Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/15/2005
Publication Date: 10/1/2005
Citation: Ehrlich, K., Montalbano, B.G., Boue, S.M., Bhatnagar, D. 2005. An aflatoxin biosynthesis cluster gene encodes a novel oxidase required for conversion of versicolorin a to sterigmatocystin. Applied and Environmental Microbiology. 71:8963-8965. Interpretive Summary: Aflatoxins and sterigmatocystins are a group of highly toxic and cancer-causing chemicals made by Aspergillus species. They are dangerous to humans because they often contaminate crops that provide products that we eat, such as corn or peanuts. Our goal is to devise methods to prevent aflatoxin and sterigmatomcystin formation before the crops become contaminated. These molecules are toxic and carcinogenic because they have an unusual chemical structure. In this paper we have proven that a previously unknown protein is necessary for a critical step in the formation of aflatoxins, namely, the conversion of the aflatoxin precursor, versicolorin A, which is not toxic, to sterigmatomcystin. This protein, called HypA, is an enzyme that causes the rearrangement of the versicolorin A-type structure, to the sterigmatomcystin-type structure which is associated with the toxicity and cancer-causing ability of these types of chemicals. With the knowledge presented in this paper, we suggest that it may be possible to devise a safe and effective means to chemically prevent the formation of sterigmatocystins and aflatoxins, by preventing the ability of this novel enzyme from performing the catalysis. Since this type of enzyme is only found in fungi, it is likely that a chemical could be devised that would be harmless to plants and animals. This research will be beneficial to researchers and growers.
Technical Abstract: The conversion of the polyketide metabolite, versicolorin A (VERA), to sterigmatomcystin (ST) has been shown to require enzymes encoded by the aflatoxin biosynthesis cluster genes, verA, ver-1, and omtB. We now show by gene disruption and metabolite feeding studies that hypA, a previously uncharacterized gene at the end of the aflatoxin biosynthesis gene cluster, is also required for this conversion. Disruption of hypA in an Aspergillus parasiticus BN009E niaD- mutant gave transformants that accumulated VERA, and no longer produced aflatoxin. Northern blot and PCR analyses confirmed that only hypA was disrupted in these transformants. Co-cultivation of a previously characterized ver-1 mutant Aspergillus strain with the hypA disruptant restored AF production as did feeding ST, but not feeding VERA or earlier precursor metabolites. This is the first report of a function for the predicted protein, HypA, and indicates that it is member of a novel class of oxidative enzymes found in fungi.