Genome
Mining for Sesterterpenes Using Bifunctional
Terpene Synthases Reveals a Unified Intermediate of Di/Sesterterpenes
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Abstract
Genome mining is a promising method
to discover novel secondary
metabolites in the postgenomic era. We applied the Aspergillus oryzae heterologous expression system
to functionally characterize cryptic bifunctional terpene synthase
genes found in fungal genomes and identified the sesterfisherol synthase
gene (<i>NfSS</i>) from Neosartorya fischeri. Sesterfisherol contains a characteristic 5–6–8–5
tetracyclic ring system and is modified by cytochrome P450 monooxygenase
(NfP450) to sesterfisheric acid. The cyclization mechanism was proposed
on the basis of the analysis of in vivo and in vitro enzymatic reactions
with isotopically labeled precursors. The mechanism involves C1 cation–olefin
IV–olefin V cyclization followed by five hydride shifts, allowing
us to propose a unified biogenesis for sesterterpenes branching from
bicyclic (5–15), tricyclic (5–12–5), and tetracyclic
(5–6–8–5) cation intermediates. Furthermore,
the mechanism is distinct from that of a separate class of di/sesterterpenes
including fusicoccins and ophiobolins. The difference between mechanisms
is consistent with phylogenetic analysis of bifunctional terpene synthases,
suggesting that the amino acid sequence reflects the initial cyclization
mode, which is most likely related to the initial conformation of
a linear prenyl diphosphate