Fungal Highly Reducing Polyketide Synthases Biosynthesize Salicylaldehydes That Are Precursors to Epoxycyclohexenol Natural Products

Fungal highly reducing polyketide synthases (HRPKSs) are highly programmed multidomain enzymes that synthesize reduced polyketide structures. Recent reports indicated salicylaldehydes are synthesized by HRPKS biosynthetic gene clusters, which are unexpected based on known enzymology of HRPKSs. Using genome mining of a Trichoderma virens HRPKS gene cluster that encodes a number of redox enzymes, we uncover the strategy used by HRPKS pathways in the biosynthesis of aromatic products such as salicylaldehyde 4, which can be oxidatively modified to the epoxycyclohexanol natural product trichoxide 1. We show selective β-hydroxyl groups in the linear HRPKS product are individually reoxidized to β-ketones by short-chain dehydrogenase/reductase enzymes, which enabled intramolecular aldol condensation and aromatization. Our work expands the chemical space of natural products accessible through HRPKS pathways.