold at 4°C after the final cycle. PCR products were resolved in 1% agarose gels. Reactions producing single bands with an accurate size predicted by their respective primer combination were cloned using Invitrogen's Topo-TA cloning® Kit and sequenced. This round of PCR yielded a partial- length cDNA clone of about 600 bp (SEQ ED NO: 111). The partial length clone (SEQ ED NO: 111) was used as a 32 P-radiolabeled probe with the aim of isolating the full length sequence. 500,000 pfu of L. usitatissimum amplified cDNA library were plated, plaques were blotted onto Magna Nylon membrane circles, and the library was screened as follows. The blotted membranes were placed between two layers of Whatman 3MM Chr. Paper. cDNA library phage DNA was fixed to the membranes and denatured in one step by autoclaving for 4 min at 100°C with fast exhaust. The membranes were washed for 30 min with gentle shaking at 37°C in 6X SSC and 0.1% SDS, and prehybridized for 5 h with gentle shaking at 57°C in preheated 6X SSC, 0.5% SDS, and 5X Denhardt's reagent (hybridization solution, 220 ml) in a crystallization dish (190 X 75 mm). The 32 P-radiolabeled probe (SEQ TD NO: 111) was denatured (98°C for 10 minutes), quickly cooled (on ice for 15 minutes), and added to a preheated hybridization solution (50 ml at 57°C) in a crystallization dish (150 X 75 mm). The prehybridized membranes were next added to this dish, which was then covered with plastic wrap. Hybridization was performed for 17 h at 57°C with gentle shaking. The membranes were washed in 4X SSC and 0.5% SDS (250 ml) for 5 min at room temperature, transfened to preheated 2X SSC and 0.5% SDS (250 ml), and incubated at 57°C for 20 min with gentle shaking. After the membranes were removed from the dish and wrapped with plastic wrap, they were exposed to Kodak X-OMAT AR film for 24 h at -80°C between intensifying screens. A larger cDNA clone (SEQ ED NO: 112) was attained, but in comparison to F. intermedia cDNA plr-Fil (SEQ ED NO:47) it still lacked -258 bp at the N-terminal. To achieve a full length clone, gene specific primer 1 (5 1 - AACATTTCCGGCCTCTTTGATGGCCTCGAC-3')(SEQ ED NO: 113) and gene specific primer 2 (5'-AAGGTAGATCATCAGATAATCTTTCATACG-3')(SEQ ED NO: 114) were designed and used in combination with the Stratagene Uni-ZAP® XR vector T7 (5 - GTAATACGACTCACTATAGGGC-3')(SEQ ED NO: 115) and T3 (5'- AATTAACCCTCACTAAAGGG-3'XSEQ ED NO: 116) primers. A 936 bp gene (SEQ D NO: 107), containing identical sequence information from all previous truncated clones, displayed an -74% similarity and -61% identity to F. intermedia cDNA plr-Fil (SEQ ID NO:47) at the amino acid level. EXAMPLE 22 Cloning a Pinoresinol/Lariciresinol Reductase cDNA from Schisandra chinensis A cDNA (SEQ TD NO: 117), encoding a pinoresinol/lariciresinol reductase protein (SEQ TD NO: 118) was cloned from mRNA extracted from Schisandra chinensis in the following manner. Degenerate primers PLR4 (SEQ TD NO: 109) and PLR6 (SEQ ID NO: 110) were made from regions of high homology between known reductase clones, and these were used to screen a Schisandra chinensis leaf and stem tissue cDNA library using a PCR-guided strategy. The Schisandra chinensis leaf cDNA library was first used as a template under the following conditions: five cycles of 95°C for 1 minute, 37°C for 1 minute and 72°C for 3 minutes, then 25 cycles of 94°C for 25 minutes, 48°C for 1 minute and 72°C 2 minutes. No cDNA bands were found upon gel analysis of the resulting products which were then used as template for another round of PCR under the following conditions: five cycles of 95°C for 1 minute, 37°C 1 minute, 72°C 3 minutes, then 25 cycles of 94°C for 25 minutes, 48°C for 1 minute, 72°C for 2 minutes. A 600 bp PCR product was obtained, which was then cloned into a TOPO-TA vector and partially sequenced to yield the sequence of SEQ ED NO: 119. Sequence analysis showed it to be 55.7% identical and 67.0% similar to F. intermedia cDNA plr- Fil (SEQ ID NO:47) at the amino acid level. This PCR product (including the nucleic acid sequence set forth in SEQ TD NO: 119) was then used to make a 32 P-CTP probe, and the cDNA library was once again probed using three rounds of screening. For the primary screen, cDNA library plaques were transfened onto nylon transfer membrane for 3 minutes, then fixed to the membranes by autoclaving for 3 minutes at 100°C. Debris was washed off membranes with 2X SSC for 30 minutes at 37°C, then the membranes were incubated in prehybridization solution consisting of 6X SSC, 5X Denhardt's reagent, and 0.5% SDS, for 6 hours at 48°C The filters were then hybridized for 24 hours at 45°C in hybridization solution (6X SSC, 5X Denhardt's reagent, 0.5% SDS), including 100 μl 5mg/ml salmon sperm DNA, 850 μl ddH 2 O and 9.6 x 10 5 cpm probe. The filters were then washed (at room temperature) with 4X SSC, 0.5% SDS, for 5 minutes, then with 2X SSC, 0.5% SDS, for 20 minutes, then rinsed briefly with 4X SSC and exposed to autoradiography film 5 days. The secondary screen was conducted under the same conditions as the primary screen except that the filters were prehybridized for 24 hours at 48°C, then hybridized for 24 hours at 48°C in the presence of 1.37 x 10 6 cpm probe. The filters were washed at room temperature with 4X SSC, 0.5% SDS, for 5 minutes then briefly rinsed in 6X SSC at room temperature and exposed to film overnight. The tertiary screen was conducted under the same conditions as the primary screen except that the filters