Submitted to: International Journal of Molecular Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/4/2008
Publication Date: 9/9/2008
Citation: Ehrlich, K., Scharfenstein, L.L., Montalbano, B.G., Chang, P. 2008. Are the Genes nadA and norB Involved in Formation of Aflatoxin G1. International Journal of Molecular Sciences. 9:1717-1729. Interpretive Summary: Some Aspergillus species accumulate both aflatoxin B1 and G1, compounds that are very dangerous contaminants of food products. Aflatoxin G1, though slightly less toxic and carcinogenic than B1, is still a serious problem for agriculture, particularly when it accumulates due to fungal contamination of peanuts and tree nuts. We now show that genes at both ends of the group of genes that is necessary for formation of these aflatoxins. The genes make a protein (NadA) that changes the precursor molecule from an oxidized to a reduced form. This allows the second protein (NorB) to reoxidize the product to aflatoxin G1. One of the proteins was originally thought to be involved in sugar metabolism. Only species of Aspergillus fungi have these proteins. The more common contaminants of agricultural crops, Aspergillus flavus, only make aflatoxin B1, and the proteins do not form correctly in this species. With these results, we have a clearer understanding of what proteins are used for forming both B and G aflatoxins.
Technical Abstract: Formation of G-group aflatoxins (AFs) from O-methylsterigmatocystin (OMST) by certain Aspergillus species initially involves epoxidation reactions by cytochrome P450 monooxygenases, OrdA, and CypA. We now show that the genes, norB and nadA, at opposite ends of the AF biosynthesis gene cluster are also involved in formation of AFG1. A 386 Da lactone is expected after sequential oxidation of OMST by OrdA and CypA. A compound with this molecular weight accumulates in culture extracts of A. parasiticus mutants in which nadA (predicted to encode an OYE-FMN binding domain reductase) is disrupted. Extracts of norB (predicted to encode an aryl alcohol dehydrogenase) mutants contained increased amounts of a 362 Da compound. These results suggest that NadA catalyzes the reduction of the 386 Da lactone. Subsequent rearrangement (hydration and decarboxylation) yields an alcohol that is then oxidized by NorB to AFG1.