Biosynthesis of trans -AT PKS-Derived Shuangdaolides Featuring a trans -acting Enzyme for Online Epoxidation.

Bacterial trans -acyltransferase polyketide synthases ( trans -AT PKSs) synthesize natural products with intricate structures and potent biological activities. They generally contain various unusual modules or trans -acting enzymes. Herein, we report the trans -AT PKS-derived biosynthetic pathway of the shuangdaolide with a rare internal 2-hydroxycyclopentenone moiety. The multidomain protein SdlR catalyzes the synthesis of 16,17-epoxide during polyketide chain elongation. The SdlR contains a ketoreductase, an acyl carrier protein, a flavoprotein monooxygenase, and a serine hydrolase domain. This online epoxidation occurs at unusual positions away from the thioester. Then, two tailoring enzymes, SdlB and SdlQ, convert a methylene to a carbonyl group and oxidize a hydroxyl group to a carbonyl group, respectively. The following spontaneous opening of 16,17-epoxide induces the formation of a new C-C bond to generate the 2-hydroxycyclopentenone moiety. The characterization of the shuangdaolide pathway extends the understanding of the trans -AT PKSs, facilitating the mining and identification of this class of natural products.