Abstract
In this study, mono- and pentacyclic C_<35> terpenes from Bacillus subtilis were biosynthesized via the cyclization of C_<35> isoprenoid by using purified enzymes, including the first identified new terpene cyclase, tetrapreny1-13-curcumene synthase, that shows no sequence homology to any of the known terpene cyclases. Based on these findings, we propose that these C_<35> terpenes are called the new family of "sesquarterpenes." This study demonstrated that a tetrapreny1-β-curcumene cyclase (TC) from B. subtilis, which was originally identified as a sesquarterpene cyclase converting a head-to-tail type of monocycle to a pentacycle, also cyclized a tail-to-tail type of linear squalene into a bicyclic triterpenol, 8α-Hydroxypolypoda-13,17,21-triene. The 8α-Hydroxypolypoda-13,17,21-triene was found to be a natural triterpene from B. megaterium, suggesting that the TC is bifunctional, cyclizing both tetrapreny1-α-curcumene and squalene in vivo. This is the first report describing the bifunctional terpene cyclase, which biosynthesizes 2 classes of cyclic terpenes with different numbers of carbons as natural products in the organism. Non-pathogenic Mycobacterium species also produce cyclic sesquarterpenes, which are biosynthesized via cyclization of Z-type C_<35> polyprenyl diphosphate. To provide deeper insight into the biosynthesis of sesquarterpenes, we carried out functional analyses of three Z-prenyltransferase homologues in M vanbaalenii identified by genomic analysis. Mvan_3822, a novel bi-functional Z-prenyltransferase, biosynthesizes C_<35>-heptaprenyl diphosphate as a main product from E,E-FPP and E,E,E-GGPP, but produces a C_<50>-decaprenyl diphosphate from GPP. Mvan_1705 is a novel Z,E,E-GGPP synthase. In addition, novel cyclic C_<35>-terpenes, 14E- and 14Z-dehydroheptaprenylcycline, were identified as minor metabolites in non-pathogenic Mycobacterium cells. Sesquarterpenes could be biosynthesized by two routes, in which E- and Z-geometric isomers of heptaprenyl diphosphate are produced from E,E-FPP and E,E,E-GGPP, and the prenylreductase responsible for the biosynthesis of sesquarterpenes may work to reduce both E- and Z-prenyl residues. The studies on sesquarterpenes promise to be an attractive field for expanding our understanding of the terpene world.
