The aspergillic acid biosynthetic gene cluster predicts neoaspergillic acid production inAspergillus sectionCircumdati

Abstract

The fungusAspergillus flavus is an opportunistic crop pathogen that produces aflatoxins. Aflatoxins are potent carcinogenic and hepatotoxic secondary metabolites that are highly regulated in most countries.A. flavus also produces many other secondary metabolites and harbours more than 50 putative secondary metabolite biosynthetic gene clusters that have yet to be characterised. Bioactive secondary metabolites that augment the ability of the fungus to infect crops are of particular interest. Biosynthetic gene cluster 11 inA. flavus has been recently shown to encode for the biosynthesis of aspergillic acid, a toxic hydroxamic acid-containing pyrazinone compound that can bind iron, resulting in a red-orange pigment known as ferriaspergillin. A decrease inA. flavus pathogenicity and aflatoxin contamination was observed when aspergillic acid biosynthesis was blocked during maize seed infection. In this study, we probe the available genomes ofAspergillus species for biosynthetic gene cluster 11 homologs. We find that all species possessing gene cluster 11 produce aspergillic acid or a closely related isomer. We demonstrate that theAspergillus sectionFlavi species harbouring biosynthetic gene cluster 11 produce a mixture of aspergillic acid, hydroxyaspergillic acid, and aspergillic acid analogs differing only in the amino acid precursors. Interestingly, manyAspergillus sectionCircumdati species, known mainly for their production of the problematic mycotoxin ochratoxin A, also harbour gene cluster 11 homologs, but do not produce aspergillic acid. Instead, these species produce neoaspergillic acid and its hydroxylated analog neohydroxyaspergillic acid, indicating that cluster 11 is responsible for neoaspergillic acid biosynthesis inAspergillus sectionCircumdati.