Biosynthesis of a bacterial meroterpenoid reveals a non-canonical class II meroterpenoid cyclase

Meroterpenoids are hybrid natural products that arise from the integration of terpenoid and non-terpenoid biosynthetic pathways. While the biosynthesis of fungal meroterpenoids typically follows a well-established sequence of prenylation, epoxidation, and cyclization, the pathways for bacterial perhydrophenanthrene meroterpenoids remain poorly understood. In this study, we report the construction of an engineered metabolic pathway in Streptomyces for the production of the bacterial meroterpenoid, atolypene A ( 1 ). Our research reveals a novel biosynthetic pathway wherein the structure of 1 is assembled through a distinct sequence of epoxidation, prenylation, and cyclization, divergent from its fungal counterparts. We demonstrate that the noncanonical class II meroterpenoid cyclase (MTC) AtoE initiates cyclization by protonating the epoxide via the E314 residue, which acts as a Brønsted acid within the characteristic xxxE 314 TAE motif. Additionally, bioinformatic analysis of biosynthetic gene clusters (BGCs) that contain AtoE-like MTCs supports that bacteria have the potential to produce a wide array of meroterpenoids. Atolypene A assembles through sequential epoxidation, prenylation, and cyclization. In vitro studies revealed that AtoE, a noncanonical class II meroterpenoid cyclase, utilizes the first Glu in the atypical xxxE 314 TAE motif to protonate an oxirane.