Indole-diterpenes represented by paxilline share a common pentacyclic core skeleton derived from indole and geranylgeranyl diphosphate. To shed light on the detailed biosynthetic mechanism of the paspaline-type hexacyclic skeleton, we examined the reconstitution of paxilline biosynthetic machinery in Aspergillus oryzae NSAR1. Stepwise introduction of the six pax genes enabled us to isolate all biosynthetic intermediates and to synthesize paxilline. In vitro and in vivo studies on the key enzymes, prenyltransferase PaxC and cyclase PaxB, allowed us to elucidate actual substrates of these enzymes. Using the isolated and the synthesized epoxide substrates, the highly intriguing stepwide epoxidation/cyclization mechanism for the construction of core structure has been confirmed. In addition, we also demonstrated "tandem transformation" to simultaneously introduce two genes using a single vector (paxG/paxB, pAdeA; paxP/paxQ, pUNA). This may provide further option for the reconstitution strategy to synthesize more complex fungal metabolites.

During a screening of putative diterpene synthase genes found in public databases using the Aspergillus oryzae expression system, it was found that a single transformant with the ACLA_76850 gene from A. clavatus produced a sesterterpene alcohol, ophiobolin F, and three minor sesterterpene hydrocarbons. The sesterterpene synthase has two catalytically independent domains (prenyltransferase/terpene cyclase) which are homologous to those of diterpene synthase, fusicoccadiene synthase. Coevolution of both domains and reaction mechanisms of these terpene synthases are discussed.

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 (NfSS) 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.

Covering: 2000 to 2018In the late 1960s, structurally unique fusicoccane- and ophiobolane-type di/sesterterpenes were isolated and their homologs were found to be widely distributed in various organisms. Nearly a half century later, the first terpene synthase PaFS was identified, which triggered the discovery of a number of di/sesterterpene synthases, which were named as cyclopentane-forming terpene synthases (CPF-TSs). In the past 10 years, CPF-TSs have emerged as a new type of class I terpene synthases, which afford di/sesterterpenes with characteristic polycyclic molecular skeletons; they catalyze two different types of cyclizations, defined as type A and B, which are relevant to the initial cyclization mode of a polyprenyl diphosphate. This review summarizes the characteristic features of CPF-TSs from various sources and detailed cyclization mechanisms; we have also discussed the structural diversification strategy of these novel enzymes.