Kenichi Yokoyama

Antifungal nucleosides: Mechanism of biosynthesis and potentials for genomics-guided discovery Nikkomycins and polyoxins constitute an important class of antifungal peptidylnucleoside (PN) antibiotics active against human and plant pathogens. The nucleoside moiety (aminohexuronic acid, AHA) of PNs is the pharmacophore essential for the antifungal activity, and is biosynthesized via a C5′-extended bicyclic high-carbon nucleoside. Here, we report biochemical evidence suggesting that this C5’-extension proceeds by a unique mechanism by a novel radical S-adenosyl-L-methionine (SAM) enzyme, PolH. PolH was identified based on the functional characterization of enzymes conserved between the nikkomycin and polyoxin biosynthetic gene clusters, and found to catalyze the conversion of enolpyruvyl uridine 5′-monophosphate (EP-UMP) into octosyl acid 5′-phosphate (OAP) by a free radical-mediated C-C bond formation using the redox active C209 residue to stereospecifically reduce a free radical intermediate. This mechanism is distinct from the nucleophilic mechanism known for C5′-extension in antibacterial nucleoside biosynthesis, and hence represents a novel mechanism of nucleoside natural product biosynthesis. Further bioinformatic analysis revealed homologs of PolH, and potentials for genome-mining discovery of OAP-related antifungal natural products.