Two dioxygenases generate the complex framework of a fungal meroterpenoid anditomin

Anditomin and its precursors, andilesins, are fungal meroterpenoids isolated from Aspergillus variecolor, and have unique chemical structures with a complex bridged-ring system. Previous isotope-feeding studies revealed their origins as 3,5-dimethylorsellinic acid and farnesyl pyrophosphate and suggested the possible involvement of a Diels-Alder reaction to afford the congested bicyclo[2.2.2]octane core structure of andilesins. Despite the intriguing structures, however, no molecular basis for the biosynthesis of anditomin has been reported to date. In this study, we elucidated the complete biosynthetic pathway of anditomin and identified key enzymes responsible for the unique framework of anditomin.

First, we performed the whole genome sequencing of an anditomin-producing strain and discovered the putative biosynthetic gene cluster of anditomin. The function of each gene was investigated by in vivo reconstitution of the biosynthesis in a heterologous fungus, Aspergillus oryzae, and by in vitro reactions with purified enzymes, which allowed the determination of the complete pathway leading to anditomin.

Interestingly, the anditomin pathway actually does not employ a Diels-Alder reaction, but involves the non-heme iron-dependent dioxygenase AndA to synthesize the bridged-ring by an unprecedented skeletal reconstruction. Another dioxygenase, AndF, is also responsible for the structural complexification, generating the end product anditomin by an oxidative rearrangement.

In conclusion, we have established the molecular basis for the anditomin biosynthesis and characterized two dioxygenases with exciting activities. As the molecular bases for several DMOA-derived meroterpenoids are now well-understood, in future studies, novel molecules with useful activities could be obtained by rationally or randomly combining the enzymes derived from different pathways.