were prehybridized for 24 hours at 48°C, then hybridized for 37 hours at 48°C in the presence of 2.2 x 10 6 cpm probe. The filters were washed at room temperature under the following conditions: 4X SSC, 0.5% SDS, for 5 minutes, then with 2X SSC, 0.5% SDS, for 5 minutes, rinsed briefly in 6X SSC and exposed to film overnight. One positive plaque was identified and isolated after the foregoing three rounds of screening. The plaque was excised into a pBluescript plasmid, cloned into SOLR cells, and sequenced (SEQ ID NO: 119). Sequence analysis showed this clone (SEQ D NO: 119) encoded a partial length reductase protein (SEQ ED NO: 120) which was 57.4% identical and 78.7% similar to F. intermedia PLR at the peptide level. However, a one- base frame-shift mutation (deletion of an adenine at position 579) was present in the full- length clone (SEQ ED NO: 119) which prevented expression of full-length protein. This full-length clone (SEQ D NO: 119) was next used to make a 32 P-dCTP probe, and the library was further screened in the following manner. For the primary screen, cDNA library plaques were transferred onto nylon transfer membrane for 3 minutes, then fixed to the membranes by autoclaving for 3 minutes at 100°C Debris was washed off membranes with 2X SSC for 30 minutes at 37°C, then the membranes were incubated in prehybridization solution consisting of 6X SSC, 5X Denhardt's reagent, and 0.5% SDS, for 18 hours at 47°C The filters were then hybridized for 24 hours at 47°C in hybridization solution (6X SSC, 5X Denhardt's reagent, 0.5% SDS), including 100 μl 5mg/ml salmon sperm DNA, 850 μl deionized, distilled H O and 9.3 x 10 6 cpm probe. The filters were then washed at room temperature with 4X SSC, 0.5% SDS, for 5-10 minutes, then rinsed briefly at room temperature with 6X SSC and exposed to autoradiography film overnight. The secondary screen was conducted under the same conditions as the primary screen except that the filters were prehybridized for eight and a third hours at 45-47°C, then hybridized overnight at 40-47°C in the presence of 7.5 x 10 6 cpm probe. The filters were washed with 4X SSC, 0.5% SDS, for 10 minutes, then with 2.5X SSC, 0.5% SDS for 10 minutes at room temperature, then washed with 2.5X SSC, 0.5% SDS, for 10 minutes with the temperature rising to 42°C during that period, and finally rinsing in 6X SSC briefly at room temperature and exposed to film overnight. The tertiary screen was conducted under the same conditions as the primary screen except that the filters were prehybridized for 11 hours at 47°C, then hybridized overnight (approximately 14 hours) at 47°C in the presence of 1.8 x 10 6 cpm probe. The filters were washed at room temperature with 4X SSC, 0.5% SDS, for 10 minutes, then rinsed at room temperature in 6X SSC briefly and exposed to film for approximately 24 hours. Two positive plaques were identified after the foregoing three rounds of screening. Each was excised, transformed into SOLR cells, and sequenced. Of these, one clone (SEQ JD NO: 117) was a full-length reductase identical to the clone having the sequence set forth in SEQ TD NO:l 19, with the exception that it was in frame. The other clone was not full-length. The full-length reductase cDNA (SEQ ID NO: 119) was cloned into a pBad-TOPO-TA expression vector. Based on the characterization and sequence comparison of the foregoing pinoresinol/lariciresinol reductase cDNAs and proteins, presently preferred pinoresinol/lariciresinol reductase proteins of the invention utilize NADPH as a cofactor and include the conserved amino acid sequence domain Gly Xaa Gly Xaa Xaa Gly (SEQ TD NO:76). EXAMPLE 23 Cloning Additional Nucleic Acid Molecules That Encode a Dirigent Protein Additional nucleic acid molecules encoding a dirigent protein can be cloned utilizing a variety of strategies. In one approach, genomic DNA or cDNA can be amplified by PCR using the following primer pair: primer PS-6For 5'- KGTGTTYGAYGATCCYATTACYBTWGACAAC-3' (SEQ ID NO: 120) and primer PS-2Rev 5'-TGRCTAMGTAWACTYCCTCTACAAATAAAG-3' (SEQ ED NO: 121). The sequence of PCR reactions is set forth in Table 4 below. Representative PCR reaction conditions are as set forth in Table 6 below, except that primers PS-6For (SEQ TD NO: 120) and PS-6Rev (SEQ ID NO: 121) are utilized instead of primers PLR4 forward (SEQ ID NO:109) andPLR6 reverse (SEQ ID NO:110). Table 4 The resulting nucleic acid molecule(s) can be used to screen a cDNA library or genomic DNA library to isolate a full-length, or substantially full-length, dirigent protein cDNA or gene. Representative library screening conditions are hybridization in 6 X SSC, 5 X Denhardt's, 0.5% SDS at 55-58°C for 12 hours, followed by washing in 2 X SSC, 0.5% SDS at 55-58°C for 30 minutes. An optional further wash can be conducted in 1 X SSC, 0.5% SDS at 55-58°C for 30 minutes, followed, if so desired, by an additional, optional, wash in 0.5 X SSC, 0.5% SDS at 55-58°C for 30 minutes. In another approach, genomic DNA molecules encoding a dirigent protein can be cloned by PCR utilizing either primer PS-6For (SEQ ID NO: 120) or primer PS-2Rev (SEQ TD NO: 121) and an adapter primer that is complementary to one strand of an oligonucleotide adapter ligated to the ends of the genomic DNA fragments that form the target population of nucleic acid molecules in the PCR reaction. Adapter molecules are well known to those of ordinary skill in the art and are described, for example, in Molecular Cloning, A Laboratory Manual (2 nd edition), Sambrook et al (eds), Chapter 8, which chapter is incorporated herein by reference. Table 5 sets forth three, representative PCR cycling regimes that can be utilized in this aspect of the invention to clone genomic DNA molecules encoding a dirigent protein. Representative PCR reaction conditions are set forth in Table 6, except that primers PS-6For (SEQ TD NO: 120) or primer PS-2Rev (SEQ TD NO: 121) and an adapter primer are utilized instead of primers PLR4 forward (SEQ ED NO.