Submitted to: Biochimica et Biophysica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2002
Publication Date: N/A
Interpretive Summary: Aflatoxin is a severely toxic and carcinogenic metabolite of some Aspergillus species. Aflatoxin contamination by Aspergillus molds in economically important crops, such as corn, cotton, peanut, and tree nut, can be a severe problem during certain years leading to losses for the economy as high as $100 million. This report describes how environmental conditions could influence aflatoxin production by Aspergillus species by affecting the biosynthesis of aflatoxin. Aflatoxin manufacture by the mold involves turning on about 23 genes which normally are not turned on during fungal growth. One of these genes, a polyketide synthase, is perhaps the most important gene in the process of aflatoxin production and large amounts of polyketide synthase enzyme are necessary to manufacture aflatoxins. We have examined the regulatory switch for this gene, the promoter, and found that it contains features that indicate that environmental factors and the developmental status of the fungus are involved in turning on the gene. This is the first time a convincing direct relationship between environmental factors and signals that respond to them have been linked to aflatoxin production, although there has been much circumstantial evidence for such a relationship in previous studies. With this knowledge, we are one step closer to finding a way to interrupt the biosynthesis and thereby prevent aflatoxin accumulation in plants.
Technical Abstract: The gene, pksA, catalyzes the formation of the polyketide backbone necessary for aflatoxin biosynthesis. Based on reporter assays and sequence comparisons of the nor1-pksA intergenic region in different aflatoxin-producing Aspergillus species, cis-acting elements for the global-acting transcription factors, BrlA, PacC, and CreA, and the pathway-specific transcription factor, AflR, are necessary for pksA expression. These results show for the first time that expression of a gene involved in mycotoxin biosynthesis is regulated by global-acting transcription factors. Binding sites for CreA and BrlA are present in some strains, but not in others, suggesting that regulation of aflatoxin production by carbon source (CreA) and developmental processes (BrlA) may be variable among aflatoxin-producing fungi. Mutagenesis of one of the two promoter PacC-binding sites led to an increase in reporter gene expression at acid pH, but a decrease in expression at alkaline pH. The finding that PacC (a factor involved in pH regulation) has positive effect on pksA expression under acid conditions is unprecedented.