Submitted to: Toxins
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2014
Publication Date: 6/23/2014
Citation: Ehrlich, K., Mack, B.M. 2014. Comparison of expression of secondary metabolite biosynthesis cluster genes in Aspergillus flavus, A. parasiticus, and A. oryzae. Toxins. 6:1916-1928.
Interpretive Summary: Aspergillus flavus is a fungus responsible for food contamination of corn, cotton, peanut and tree nuts. It causes loss of crops due the presence of aflatoxins produced by the fungus, the ingestion of which is severely limited to the aflatoxin’s toxicity and carcinogenicity. Aflatoxin is only one of many toxic metabolites produced by the fungus. Although a biocontrol strategy is being developed to control aflatoxin in food and animal feeds, no strategy for control of these other toxins is available. The work reported here is a general way to assess if the toxins may be produced by different closely related Aspergillus isolates including the one used for food fermentations, A. oryzae, one found mainly in the soil of peanut fields, A. parasiticus, and two variant A. flavus that are found in corn-growing regions of the southern united states and in the cotton growing areas of Arizona. Some of these other toxins could act together with aflatoxin to cause illness in humans and animals or act independently to contribute to chronic toxic effects. Our method will help to determine the likelihood that some of these other toxins are produced and could be used with other methods to determine the expected quantities.
Technical Abstract: More than 55 secondary metabolite biosynthesis gene clusters are predicted to be present in the Aspergillus flavus genome. In spite of this the biosynthesis of only a few metabolites, such as the aflatoxin, cyclopiazonic acid and aflatrem, has been correlated with a particular gene cluster. Using RNAseq analysis we find that expression of many putative secondary metabolite genes in gene clusters in A. parasiticus, A. oryzae, and A. flavus S and L isolates is markedly variable for these fungi grown on potato dextrose agar medium. This variability could explain the differences in metabolite production by these closely related fungi. The gene clusters whose genes are most highly expressed can be correlated with the biosynthesis of some of the known metabolites from these fungi. For most, the genes involved in their biosynthesis remain to be proven. Our results suggest that the S-strain A. flavus, a prevalent contaminant of maize in central Africa, is capable of producing several indole diterpene metabolites that could contribute to the toxicity of A. flavus toward humans. This finding may be important for estimating the toxicity of A. flavus for plants and animals.