-109) and PLR6 reverse (SEQ ED NO:110). Table 5 In yet another approach, nucleic acid molecules encoding a dirigent protein can be cloned by PCR utilizing genomic DNA or cDNA as template and pools of degenerate primers. Exemplary PCR cycling regimes useful in this aspect of the invention are those set forth as Program I in Table 5 above, except that the annealing temperature is 45°C to 50°C. Representative PCR reaction conditions are set forth in Table 6, except that pools of degenerate primers are utilized instead of primers PLR4 forward (SEQ ID NO: 109) and PLR6 reverse (SEQ ID NO: 110). In a further approach, cDNA molecules encoding a dirigent protein can be cloned by initially cloning a target population of cDNA molecules into a vector that includes binding sites for the T3 primer (SEQ ED NO: 116) and the T7 primer (SEQ ID NO: 117) flanking each cDNA insert. The cloned population of cDNA molecules is then utilized as template in a PCR reaction that utilizes either primer PS-6For (SEQ ID NO: 120) or primer PS-2Rev (SEQ ED NO: 121) and either the T3 primer (SEQ ID NO:116) or the T7 primer (SEQ ID NO: 117). A representative PCR cycling regime is set forth as Program I in Table 5 above. Representative PCR reaction conditions are set forth in Table 6, except that primers PS-6For (SEQ ID NO: 120) or primer PS-2Rev (SEQ ED NO: 121) and either the T3 primer (SEQ ED NO: 116) or the T7 primer (SEQ ED NO: 117) are utilized instead of primers PLR4 forward (SEQ ED NO: 109) and PLR6 reverse (SEQ ID NO:l 10). Thus, in one aspect, the present invention provides isolated nucleotide sequences encoding dirigent proteins, the nucleotide sequences being capable of remaining hybridized to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NOS.T2, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105 under wash conditions of 1 X SSC at 58°C The present invention also provides vectors, such as replicable expression vectors, that include one or more nucleic acid molecules of the invention, and host cells that include one or more vectors of the invention. The present invention also provides methods of enhancing the expression of dirigent protein in a host cell. The methods of enhancing the expression of dirigent protein in a suitable host cell include the steps of (a) introducing into the host cell a replicable expression vector that comprises a nucleic acid sequence encoding a dirigent protein, the nucleic acid sequence being capable of remaining hybridized to the antisense complement of a nucleic acid sequence selected from the group consisting of SEQ ID NOS:12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105 under stringent wash conditions, and (b) expressing the encoded dirigent protein. The present invention also provides methods of inhibiting the expression of dirigent protein in a host cell. The methods include the steps of (a) introducing into the host cell a replicable vector that includes a nucleic acid sequence capable of hybridizing to a nucleic acid sequence selected from the group consisting of SEQ ID NOS: 12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105 under stringent wash conditions, and (b) transcribing the nucleic acid sequence. In another aspect, the present invention provides methods of producing optically- pure lignans. The methods include the steps of (a) introducing into a host cell an expression vector that includes a nucleic acid sequence encoding a dirigent protein capable of directing a bimolecular phenoxy coupling reaction to produce an optically pure lignan, the nucleic acid sequence being capable of remaining hybridized to the antisense complement of a nucleic acid sequence selected from the group consisting of SEQ D NOS: 12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105 under stringent wash conditions, (b) expressing the encoded dirigent protein and (c) purifying optically pure lignan from the host cell. EXAMPLE 24 Cloning Additional Nucleic Acid Molecules That Encode a Pinoresinol Lariciresinol Reductase Additional nucleic acid molecules encoding a pinoresinol/lariciresinol reductase can be cloned utilizing a variety of strategies. In one approach, a DNA probe is generated from cDNA or genomic DNA by PCR, and the probe is then used to screen a cDNA or genomic DNA library. Representative PCR reaction conditions are set forth in Table 6 below. The primers utilized are PLR4 forward (SEQ JD NO: 109) and PLR6 reverse (SEQ JD NO: 110) Table 6 A representative PCR thermocycler program useful in this aspect of the invention is set forth in Table 7. Table 7 The DNA fragment generated by the foregoing PCR reactions can be used as a probe to screen a cDNA or genomic DNA library in order to isolate a full-length, or substantially full-length, pinoresinol/lariciresinol reductase clone. Representative hybridization conditions are hybridization in 6 X SSC, 5 X Denhardt's and 0.5 % SDS at 57°C Representative wash conditions are one wash in 4 X SSC, 0.5 % SDS, at room temperature (typically 