To elucidate the general biosynthetic pathway of fungal dimeric anhydrides, a gene cluster for the biosynthesis of the antihy-percholesterolemic agent phomoidride was identified by heterologous expression of candidate genes encoding the highly reducing polyketide synthase, alkylcitrate synthase (ACS), and alkylcitrate dehydratase (ACDH). An in vitro analysis of ACS and ACDH revealed that they give rise to anhydride monomers. Based on the established monomer biosynthesis, we propose a general biogenesis of dimeric anhydrides involving a single donor unit and four acceptor units.

To elucidate the biosynthesis of a fungicidal dimeric anhydride zopfiellin, the putative biosynthetic gene cluster was identified. We conducted heterologous expression of candidate genes for the synthesis of maleic anhydride and its dimerization and identified the two isomeric dimers with 9-membered rings as products. Notably, α-ketoglutarate-dependent dioxygenase ZopK oxidized one of the dimers, giving the 8-membered ring of zopfiellin. The mechanism of oxidative rearrangement is proposed by analyzing the incorporation of 13C-labeled precursors.

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