Molecular cloning and functional expression of cDNAs encoding oxidosqualene cyclases from Costus speciosus

Costus speciosus produces a large quantity of steroidal glycosides derived from the sole aglycone, diosgenin. Cycloartenol, a product of oxidosqualene cyclase (OSC), is postulated to be a common intermediate for phytosterols of primary metabolism and diosgenin of secondary metabolism, possibly providing a metabolic branch point. Two cDNAs, CSOSC1 and CSOSC2, were cloned from C. speciosus by RT-PCR and cDNA library screening. Both cDNAs encode 759 amino acids with high mutual identity (74%), resembling (>55% identity) the known OSCs. Phylogenetic tree analysis indicated that the gene products occupy distinct positions from those of cycloartenol synthases (CASs) and triterpene synthases from dicotyledonous plants. By functional expression in yeast, CSOSC1 and CSOSC2 were proved to encode a CAS and a multifunctional triterpene synthase, respectively. The present result is the first demonstration of the functional expression of OSCs from monocotyledonous plants.