20°C to 30°C) for 5 minutes, followed by one wash in 2 X SSC, 0.5 % SDS at 57 °C for 20 minutes. An optional further wash can be conducted in 1 X SSC, 0.5% SDS at 57°C for 30 minutes, followed, if so desired, by an additional, optional, wash in 0.5 X SSC, 0.5% SDS at 57°C for 30 minutes. In another approach, DNA molecules encoding a pinoresinol/lariciresinol reductase protein can be cloned from mRNA by RT-PCR, i.e., a PCR reaction that utilizes mRNA as the initial substrate. First-strand DNA is synthesized from the purified mRNA of interest. Purified mRNA (150 ng) is mixed with 1.4 μg linker-primer (5'- CTCGAGTTTTTTTTTTTT-3') (SEQ ID NO: 122) from ZAP-cDNA ® synthesis kit (Stratagene), heated to 70°C for 10 min, and quickly chilled on ice. The mixture of denatured mRNA template and linker-primer (SEQ JD NO: 122) is then mixed with First Strand Buffer (Life Technologies), 10 mM DTT, 0.5 mM each dNTP, and 200 units of Superscript™ H (Life Technologies) in a final volume of 20 μl. The reaction is carried out at 42°C for 50 minutes and then stopped by heating (70°C, 15 min). E. coli Rnase H (1.5 units, 1 μl) is added to the solution and incubated at 37°C for 20 min. The first-strand reaction (2 μl) is next used as the template in 100-μl PCR reactions (10 mM Tris-HCl, pH 9.0, 50 mM KCl, 0.1 % Triton X-100, 1.5 mM MgCl 2 , 0.2 mM each dNTP, and 5 units of Taq DNA polymerase) with primer CR6-NT (5'GCACATAAGAGTATGGATAAG3') (SEQ JD NO:60) (10 pmol) and primer Xhol- Poly(dT) (5'GTCTCGAGTTTTTTTTITrTTTTTTT3')(SEQ D NO:59)(10 pmol). PCR amplification is carried out in a thermocycler as described in Dinkova-Kostova, A.T., et al., J. Biol. Chem. 271:29473-29482 (1996)(which publication is incorporated herein by reference), except for the annealing temperature at 52°C In brief, the PCR amplification regime includes: 35 cycles, each cycle including one minute at 94°C, two minutes at 52°C, and three minutes at 72°C; followed by 5 minutes at 72°C and an indefinite hold at 4°C after the final cycle. PCR products are resolved in 1.3 % agarose gels to reveal a band of about 1,200-bp. The gel-purified PCR product (-50 ng) is then ligated into the pT7Blue T-vector (50 ng) and transformed into competent NovaBlue cells, according to Novagen's instructions. The inserted cDNA is next sequenced using overlapping sequencing primers. The cDNA so obtained can be used to screen cDNA or genomic DNA libraries for full-length, or substantially full-length, pinoresinol/lariciresinol reductase clones. Representative hybridization conditions for screening cDNA or genomic DNA libraries are 6 X SSC, 5 X Denhardt's, 0.5 % SDS at 57-58 °C. Representative wash conditions are one wash in 4 X SSC, 0.5 % SDS at room temperature (typically 20°C to 30°C) for 5 minutes, followed by one wash in 2 X SSC, 0.5 % SDS at 57-58°C for 20 minutes. An optional further wash can be conducted in 1 X SSC, 0.5% SDS at 57-58°C for 30 minutes, followed, if so desired, by an additional, optional, wash in 0.5 X SSC, 0.5% SDS at 57-58°C for 30 minutes. In yet another approach, the entire Forsythia intermedia plr_Fil cDNA (SEQ TD NO:47) can be used as a probe to screen cDNA or genomic DNA libraries for full-length, or substantially full-length, pinoresinol/lariciresinol reductase clones. Representative hybridization conditions are 6 X SSC, 5 X Denhardt's, 0.5 % SDS (Sigma) at 47 °C for3 hours, followed by one wash at 6 X SSC, 0.5 % SDS (Sigma) at room temperature (typically 20°C to 30°C) for 3 minutes, then 4 X SSC, 0.5 % SDS (Sigma) at 47°C for 2 minutes. An optional further wash can be conducted in 1 X SSC, 0.5% SDS at 55°C for 30 minutes, followed, if so desired, by an additional, optional, wash in 0.5 X SSC, 0.5% SDS at 55°C for 30 minutes. Thus, in one aspect the present invention provides isolated nucleotide sequences encoding pinoresinol/lariciresinol reductase proteins, the nucleotide sequences being capable of remaining hybridized to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ TD NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117 under wash conditions of 1 X SSC at 55°C The present invention also provides vectors, such as replicable expression vectors, that include one or more nucleic acid molecules of the invention, and host cells that include one or more vectors of the invention. The present invention also provides methods of enhancing the expression of pinoresinol/lariciresinol reductase protein in a host cell. The methods include the steps of (a) introducing into the host cell a replicable expression vector comprising a nucleic acid sequence encoding a pinoresinol/lariciresinol reductase protein, said nucleic acid sequence being capable of remaining hybridized to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ID NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117, under stringent wash conditions, and (b) expressing the encoded pinoresinol/lariciresinol reductase protein. The present invention also provides methods of inhibiting the expression of pinoresinol/lariciresinol reductase protein in a host cell. The methods include the steps of (a) introducing into the host cell a replicable vector that comprises a nucleic acid sequence capable of remaining hybridized to a nucleic acid sequence selected from the group consisting of SEQ TD NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117 under stringent wash conditions and (b) transcribing said nucleic acid sequence. While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.

