Deciphering Early Oxidative Steps in Spirotetronate Biosynthesis Reveals a Two-Enzyme Cascade for Macrocyclic Lactone Formation.

We decipher functions of three monooxygenases (PasO1, PasO3, and PasO4) in early biosynthetic steps in spirotetronate PA-46101 and demonstrate a PasO4/PasO3 cascade for forging the signature macrocyclic lactone. The P450 PasO4 oxidizes a methyl group to a carboxylate, enabling a regiospecific Baeyer-Villiger oxidation by PasO3. Structural analysis of a PasO3 homologue identifies a carboxylate-binding pocket essential for this strict substrate specificity. This two-enzyme cascade can serve as a portable biocatalytic tool for diversifying spirotetronates.