Elucidation of ustilaginoidin biosynthesis reveals a previously unrecognised class of ene-reductases

Abstract

Ustilaginoidins are a type of mycotoxin featuring a dimeric naphtho-γ-pyrone skeleton, produced by the rice false smut pathogen <i>Ustilaginoidea virens</i>. Here we used gene disruption, heterologous expression in <i>Aspergillus oryzae</i>, feeding experiments, and <i>in vitro</i> experiments to fully elucidate the biosynthesis of ustilaginoidins. A new route to dimeric 2,3-unsaturated naphtho-γ-pyrones <i>via</i> dimerization of YWA1 (and 3-methyl YWA1) followed by dehydration was discovered. Intriguingly, the reduction of the 2,3-double bond of the pyrenone ring was catalyzed by a phospholipid methyltransferase-like enzyme (UsgR). The reductase was specific for reduction of monomeric, linear naphtho-γ-pyrenones, but not for the dimers. Atroposelective coupling of various monomers by the laccase (UsgL) led to diverse ustilaginoidins. Moreover, 3-epimerism of the 3-methyl-2,3-dihydro-naphtho-γ-pyrones adds additional complexity to the biosynthesis.