Discovery of a Unique Flavonoid Biosynthesis Mechanism in Fungi by Genome Mining

Abstract

Flavonoids are important plant natural products with variable structures and bioactivities. All known plant flavonoids are generated under the catalysis of a type III polyketide synthase (PKS) followed by a chalcone isomerase (CHI) and a flavone synthase (FNS). In this study, the biosynthetic gene cluster of chlorflavonin, a fungal flavonoid with acetolactate synthase inhibitory activity, was discovered using a self-resistance-gene-directed strategy. A novel flavonoid biosynthetic pathway in fungi was revealed. A core nonribosomal peptide synthetase-polyketide synthase (NRPS-PKS) is responsible for the generation of the key precursor chalcone. Then, a new type of CHI catalyzes the conversion of a chalcone into a flavanone by a histidine-mediated oxa-Michael addition mechanism. Finally, the desaturation of flavanone to flavone is catalyzed by a new type of FNS, a flavin mononucleotide (FMN)-dependent oxidoreductase.