Motivation: Ustiloxins A and B are toxic cyclic tetrapeptides, Tyr-Val/Ala-Ile-Gly (Y-V/A-I-G), that were originally identified from Ustilaginoidea virens, a pathogenic fungus affecting rice plants. Contrary to our report that ustiloxin B is ribosomally synthesized in Aspergillus flavus, a recent report suggested that ustiloxins are synthesized by a non-ribosomal peptide synthetase in U. virens. Thus, we analyzed the U. virens genome, to identify the responsible gene cluster.
Results: The biosynthetic gene cluster was identified from the genome of U. virens based on homologies to the ribosomal peptide biosynthetic gene cluster for ustiloxin B identified from A. flavus. It contains a gene encoding precursor protein having five Tyr-Val-Ile-Gly and three Tyr-Ala-Ile-Gly motifs for ustiloxins A and B, respectively, strongly indicating that ustiloxins A and B from U. virens are ribosomally synthesized.