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Molecular Basis for the Biosynthesis of an Unusual Chain-Fused Polyketide Gregatin A.

Gregatin A (1) is a fungal polyketide featuring an alkylated furanone core, but the biosynthetic mechanism to furnish the intri-guing molecular skeleton has yet to be elucidated. Herein, we have identified the biosynthetic gene cluster of gregatin A (1) in Penicillium sp. sh18, and investigated the mechanism that produces the intriguing structure of 1 by in vivo and in vitro recon-stitution of its biosynthesis. Our study established the biosynthetic route leading to 1, and illuminated that 1 is generated by the fusion of two different polyketide chains, which are, amazingly, synthesized by a single PKS GrgA with the aid of a trans-acting enoylreductase GrgB. Chain fusion, as well as chain hydrolysis, is catalyzed by an α/β hydrolase GrgF, hybridizing the C11 and C4 carbon chains by Claisen condensation. Finally, structural analysis and mutational experiments using GrgF provided insight into how the enzyme facilitates the unusual chain-fusing reaction. In unraveling a new biosynthetic strategy involving a bifunc-tional PKS and a polyketide fusing enzyme, our study expands our knowledge concerning fungal polyketide biosynthesis.