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 <i>Streptomyces</i> 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 <i>via</i> the E314 residue, which acts as a Brønsted acid within the characteristic xxxE³¹⁴TAE 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.