Claims: The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 1. An isolated protein from a lignan biosynthetic pathway, said protein being selected from the group consisting of dirigent protein and pinoresinol/lariciresinol reductase. 2. An isolated dirigent protein of Claim 1. 3. An isolated Forsythia dirigent protein of Claim 2. 4. An isolated Forsythia intermedia dirigent protein of Claim 3. 5. An isolated Forsythia intermedia dirigent protein of Claim 3, said protein being selected from the group consisting of SEQ TD NO: 13 and SEQ ED NO: 15. 6. An isolated Tsuga dirigent protein of Claim 2. 7. An isolated Tsuga heterophylla dirigent protein of Claim 6. 8. An isolated Tsuga heterophylla dirigent protein of Claim 7, said protein being selected from the group consisting of SEQ ED NO: 17 and SEQ ED NO: 19. 9. An isolated Thuja dirigent protein of Claim 2. 10. An isolated Thuja plicata dirigent protein of Claim 9. 11. An isolated Thuja plicata dirigent protein of Claim 10, said protein being selected from the group consisting of SEQ TD NOS:21, 23, 29, 31, 33, 35, 78, 87 and 94. 12. An isolated Eucommia dirigent protein of Claim 2. 13. An isolated Eucommia ulmoides dirigent protein of Claim 12. 14. An isolated Eucommia ulmoides dirigent protein of Claim 13, said protein consisting of the amino acid sequence set forth in SEQ TD NO:99. 15. An isolated Schisandra dirigent protein of Claim 2. 16. An isolated Schisandra chinensis dirigent protein of Claim 15. 17. An isolated Schisandra chinensis dirigent protein of Claim 16, said protein consisting of the amino acid sequence set forth in SEQ ED NO: 106. 18. An isolated pinoresinol/lariciresinol reductase protein of Claim 1. 19. An isolated Forsythia pinoresinol/lariciresinol reductase protein of Claim 18. 20. An isolated Forsythia intermedia pinoresinol/lariciresinol reductase protein of Claim 19. 21. An isolated Forsythia intermedia pinoresinol/lariciresinol reductase protein of Claim 20, said protein being selected from the group consisting of SEQ ED NOS:48, 50, 52, 54, 56 and 58. 22. An isolated Tsuga pinoresinol/lariciresinol reductase protein of Claim 18. 23. An isolated Tsuga heterophylla pinoresinol/lariciresinol reductase protein of Claim 22. 24. An isolated Tsuga heterophylla pinoresinol/lariciresinol reductase protein of Claim 23, said protein being selected from the group consisting of SEQ TD NO:70 and SEQ ED NO:72. 25. An isolated Thuja pinoresinol/lariciresinol reductase protein of Claim 18. 26. An isolated Thuja plicata pinoresinol/lariciresinol reductase protein of Claim 25. 27. An isolated Thuja plicata pinoresinol/lariciresinol reductase protein of Claim 26, said protein being selected from the group consisting of SEQ ED NO:62, 64, 66 and 68. 28. An isolated Linum pinoresinol/lariciresinol reductase protein of Claim 18. 29. An isolated Linum usitatissimum pinoresinol/lariciresinol reductase protein of Claim 28. 30. An isolated Linum usitatissimum pinoresinol/lariciresinol reductase protein of Claim 29, said protein consisting of the amino acid sequence set forth in SEQ TD NO: 108. 31. An isolated Schisandra pinoresinol/lariciresinol reductase protein of Claim 18. 32. An isolated Schisandra chinensis pinoresinol/lariciresinol reductase protein of Claim 31. 33. An isolated Schisandra chinensis pinoresinol/lariciresinol reductase protein of Claim 32, said protein consisting of the amino acid sequence set forth in SEQ ID NO:118. 34. An isolated nucleotide sequence capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ ED NOS.T2, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NOS:12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105, under stringent wash conditions. 35. An isolated nucleotide sequence of Claim 34, said isolated nucleotide sequence being capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ TD NO: 12 and SEQ ED NO: 14, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ID NO: 12 and SEQ TD NO: 14, under stringent wash conditions. 36. An isolated nucleotide sequence of Claim 35 wherein said nucleotide sequence encodes an amino acid sequence selected from the group consisting of SEQ TD NO: 13 and SEQ TD NO: 15. 37. An isolated nucleotide sequence of Claim 36 consisting of a nucleotide sequence selected from the group consisting of SEQ TD NO: 12 and SEQ ED NO: 14. 38. An isolated nucleotide sequence of Claim 34, said isolated nucleotide sequence being capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ ED NO: 16 and SEQ ED NO: 18, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NO:16 and SEQ TD NO:18, under stringent wash conditions. 39. An isolated nucleotide sequence of Claim 38 wherein said nucleotide sequence encodes an amino acid sequence selected from the group consisting of SEQ ID NO:17 and SEQ D NO:19. 40. An isolated nucleotide sequence of Claim 38 consisting of a nucleotide sequence selected from the group consisting of SEQ TD NO:16 and SEQ ED NO:18. 41. An isolated nucleotide sequence of Claim 34, said isolated nucleotide sequence being capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ TD NOS:20, 22, 28, 30, 32, 34, 77, 86 and 93, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NOS:20, 22, 28, 30, 32, 34, 77, 86 and 93, under stringent wash conditions. 