Sesame (Sesamum indicum) is an annual plant in the family Pedaliaceae belonging to the genus Sesamum. Sesame is said to be indigenous to Central Africa. Supposedly sesame is the oldest domesticated oil seed crop having about a 6000 year history and has been cultivated throughout the world. Sesame is a valuable food from ancient times and known as representing healthy foods. In particular, sesame seeds, oil pressed from sesame seeds and extracts from sesame seeds are utilized (see, e.g., Goma: SONO-KAGAKU-TO-KINOSEI (Sesame: Science and Function), edited by Mitsuo Namiki, Maruzen Planet Publishing Co. (1998)). The components contained in sesame seeds are about 50% of lipids and about 20% of proteins. The major components of lipids contained in sesame are essentially triglycerides mainly composed of oleic acid and linoleic acid. Furthermore, sesame contains vitamins B1, B2, E, etc. In addition to the components described above, secondary metabolites (e.g., sesamin, sesamolin, etc.) of plants collectively referred to as lignans are contained in sesame, and a sesame lignan is a distinctive component in sesame seeds (see, e.g., Bedigian, D., et al., Biochemical Systematics and Ecology, 13, 133-139 (1985)).
Lignans are compounds in which two phenylpropanoid molecules having the C6C3 skeleton are dimerized mostly through the 8-8′ position (a 8,8′-linkage). Lignans are considered to contribute to biological defense mechanisms in plants.
Representative lignans include sesamin, sesaminol, sesamolin and sesamolinol contained in sesame (Sesamum indicum); (+)-pinoresinol, (−)-arctigenin and (−)-matairesinol contained in Forsythia intermedia; (−)-pinoresinol and (−)-lariciresinol contained in Daphne tangutica; (+)-secoisolariciresinol contained in Linum usitatissimum; etc. Molecular structures of these lignans are diverse.
Sesamin, which is one of sesame lignans, displays an abundance of biological activities and are effective for improving cholesterol metabolism, liver function and immune function (see, e.g., Goma: SONO-KAGAKU-TO-KINOSEI (Sesame: Science and Function), edited by Mitsuo Namiki, Maruzen Planet Publishing Co. (1998)). Methods for the separation and purification of sesamin from sesame seeds or sesame lees have already been launched (see, e.g., Japanese Patent Laid-open Publication (Kokai) No. 2001-139579 (laid open to public inspection on May 22, 2001) and Japanese Patent Laid-open Publication (Kokai) No. 10-7676 (laid open to public inspection on Jan. 13, 1998)), and sesamin-based liver function improvers/potentiators having an alcoholysis-promoting activity are commercially available (trade name: Sesamin, from sales agency Suntory, Ltd.). It is reported that lignans other than sesamin (see, e.g., sesaminol, sesamolin, etc.) also have biological activities (see, e.g., J. Bioscience, Biotechnology and Biochemistry, 76: 805-813 (2002)).
As to biosynthesis of lignans, reference is made to, e.g., Lignans: Biosynthesis and Function, Comprehensive Natural Products Chemistry, 1: 640-713 (1999). It is shown in Lignans: Biosynthesis and Function, Comprehensive Natural Products Chemistry, 1: 640-713 (1999) that pinoresinol synthesized by polymerization of coniferyl alcohol is the first lignan in the biosynthesis and a variety of lignans are synthesized from pinoresinol via biosynthetic pathways specific to individual plant species. General biosynthesis of lignans is illustratively shown in the schematic diagram below.

Piperitol synthase acts on (+)-pinoresinol to synthesize piperitol. Next, sesamin synthase acts on this piperitol to synthesize sesamin.
As the enzymes involved in the biosynthesis of lignans, dirigent proteins which take part in pinoresinol synthesis are reported in Forsythia intermedia, etc. (see, e.g., Plant Physiol., 123: 453 (2000) and Japanese National Phase PCT Laid-open Publication No. 2001-507931 (laid open to public inspection on Jun. 19, 2001)). As genes for enzymes involved in the lignan biosynthesis and their utilization, there are further reported the gene for pinoresinol/lariciresinol reductase in Forsythia intermedia (see, e.g., J. Biol. Chem., 271:29473 (1996) and Japanese National Phase PCT Laid-open Publication No. 2001-507931 (laid open to public inspection on Jun. 19, 2001)), the gene for pinoresinol/lariciresinol reductase in Thuja plicata (see, e.g., J. Biol. Chem., 274: 618 (1999) and recombinant secoisolariciresinol dehydrogenase and the method of its use (see, e.g., J. Biol. Chem., 276 (16): 12614-23 (2001) and Japanese National Phase PCT Laid-open Publication No. 2002-512790 (laid open to public inspection on May 18, 2002)). Besides, the gene for larreatricin hydroxylase has been cloned from Larrea tridentata (see, e.g., Proc. Nat. Acad. Sci. USA, 100: 10641 (2003)).
