Stemarene

Abstract: We have cloned a novel diterpene synthase (OsKSL11) from rice that produces stemod-13(17)-ene from syn-copalyl diphosphate. Notably, this gene sequence was not predicted from the extensive sequence information available for rice, nor, despite extensive phytochemical investigations, has this diterpene or any derived natural product previously been reported in rice plants. OsKSL11 represents the first identified stemodene synthase, which catalyzes the committed step in biosynthesis of the stemodane family of diterpenoid natural products, some of which possess antiviral activity. In addition, OsKSL11 is highly homologous to the mechanistically similar stemarene synthase recently identified from rice, making this pair of diterpene cyclases an excellent model system for investigating the enzymatic determinants for differential product outcome. The unexpected nature of this cyclase and its product parallels recent observations of previously unrecognized natural products metabolism in Arabidopsis thaliana, suggesting that many, if not all, plant species will prove to have extensive biosynthetic capacity. Introduction The rice (Oryza sativa) draft genome sequences have provided researchers with a valuable tool for investigating the biochemical machinery of this vital cereal crop plant [1,2]. One area of particular interest is the production of natural products, which govern essentially all aspects of plant biology, ranging from growth and development to defense [3]. Many of these low-molecular weight organic compounds are terpenoids, which comprise the largest class of natural products with nearly 50,000 known members [4]. The labdane-related diterpenoids, characterized by minimally containing the labdane bicyclic core structure,