42. An isolated nucleotide sequence of Claim 41 wherein said nucleotide sequence encodes an amino acid sequence selected from the group consisting of SEQ D NOS:21, 23, 29, 31, 33, 35, 78, 87 and 94. 43. An isolated nucleotide sequence of Claim 41 consisting of a nucleotide sequence selected from the group consisting of SEQ ED NOS:20, 22, 28, 30, 32, 34, 77, 86 and 93. 44. An isolated nucleotide sequence of Claim 34, said isolated nucleotide sequence being capable of remaining hybridized to the nucleotide sequence of SEQ ED NO:98, or to the antisense complement of the nucleotide sequence of SEQ ED NO:98, under stringent wash conditions. 45. An isolated nucleotide sequence of Claim 44 wherein said nucleotide sequence encodes the amino acid sequence of SEQ ED NO:99. 46. An isolated nucleotide sequence of Claim 44 wherein said nucleotide sequence consists of the nucleotide sequence of SEQ TD NO:98. 47. An isolated nucleotide sequence of Claim 34, said isolated nucleotide sequence being capable of remaining hybridized to the nucleotide sequence of SEQ TD NO: 105, or to the antisense complement of the nucleotide sequence of SEQ TD NO: 105, under stringent wash conditions. 48. An isolated nucleotide sequence of Claim 47 wherein said nucleotide sequence encodes the amino acid sequence of SEQ TD NO: 106. 49. An isolated nucleotide sequence of Claim 47 wherein said nucleotide sequence consists of the nucleotide sequence of SEQ ED NO: 105. 50. An isolated nucleotide sequence, said isolated nucleotide sequence being capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ ED NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ID NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117, under stringent wash conditions. 51. An isolated nucleotide sequence of Claim 50, said isolated nucleotide sequence being capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ ED NOS:47, 49, 51, 53, 55, and 57, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NOS:47, 49, 51, 53, 55, and 57, under stringent wash conditions. 52. An isolated nucleotide sequence of Claim 51 wherein said nucleotide sequence encodes an amino acid sequence selected from the group consisting of SEQ TD NOS:48, 50, 52, 54, 56, and 58. 53. An isolated nucleotide sequence of Claim 51 consisting of a nucleotide sequence selected from the group consisting of SEQ ED NOS:47, 49, 51, 53, 55,and 57. 54. An isolated nucleotide sequence of Claim 50, said isolated nucleotide sequence being capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ TD NOS:61, 63, 65 and 67, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NOS:61, 63, 65 and 67, under stringent wash conditions. 55. An isolated nucleotide sequence of Claim 54 wherein said nucleotide sequence encodes an amino acid sequence selected from the group consisting of SEQ ED NOS:62, 64, 66 and 68. 56. An isolated nucleotide sequence of Claim 54 consisting of a nucleotide sequence selected from the group consisting of SEQ TD NOS:61, 63, 65 and 67. 57. An isolated nucleotide sequence of Claim 50, said isolated nucleotide sequence being capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ TD NO:69 and SEQ ID NO:71, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ TD NO:69 and SEQ ED NO:71, under stringent wash conditions. 58. An isolated nucleotide sequence of Claim 57 wherein said nucleotide sequence encodes an amino acid sequence selected from the group consisting of SEQ TD NO:70 and SEQ TD NO:72. 59. An isolated nucleotide sequence of Claim 57 consisting of a nucleotide sequence selected from the group consisting of SEQ TD NO:69 and SEQ ED NO:71. 60. An isolated nucleotide sequence of Claim 50, said isolated nucleotide sequence being capable of remaining hybridized to the nucleotide sequence of SEQ TD NO:107, or to the antisense complement of the nucleotide sequence of SEQ TD NO: 107, under stringent wash conditions. 61. An isolated nucleotide sequence of Claim 60 wherein said nucleotide sequence encodes the amino acid sequence of SEQ ED NO:108. 62. An isolated nucleotide sequence of Claim 60 consisting of the nucleotide sequence of SEQ ID NO: 107. 63. An isolated nucleotide sequence of Claim 50, said isolated nucleotide sequence being capable of remaining hybridized to the nucleotide sequence of SEQ TD NO: 117, or to the antisense complement of the nucleotide sequence of SEQ ED NO: 117, under stringent wash conditions. 64. An isolated nucleotide sequence of Claim 63 wherein said nucleotide sequence encodes the amino acid sequence of SEQ ED NO:l 18. 65. An isolated nucleotide sequence of Claim 63 consisting of the nucleotide sequence of SEQ TD NO: 117. 66. A double stranded nucleic acid molecule encoding a dirigent protein, said double stranded nucleic acid molecule comprising a coding strand and a complementary non-coding strand, wherein the coding strand hybridizes to the oligonucleotide molecule of SEQ ID NO: 120 in 50 mM KCL at 60°C, and the non-coding strand hybridizes to the oligonucleotide molecule of SEQ TD NO: 121 in 50 mM KCL at 60°C 67. A replicable vector comprising a nucleotide sequence capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ ED NOS:12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ TD NOS:12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105, under stringent wash conditions. 