Quinolactacin Biosynthesis Involves NRPSs Catalyzed Dieckmann Condensation to Form the Quinolone‐γ‐lactam Hybrid

Quinolactacins are novel fungal alkaloids that feature a quinolone- γ -lactam hybrid, which is a potential pharmacophore for the treatment of cancer and Alzheimer's disease. Here, we report the identification of the quinolactacin A2 biosynthetic gene cluster and elucidate the enzymatic basis for the formation of the quinolone- γ -lactam structure. We reveal an unusual β -keto acid ( N -methyl-2-aminobenzoylacetate) precursor that is derived from the primary metabolite L-kynurenine via methylation, oxidative decarboxylation and amide hydrolysis reactions. In vitro assays reveal two single-module NRPSs that incorporate the β -keto acid and L-isoleucine, followed by Dieckmann condensation, to form the quinolone- γ -lactam. Notably, the bioconversion from L-kynurenine to the β -keto acid is a unique strategy employed by Nature to decouple R* domain-containing NRPS from PKS machinery, which expands the paradigm for the biosynthesis of quinolone- γ -lactam natural products via Dieckmann condensation.