Isolation of Verrucosins A–E from a Marine <i>Verrucosispora</i> sp. Reveals a Unifying Biosynthetic Hypothesis for Linear and Macrocyclic Polyketides

As part of our long-standing program evaluating the biosynthetic complexity and biomedical potential of natural products from marine microbes, our attention was drawn to culture extracts from a <i>Verrucosispora</i> sp. (strain TAA-831), which produced multiple compounds with unique UV absorbance signatures and HRMS data. Large-scale fermentation and targeted isolation afforded verrucosins A-E (<b><b>1-5</b></b>), a mixture of linear and macrocyclic polyketides whose structures were determined through a synergistic combination of experimental, computational, and genomic approaches. The conserved sequence of methyl malonate and malonate motifs across the verrucosins implied a shared biosynthetic origin despite structural divergence as linear and cyclic congeners. Targeted genome mining revealed a lone type I/type III hybrid polyketide synthase biosynthetic gene cluster, <i>vrs</i>, that is likely responsible for verrucosin production. This revelation demonstrates for the first time that linear 3,5-dihydroxybenzenic (<b>1</b> and <b>2</b>) and cyclic ansamycin (<b>3-5</b>) polyketides can be naturally produced by a single biosynthetic gene cluster. The identification of the <i>vrs</i> cluster and bioinformatic prediction of the stereoselectivity of the embedded reductive domains within the modular type I polyketide synthase reinforced the NMR and computational stereochemical assignments for the co-isolates, particularly the stereochemically complex linear verrucosins (<b>1</b> and <b>2</b>).