68. A replicable vector of Claim 67 wherein said nucleotide sequence is capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ ID NO: 12 and SEQ ED NO: 14, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NO: 12 and SEQ ED NO: 14, under stringent wash conditions. 69. A replicable vector of Claim 67 wherein said nucleotide sequence is capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ TD NO:16 and SEQ ED NO:18, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ TD NO: 16 and SEQ TD NO: 18, under stringent wash conditions. 70. A replicable vector of Claim 67 wherein said nucleotide sequence is capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ ED NOS:20, 22, 28, 30, 32, 34, 77, 86 and 93, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ TD NOS:20, 22, 28, 30, 32, 34, 77, 86 and 93, under stringent wash conditions. 71. A replicable vector of Claim 67 wherein said nucleotide sequence is capable of remaining hybridized to the nucleotide sequence of SEQ ED NO:98, or to the antisense complement of the nucleotide sequence of SEQ ED NO:98, under stringent wash conditions. 72. A replicable vector of Claim 67 wherein said nucleotide sequence is capable of remaining hybridized to the nucleotide sequence of SEQ ED NO: 105, or to the antisense complement of the nucleotide sequence of SEQ ED NO:98, under stringent wash conditions. 73. A replicable vector comprising a nucleotide sequence capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ ED NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117, under stringent wash conditions. 74. A replicable vector of Claim 73 wherein said nucleotide sequence is capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ TD NOS:47, 49, 51, 53, 55, and 57, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NOS:47, 49, 51, 53, 55, and 57, under stringent wash conditions. 75. A replicable vector of Claim 73 wherein said nucleotide sequence is capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ TD NOS:61, 63, 65 and 67, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ TD NOS:61, 63, 65 and 67, under stringent wash conditions. 76. A replicable vector of Claim 73 wherein said nucleotide sequence is capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ TD NO:69 and SEQ ED NO:71, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NO:69 and SEQ ED NO:71, under stringent wash conditions. 77. A replicable vector of Claim 73 wherein said nucleotide sequence is capable of remaining hybridized to the nucleotide sequence of SEQ ID NO: 107, or to the antisense complement of the nucleotide sequence of SEQ TD NO: 107, under stringent wash conditions. 78. A replicable vector of Claim 73 wherein said nucleotide sequence is capable of remaining hybridized to the nucleotide sequence of SEQ TD NO: 117, or to the antisense complement of the nucleotide sequence of SEQ ID NO: 117, under stringent wash conditions. 79. A host cell comprising a replicable vector comprising a nucleotide sequence capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ TD NOS:12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NOS:12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105, under stringent wash conditions. 80. A host cell of Claim 79 wherein said nucleotide sequence is capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ ID NO: 12 and SEQ ID NO: 14, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ID NO: 12 and SEQ ID NO: 14, under stringent wash conditions. 81. A host cell of Claim 79 wherein said nucleotide sequence is capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ TD NO: 16 and SEQ TD NO: 18, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NO: 16 and SEQ DD NO: 18, under stringent wash conditions. 82. A host cell of Claim 79 wherein said nucleotide sequence is capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ TD NOS:20, 22, 28, 30, 32, 34, 77, 86 and 93, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ TD NOS:20, 22, 28, 30, 32, 34, 77, 86 and 93, under stringent wash conditions. 83. A host cell of Claim 79 wherein said nucleotide sequence is capable of remaining hybridized to the nucleotide sequence of SEQ ED NO:98, or to the antisense complement of the nucleotide sequence of SEQ ID NO:98, under stringent wash conditions. 84. A host cell of Claim 79 wherein said nucleotide sequence is capable of remaining hybridized to the nucleotide sequence of SEQ ID NO: 105, or to the antisense complement of the nucleotide sequence of SEQ ID NO: 105, under stringent wash conditions. 85. A host cell comprising a replicable vector comprising a nucleotide sequence capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ ED NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ TD NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117, under stringent wash conditions. 86. A host cell of Claim 85 wherein said nucleotide sequence is capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ ID NOS:47, 49, 51, 53, 55, and 57, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NOS:47, 49, 51, 53, 55, and 57, under stringent wash conditions. 