The aspergillic acid biosynthetic gene cluster predicts neoaspergillic acid production in Aspergillus section Circumdati

Authors: Lebar, Matthew; Mack, Brian; Carter-Wientjes, Carol; Gilbert, Matthew

Submitted to: World Mycotoxin Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/12/2019
Publication Date: 5/15/2019
Citation: Lebar, M.D., Mack, B.M., Carter-Wientjes, C.H., Gilbert, M.K. 2019. The aspergillic acid biosynthetic gene cluster predicts neoaspergillic acid production in Aspergillus section Circumdati. World Mycotoxin Journal. 12(3):213-222. https://doi.org/10.3920/WMJ2018.2397.
DOI: https://doi.org/10.3920/WMJ2018.2397

Interpretive Summary: Fungi produce a number of toxic compounds that can contaminate the food supply and cause great harm (e.g. aflatoxins and trichothecenes). The genes responsible for producing these toxic metabolites are often clustered together on the chromosome. We have identified the metabolic cluster in the fungus Aspergillus flavus that is responsible for the production of a toxic compound named aspergillic acid. We searched the genomes of all fungal species available and found this gene cluster in many other Aspergillus species. We confirmed, using chemical analysis, that all of the fungal species containing this gene cluster can produce aspergillic acid or a related compound, neoaspergillic acid. Aspergillic acid is antimicrobial and could be used by the fungus against microbial threats or as a virulence aid. Aspergillic acid bioactivity is likely due to its ability to bind tightly to iron. Fungi use iron-binding compounds for uptake, transport, and storage of iron. Finding ways to disrupt the production of iron-binding compounds could lead to fungi control strategies.

Technical Abstract: The fungus Aspergillus 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 harbors more than 50 putative secondary metabolite biosynthetic gene clusters that have yet to be characterized. Bioactive secondary metabolites that augment the ability of the fungus ability to infect crops are of particular interest. Biosynthetic gene cluster 11 in A. 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 pigment known as ferriaspergillin. In this study, we probe the available genomes of Aspergillus species for biosynthetic gene cluster 11 homologs. We find that all species possessing gene cluster 11 produce aspergillic acid or closely related analogs. We demonstrate that the Aspergillus section Flavi species harboring biosynthetic gene cluster 11 produce a mixture of aspergillic acid, hydroxyaspergillic acid, and aspergillic acid analogs differing only in the amino acid precursors. Interestingly, many section Circumdati species, known mainly for their production of the problematic mycotoxin ochratoxin A, also harbor 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 in Aspergillus section Circumdati.