Using a heterologous expression system in Aspergillus oryzae we characterized cryptic biosynthetic genes found in Alternaria solani. This system enabled the isolation of an antifungal metabolite, didymellamide B, which was previously not detected in the extract of the A. solani. The co-production and chemical transformation of didymellamide congeners enabled us to decipher the biosynthetic pathway of didymellamide B as follows: (1) formation of a linear polyketide chain with 3-acyltetramate, (2) oxidative ring expansion to yield a 2-pyridone skeleton, and (3) non-enzymatic endo-selective [4+2] cycloaddition to afford a trans-decalin. These results demonstrate the robustness and reliability of the A. oryzae expression system.
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