87. A host cell of Claim 85 wherein said nucleotide sequence is capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ ED NOS:61, 63, 65 and 67, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NOS:61, 63, 65 and 67, under stringent wash conditions. 88. A host cell of Claim 85 wherein said nucleotide sequence is capable of remaining hybridized to a nucleotide sequence selected from the group consisting of SEQ ED NO:69 and SEQ ED NO:71, or to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NO:69 and SEQ ED NO:71, under stringent wash conditions. 89. A host cell of Claim 85 wherein said nucleotide sequence is capable of remaining hybridized to the nucleotide sequence of SEQ ID NO: 107, or to the antisense complement of the nucleotide sequence of SEQ ID NO: 107, under stringent wash conditions. 90. A host cell of Claim 85 wherein said nucleotide sequence is capable of remaining hybridized to the nucleotide sequence of SEQ TD NO: 117, or to the antisense complement of the nucleotide sequence of SEQ ID NO: 117, under stringent wash conditions. 91. A method of enhancing the expression of pinoresinol/lariciresinol reductase in a suitable host cell comprising introducing into the host cell a replicable expression vector comprising a nucleic acid sequence encoding a pinoresinol/lariciresinol reductase protein, said nucleic acid sequence being capable of remaining hybridized to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ TD NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117, under stringent wash conditions, and expressing said encoded pinoresinol/lariciresinol reductase protein. 92. A method of inhibiting the expression of pinoresinol/lariciresinol reductase in a suitable host cell comprising introducing into the host cell a replicable vector that comprises a nucleic acid sequence capable of remaining hybridized to a nucleic acid sequence selected from the group consisting of SEQ ED NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117 under stringent wash conditions and transcribing said nucleic acid sequence. 93. A method of enhancing the expression of dirigent protein in a suitable host cell comprising introducing into the host cell a replicable expression vector that comprises a nucleic acid sequence encoding a dirigent protein, said nucleic acid sequence being capable of remaining hybridized to the antisense complement of a nucleic acid sequence selected from the group consisting of SEQ ID NOS:12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105, under stringent wash conditions, and expressing said encoded dirigent protein. 94. A method of inhibiting the expression of dirigent protein in a suitable host cell comprising introducing into the host cell a replicable vector that comprises a nucleic acid sequence capable of hybridizing to a nucleic acid sequence selected from the group consisting of SEQ ID NOS:12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105 under stringent wash conditions and transcribing said nucleic acid sequence. 95. A method of producing optically-pure lignans comprising: (a) introducing into a host cell an expression vector that comprises a nucleic acid sequence encoding a dirigent protein capable of directing a bimolecular phenoxy coupling reaction to produce an optically pure lignan, said nucleic acid sequence being capable of remaining hybridized to the antisense complement of a nucleic acid sequence selected from the group consisting of SEQ TD NOS:12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105 under stringent wash conditions; (b) expressing said encoded dirigent protein; and (c) purifying optically pure lignan from the host cell. 96. The method of Claim 95 wherein the nucleic acid sequence encoding a dirigent protein is selected from the group consisting of SEQ ID NOS: 12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105. 97. An isolated nucleotide sequence encoding a dirigent protein, said isolated nucleotide sequence being capable of remaining hybridized to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ED NOS: 12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 77, 86, 93, 98 and 105 under wash conditions of 1 X SSC at 58°C. 98. An isolated nucleotide sequence encoding a pinoresinol/lariciresinol reductase- protein, said isolated nucleotide sequence being capable of remaining hybridized to the antisense complement of a nucleotide sequence selected from the group consisting of SEQ ID NOS:47, 49, 51, 53, 55, 57, 61, 63, 65, 67, 69, 71, 107 and 117 under wash conditions of 1 X SSC at 55°C

Inventors: DAVIN LAURENCE B, DINKOVA-KOSTOVA ALBENA T, FORD JOSHUA D, FUJITA MASAYUKI, GANG DAVID R, LEWIS NORMAN G, SARKANEN SIMO, DAVIN, LAURENCE, B., DINKOVA-KOSTOVA, ALBENA, T., FORD, JOSHUA, D., FUJITA, MASAYUKI, GANG, DAVID, R., LEWIS, NORMAN, G., SARKANEN, SIMO
Applicants: DAVIN LAURENCE B, DINKOVA KOSTOVA ALBENA T, FORD JOSHUA D, FUJITA MASAYUKI, GANG DAVID R, LEWIS NORMAN G, SARKANEN SIMO, UNIV MINNESOTA, UNIV WASHINGTON, DAVIN, LAURENCE, B., DINKOVA-KOSTOVA, ALBENA, T., FORD, JOSHUA, D., FUJITA, MASAYUKI, GANG, DAVID, R., LEWIS, NORMAN, G., REGENTS OF THE UNIVERSITY OF MINNESOTA, SARKANEN, SIMO, WASHINGTON STATE UNIVERSITY RESEARCH FOUNDATION
Application year: 2000
Main classification: C12N9
IPC codes: C07K14, C12N1, C12N5, C12N9, C12N15, C12P7, C12P17