Biosynthesis of Sesqui- and Diterpenes by the Gibberellin Producer Fusarium fujikuroi

The fungus Fusarium fujikuroi IMI58289 emits a complex pattern of volatile terpenoids including two major compounds, the sesquiterpene alcohol a-acorenol and the diterpene ent-kaurene. ent-Kaurene is the precursor for the phytohormone gibberellic acid (GA3) and is produced from geranylgeranyl diphosphate (GGPP) via ent-copalyl diphosphate by the bifunctional ent-copalyl diphosphate/ent-kaurene synthase (CPS/KS). Several structurally related diterpenes were identified as side products of the CPS/KS. Deletion of the cps/ks gene or the whole GA3 biosynthetic gene cluster resulted in completely abolished diterpene production. Mutants with deletions of the cytochrome P450 monooxygenase gene P450-4, which is responsible for the three oxidation steps from ent-kaurene to ent-kaurenoic acid en route to GA3, accumulate diterpene hydrocarbons. Feeding with [6,6,6-2H3] mevalonolactone gave insights into the stereochemistry of the GGPP cyclisation, which operates with a chairchair-antipodal fold. A rational biosynthetic scheme for all identified sesquiterpenes demonstrated their formation from farnesyl diphosphate (FPP) via three alternative initial cyclisations. Genome sequencing revealed the presence of five putative sesquiterpene synthase genes in the F. fujikuroi genome. The structures of several trace compounds from other classes have been identified as new natural products; these were delineated from their mass spectra and unambiguously assigned by comparison to synthetic references.