The structures of cyclic triterpenoids of higher plants, whose skeletons are constructed by oxidosqualene cyclases (OSCs), are remarkably diverse in contrast to the limited variation in sterol/triterpenoid structures of other organisms. We comprehensively analyzed the functions and structures of OSC genes of a model legume, Lotus japonicus, and compared them with those of Arabidopsis thaliana in order to elucidate the diversification process of plant cyclic triterpenoids. Eight OSC genes (OSC1-OSC8) formed two sets of clusters in chromosomes 2 and 3 of L. japonicus. OSC1 and OSC8 corresponded to the previously reported LjAMY1 and LjAMY2, respectively [I. Iturbe-Ormaetxe, K. Haralampidis, K. Papadopoulou, A.E. Osbourn, Molecular cloning and characterization of triterpene synthases from Medicago truncatula and Lotus japonicus, Plant Mol. Biol. 51 (2003) 731-743]. OSC1 (the full-length version of LjAMY1), OSC3 and OSC5 were identified as β-amyrin synthase, lupeol synthase and cycloartenol synthase, respectively, by heterologous expression of the cDNAs in an OSC-disrupted yeast mutant. A phylogenetic tree based on deduced amino acid sequences categorized eudicot OSCs into four groups and suggested an extensive diversification of the group IV OSCs.
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