ATrans-Acting Ketoreductase in Biosynthesis of a Symmetric Polyketide Dimer SIA7248

New assembly line, new compound: SIA7248, a new symmetric macrolide, was isolated from a marine-derived Streptomyces strain. Bioinformatic analyses of the identified biosynthetic gene cluster (sia) for SIA7248 suggested a polyketide biosynthesis utilizing an iteratively trans-acting ketoreductase (KR). We characterized SiaM as a trans-KR to catalyse reductions of various β-ketoacyl-thioesters with D-stereospecificity.