ECOLOGICAL BASIS FOR AFLATOXIN REDUCTION THROUGH CROP MANAGEMENT AND BIOLOGICAL CONTROL
Location: Food and Feed Safety Research
Title: Absence of the Aflatoxin Biosynthesis Gene, norA, allows accumulation of deoxyaflatoxin B1 in Aspergillus flavus cultures
Submitted to: FEMS Microbiology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 18, 2010
Publication Date: February 12, 2010
Citation: Ehrlich, K., Chang, P., Scharfenstein, L.L., Cary, J.W., Crawford, J.M., Townsend, C.A. 2010. Absence of the Aflatoxin Biosynthesis Gene, norA, allows accumulation of deoxyaflatoxin B1 in Aspergillus flavus cultures. FEMS Microbiology Letters. 305:65-70.
Interpretive Summary: Plant and animal toxins produced by fungi are called mycotoxins. The most potent toxin known is aflatoxin B1, a colorless compound that is produced by the fungus, Aspergillus flavus, on corn, cotton, treenuts, and peanuts. This toxin is the most potent cancer-causing agent yet known from a natural source. In spite of over 45 years of study, how aflatoxin is made is not well understood. With recent work showing that all of the enzymes that are necessary for the formation of aflatoxin are made by genes in a well-regulated cluster, we have a much better understanding of how aflatoxin is made by the fungus. In this paper we inactivated a gene in the cluster called norA. This gene makes an enzyme that is necessary for the final step in aflatoxin B1 formation. Without that gene, the fungus accumulates a bright blue fluorescent compound. This result was not expected and helps clarify the last step in aflatoxin formation, a step that until now has been hard to explain on a chemical basis. We now know that there must be an intermediate alcohol that is first reduced and then oxidized by the enzyme NorA, in order to make aflatoxin. By improving the understanding of this critical step we may be able to provide new insights into ways to inhibit aflatoxin formation.
Conversion of O-methylsterigmatocystin (OMST) to aflatoxin B1 (AFB1), a highly toxic and carcinogenic fungal metabolite of some Aspergillus species, begins with its oxidation catalyzed by the cytochrome P450 monooxygenase, OrdA (AflQ). The complexity of the subsequent oxidation, hydration, ring-opening, and cyclization steps suggests that several other enzymes are involved. Inactivation in A. flavus of norA (aflE), a gene encoding an aryl alcohol dehydrogenase, led to the accumulation of a 296 Da blue-fluorescent compound which was proven to be deoxyAFB1 by its identity to the synthetic compound obtained after dehydration of aflatoxicol. This result suggests that oxidation of aflatoxicol by NorA is the final step in AFB1 biosynthesis.