Biochemical and Genetic Basis of Guanacastane Diterpene Biosynthesis in Basidiomycete Fungi

Guanacastane
diterpenoids with an unusual 5/7/6 tricyclic skeleton
mainly produced by basidiomycete fungi represent a structurally intriguing
class of natural products. While the chemical synthesis of several
members has been achieved, the biochemical and genetic basis of their
biosynthesis remain unknown. Herein, we present the identification
and characterization of two crucial enzymes in the biosynthesis of
guanacastane diterpenoids in Psathyrella candolleana. Heterologous expression reveals that PsaD, a typical class I diterpene
synthase, catalyzes the cyclization of geranylgeranyl diphosphate
to form a new guanacastane-type diterpene, guanacasta-1,3-diene (7). Moreover, we demonstrate that PsaA, a cytochrome P450
monooxygenase, can catalyze multiple oxidations of 7 to
yield guanacastepene U (8). These results provide new
opportunities for genome mining and metabolic engineering of guanacastane
diterpenoids.