The small-molecule biosynthetic potential of most filamentous fungi has remained largely unexplored and represents an attractive source for the discovery of new compounds. Genome sequencing of Calcarisporium arbuscula, a mushroom-endophytic fungus, revealed 68 core genes that are involved in natural product biosynthesis. This is in sharp contrast to the predominant production of the ATPase inhibitors aurovertin B and D in the wild-type fungus. Inactivation of a histone H3 deacetylase led to pleiotropic activation and overexpression of more than 75 % of the biosynthetic genes. Sampling of the overproduced compounds led to the isolation of ten compounds of which four contained new structures, including the cyclic peptides arbumycin and arbumelin, the diterpenoid arbuscullic acid A, and the meroterpenoid arbuscullic acid B. Such epigenetic modifications therefore provide a rapid and global approach to mine the chemical diversity of endophytic fungi.

Aurovertins are fungal polyketides that exhibit potent inhibition of adenosine triphosphate synthase. Aurovertins contain a 2,6-dioxabicyclo[3.2.1]octane ring that is proposed to be derived from a polyene precursor through regioselective oxidations and epoxide openings. In this study, we identified only four enzymes required to produce aurovertin E. The core polyketide synthase produces a polyene α-pyrone. Following pyrone O-methylation by a methyltransferase, a flavin-dependent mono-oxygenase and an epoxide hydrolase can iteratively transform the terminal triene portion of the precursor into the dioxabicyclo[3.2.1]octane scaffold. We demonstrate that a tetrahydrofuranyl polyene is the first stable intermediate in the transformation, which can undergo epoxidation and anti-Baldwin 6-endo-tet ring opening to yield the cyclic ether product. Our results further demonstrate the highly concise and efficient ways in which fungal biosynthetic pathways can generate complex natural product scaffolds.