Discovery of Pyranoquinolone Diterpenoids Biosynthesized through Oxidative Indole Rearrangement and Atypical Terpene...

Pyr4-like transmembrane terpene cyclases (TCs) are key enzymes in the biosynthesis of diverse fungal meroterpenoids. However, their roles in bacterial meroterpenoid biosynthesis remain largely unexplored. Here, we charted the biosynthetic space of bacterial Pyr4-like TCs and discovered zigralone A, a quinolone diterpenoid. Functional characterization of the gene cluster established a multienzymatic assembly of its 6/6/6/5/6/6/6-heptacyclic scaffold. Key transformations include an oxidative indole rearrangement catalyzed by a thiamine pyrophosphate-dependent enzyme to form a 4-quinolone. Subsequent cytochrome P450-mediated epoxidation and hydroxylation direct carbo/oxacyclizations catalyzed by a Pyr4-like TC, generating a bilobed architecture of a pyranoquinolone and a 6/6/6-tricyclic ring system. Further P450-mediated oxidations and nonenzymatic hemiketalization furnish a spiro-fused tetrahydrofuran, completing the heptacyclic framework. Notably, zigralone A congeners exhibit potent antimicrobial activity against Staphylococcus aureus. This work provides insights into bacterial Pyr4-like TC-associated pathways, expanding the biosynthetic logic underlying the assembly of complex bioactive meroterpenoids.