Identification of csypyrone B1 as the novel product of Aspergillus oryzae type III polyketide synthase CsyB

As a novel superfamily of type III polyketide synthases (PKSs) in microbes, four genes, csyA, csyB, csyC, and csyD, were found in the genome of Aspergillus oryzae, an industrially important filamentous fungus. Although orthologs of csyA, csyC, and csyD genes are present in a closely related species, Aspergillus flavus, csyB gene is unique to A. oryzae. To identify its function, we carried out overexpression of csyB gene under the control of α-amylase promoter in A. oryzae. 3-(3-Acetyl-4-hydroxy-2-oxo-2H-pyran-6-yl)propanoic acid, named csypyrone B1, was identified as a CsyB product. Feeding experiments of 13C-labeled acetate indicated that five acetate units were incorporated into csypyrone B1. Two possible mechanisms are proposed for the biosynthesis of cycpyrone B1: (1) condensation of succinyl-CoA with three acetyl/malonyl-CoAs, and the following pyrone ring cyclization; (2) condensation of butyryl-CoA with three acetyl/malonyl-CoAs, and the following pyrone ring cyclization and side-chain oxidation.

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