Insights into the Role of Ketoreductases in the Biosynthesis of Partially Reduced Bacterial Aromatic Polyketides

Partially reduced aromatic polyketides are bioactive secondary metabolites or intermediates in the biosynthesis of deoxygenated aromatics. For the antibiotic GTRI‐02 (mensalone) in different Streptomyces spp., biosynthesis involving the reduction of a fully aromatized acetyltrihydroxynaphthalene by a naphthol reductase has been proposed and shown in vitro with a fungal enzyme. However, more recently, GTRI‐02 has been identified as a product of the ActIII biosynthetic gene cluster from Streptomyces coelicolor A3(2), for which the reduction of a linear polyketide precursor by ActIII ketoreductase, prior to cyclization and aromatization, has been suggested. We have examined three different ketoreductases from bacterial producer strains of GTRI‐02 for their ability to reduce mono‐, bi‐, and tricyclic aromatic substrates. The enzymes reduced 1‐ and 2‐tetralone but not other aromatic substrates. This strongly suggests a reduction of a cyclized but not yet aromatic polyketide intermediate in the biosynthesis of GTRI‐02. Implications of the results for the biosynthesis of other secondary polyketidic metabolites are discussed. One ring to rule: In contrast to the case of reduction of fully aromatic naphthols or a linear polyketide, monocyclized polyketides are proposed as the substrates of ketoreductases in the biosynthesis of bacterial aromatic polyketides such as GTRI‐02.