Plant-like alkaloid biosynthesis in fungi

Traditional natural products discovery efforts have focused primarily on biosynthetic genes clusters (BGCs) containing large multi-modular NRPSs; however, sequencing of fungal genomes has revealed a vast reservoir of BGCs containing smaller NRPS-like genes whose biosynthetic functions have remained elusive. Using a comparative metabolomics approach, we show that a BGC in the human pathogen Aspergillus fumigatus named fsq, which contains an NRPS-like gene, produces a family of novel isoquinoline alkaloids, the fumisoquins. Assembly of these compounds is based on an unprecedented carbon-carbon bond formation between L-serine- and L-tyrosine-derived building blocks, followed by formation of the isoquinoline ring system via a sequence that is directly analogous to the biosynthesis of an important family of isoquinoline alkaloids in plants. Fumisoquin biosynthesis requires the N-methylase FsqC and the FAD-dependent oxidase FsqB, which represent functional orthologs of coclaurine N-methyltransferase and berberine bridge enzyme in plants. Analysis of fungal genomes revealed homologs of fsq genes in many species, suggesting that plant-like isoquinoline biosynthesis is widespread in fungi. We corroborated the biosynthetic role of these fungal genes by demonstrating isoquinoline biosynthesis by a different BGC in A. flavus. Our results provide a striking example for convergent evolution of a multistep biosynthetic mechanism and show that BGCs containing NRPS-like genes may reveal new biosynthetic paradigms.