title: Isolation and characterization of an oxidosqualene cyclase gene encoding a β-amyrin synthase involved in Polygala tenuifolia Willd. saponin biosynthesis.
abstract: CONCLUSIONS
  Expression of PtBS (Polygala tenuifolia β-amyrin synthase) led to the production of β-amyrin as sole product.
  Polygala tenuifolia Willdenow is a rich source of triterpene saponins, onjisaponins and polygalasaponins, used as herbal medicine to treat phlegms and for detumescence in traditional Asian healing. The Polygala saponins share the oleanane backbone structure and are, therefore, likely synthesized via β-amyrin as a common precursor. We hypothesized that, in analogy to diverse other plant species, this central intermediate should be formed by a β-amyrin synthase catalyzing the complex cyclization of oxidosqualene. This member of the oxidosqualene cyclase (OSC) family of enzymes is thus defining an important branch point between primary and secondary metabolisms, and playing a crucial role in the control of oleanane-type triterpene saponin biosynthesis. From P. tenuifolia roots, we isolated an OSC cDNA containing a reading frame of 2,289 bp nucleotides. The predicted protein of 763 amino acids (molecular weight 87.353 kDa) showed particularly high amino acid sequence identities to known β-amyrin synthases (85-87 %) and was, therefore, named PtBS. Expression of PtBS in the triterpenoid synthase-deficient yeast mutant GIL77 led to the production of β-amyrin as sole product. qRT-PCR analysis of various P. tenuifolia organs showed that PtBS transcript levels were highest in the roots, consistent with onjisaponin accumulation patterns. Therefore, we conclude that PtBS is the β-amyrin synthase enzyme catalyzing the first committed step in the biosynthesis of onjisaponins and polygalasaponins in P. tenuifolia.
authors.affiliation: Department of Herbal Crop Research, National Institute of Horticultural and Herbal Science, RDA, Eumseong, 369-873, South Korea.
authors.forename: Mei Lan
authors.initials: ML
authors.lastname: Jin
authors.forename: Dae Young
authors.initials: DY
authors.lastname: Lee
authors.forename: Yurry
authors.initials: Y
authors.lastname: Um
authors.forename: Jeong Hoon
authors.initials: JH
authors.lastname: Lee
authors.forename: Chun Geun
authors.initials: CG
authors.lastname: Park
authors.forename: Reinhard
authors.initials: R
authors.lastname: Jetter
authors.forename: Ok Tae
authors.initials: OT
authors.lastname: Kim
mesh_terms: D019751:Intramolecular Transferases / Q000378:metabolism*; D031801:Polygala / Q000737:chemistry*; D012503:Saponins / Q000096:biosynthesis*
chemical_list: D012503:Saponins; D019751:Intramolecular Transferases; C039924:2,3-oxidosqualene-beta-amyrin-cyclase; C021924:lanosterol synthase