Cucurbitadienol synthase, the first committed enzyme for cucurbitacin biosynthesis, is a distinct enzyme from cycloartenol synthase for phytosterol biosynthesis

Three oxidosqualene cyclase (OSC) cDNAs (CPX, CPQ, CPR) were cloned from seedlings of Cucurbita pepo by homology based PCR method. Their open reading frames were expressed in lanosterol synthase deficient (erg7) Saccharomyces cerevisiae strain GIL77. Analyses of in vitro enzyme activities and in vivo accumulated products in the transformants demonstrated that CPQ and CPX encode cucurbitadienol and cycloartenol synthases, respectively. These results indicated the presence of distinct OSCs for cycloartenol and cucurbitadienol synthesis in this plant.