Abstact

Aromatic polyketides are an important class of natural products with various bioactivities, and their structural diversity arises from modifications to their aromatic frameworks. In this study, we identify a stereoselective aromatase/cyclase (ARO/CYC) FasU, which is responsible for forming the axial chiral biphenyl framework in fasamycin. FasU catalyzes sequential decarboxylation and cyclization/aromatization with strict S-stereospecificity on a previously unidentified biosynthetic intermediate. Through crystal structure analysis and site-directed mutagenesis, we reveal the enzyme's substrate binding mode, stereospecificity, and the key residues involved in catalysis. This discovery introduces a novel class of ARO/CYC enzymes in type II polyketide biosynthesis, advancing the development of biocatalysts for chiral aromatic polyketides.