An Unusual Extender Unit Is Incorporated into the Modular Polyketide Synthase of Scopranones Biosynthesis.

Scopranones, produced by sp. BYK-11038, are the novel bone morphogenetic protein inhibitors characterized by atypical two scoop-like moieties and a 3-furanone moiety. Two scoop-like moieties connected to a 3-furanone have not previously been reported in natural products, and their biosynthesis must occur via a unique pathway. Feeding experiments using C-labeled precursors indicated that scopranones were synthesized from three acetates and three butyrates in polyketide-type biosynthesis. Genome mining of sp. BYK-11038 revealed that the candidate biosynthetic gene cluster contains 21 open reading frames (ORFs), including three modular polyketide synthases (PKSs; SprA, SprB, and SprC), which were composed of 4 modules with one loading module and 18 additional ORFs (SprD to SprU) spanning a distance of 55 kbp. The characterization of in-frame deletion mutants and feeding experiments with the predicted extender units indicated that two genes, and , encoding discrete 3-oxoacyl-ACP synthases, and a gene, , encoding crotonyl-CoA reductase, were involved in assembling an unusual C branched extender unit, 2-(2-ethylbutyl)malonyl-CoA. Additionally, three ORFs, , , and , encoding cytochrome P450s and a monooxygenase, are important tailoring enzymes in post-PKS modification. SprT is an essential enzyme for decarboxylative ring contraction via oxidation, which converts the 2-pyranone to a 3-furanone.