Regarding the biosynthesis of sesame lignans, it is also reported that exhaustive analysis of about 3000 clones of the gene expressed in sesame seeds gave dirigent proteins, seed storage proteins and gene fragments associated with lipid synthesis (see, e.g., Plant Mol. Biol., 52: 1107 (2003)). Moreover, the gene involved in fatty acid synthesis (see, e.g., Biosci. Biotechnol. Biochem., 66(10): 2146-53 (2002), Plant Physiol., 128 (4): 1200-11 (2002) and Plant Cell Physiol., 37(2): 201-5 (1996)), and the gene for seed storage protein, etc. are cloned from sesame.
In recent years, attention has been drawn not only to lignans but to lignan glycosides. It is known that some of the lignan molecules described above are present as glycosides in plants. For instance, sesaminol glycosides (sesaminol 2′-O-β-D-glucopyranoside; sesaminol 2′-O-β-D-glucopyranosyl (1-2)-O-β-D-glucopyranoside; and sesaminol 2′-O-β-D-glucopyranosyl (1-2)-O-(-β-D-glucopyranosyl(1-6))-β-D-glucopyranoside)), and pinoresinol glycosides (pinoresinol 4′-O-β-D-glucopyranosyl (1-6)-β-D-glucopyranoside; pinoresinol 4′-O-β-D-glucopyranosyl (1-2)-β-D-glucopyranoside; pinoresinol 4′-O-β-D-glucopyranosyl (1-6)-O-(β-D-glucopyranosyl (1-6)) β-D-glucopyranoside; and pinoresinol di-O-β-D-glucopyranoside)), etc. are present in sesame seeds; (+)-pinoresinol 4′-O-β-D-glucoside and (−)-matairesinol-4-O-glucoside, etc. are present in Forsythia intermedia; and secolariciresinol diglucoside and pinoresinol diglucoside, etc. are present in Linum usitatissimum (see, e.g., Goma: SONO-KAGAKU-TO-KINOSEI (Sesame: Science and Function), edited by Mitsuo Namiki, Maruzen Planet Publishing Co. (1998), Journal of Natural Medicines, 32, 194 (1978), Tetrahedron, 14: 649 (2003) and Phytochemistry, 58: 587 (2001)).
Glycosides are produced through the glycosidation (glycosyl transfer) reaction of various compounds such as flavonoids being catalyzed by enzymes called glycosidases or glycosyltransferases. To date, the amino acid sequences of some glycosyltransferases and their functions have been elucidated. Genes for an enzyme (UDP-glucose: flavonoid 3-glucosyltransferase) which catalyzes a reaction to transfer a sugar onto the hydroxyl group at the position 3 of flavonoids or anthocyanidins have been obtained from maize, gentian, grapevine, etc. (see, e.g., J. Biol. Chem., 276:4338 (2001)). Also, genes for an enzyme (UDP-glucose: anthocyanidin 5-glucosyltransferase) which catalyzes a reaction to transfer a sugar onto the hydroxyl at the position 5 of anthocyanidins have been obtained from perilla, verbena, etc. (see, e.g., J. Biol. Chem. 274:7405 (1999)).
Pinoresinol glycosides and sesaminol glycosides contained in sesame (see, e.g., Goma: SONO-KAGAKU-TO-KINOSEI (Sesame: Science and Function), edited by Mitsuo Namiki, Maruzen Planet Publishing Co. (1998) and Katsuzaki, H. et al., Biosci. Biotech. Biochem., 56, 2087-2088 (1992)) show potent antioxidative properties in the water-soluble region, and are expected to yield different applications than lipophilic antioxidants (e.g., tocopherol). Also, the following mechanism of action is proposed for lignan glycosides. In lignan glycosides, the phenolic hydroxyl group, which is a functional group exhibiting antioxidative properties, are protected by sugars possessed by themselves but taken up into the body and then hydrolyzed by the action of β-glucosidase from enterobacteria to produce lipophilic lignans as the aglycone portion. This aglycone is absorbed into the intestines and carried to various organs via blood to prevent oxidative damages in biomembranes of the organs, etc. Based on this mechanism of action, lignan glycosides are expected to involve applications as preventive diets for arteriosclerosis (see, e.g., T. Osawa: Anticarcinogenesis and Radiation Protection 2: p. 327, Plenum Press, New York (1991)).
The biosynthetic pathway of secondary metabolites in plants is modified to produce useful substances and/or breed useful plants. Such a technology is called metabolic engineering. Use of such a technology enables to produce optional compounds in a large scale and/or prevent the production of unwanted substances. Accordingly, it is industrially useful to synthesize lignans and their glycosides by metabolic engineering using the genes involved in the biosynthesis of lignans and their glycosides, in view of the utility of these substances as described above. However, findings on the genes involved in the biosynthesis of lignans, especially sesame lignans are so limited as described above, and any glycosidase that catalyzes the production of lignan glycosides is not found. It has thus been desired to acquire additional genes.