title: The aflatoxin biosynthesis cluster gene, aflX, encodes an oxidoreductase involved in conversion of versicolorin A to demethylsterigmatocystin.
abstract: Biosynthesis of the toxic and carcinogenic aflatoxins by the fungus Aspergillus flavus is a complicated process involving more that 27 enzymes and regulatory factors encoded by a clustered group of genes. Previous studies found that three enzymes, encoded by verA, ver-1, and aflY, are required for conversion of versicolorin A (VA), to demethylsterigmatocystin. We now show that a fourth enzyme, encoded by the previously uncharacterized gene, aflX (ordB), is also required for this conversion. A homolog of this gene, stcQ, is present in the A. nidulans sterigmatocystin (ST) biosynthesis cluster. Disruption of aflX in Aspergillus flavus gave transformants that accumulated approximately 4-fold more VA and fourfold less aflatoxin than the untransformed strain. Southern and Northern blot analyses confirmed that aflX was the only gene disrupted in these transformants. Feeding ST or O-methylsterigmatocystin, but not VA or earlier precursor metabolites, restored normal levels of AF production. The protein encoded by aflX is predicted to have domains typical of an NADH-dependent oxidoreductase. It has 27% amino acid identity to a protein encoded by the aflatoxin cluster gene, aflO (avfA). Some of domains in the protein are similar to those of epoxide hydrolases.
authors.affiliation: Southern Regional Research Center/ARS/USDA, P.O. Box 19687, New Orleans, LA 70179, USA. jcary@srrc.ars.usda.gov
authors.forename: Jeffrey W
authors.initials: JW
authors.lastname: Cary
authors.forename: Kenneth C
authors.initials: KC
authors.lastname: Ehrlich
authors.forename: John M
authors.initials: JM
authors.lastname: Bland
authors.forename: Beverly G
authors.initials: BG
authors.lastname: Montalbano
mesh_terms: D000595:Amino Acid Sequence; D000880:Anthraquinones / Q000737:chemistry / Q000378:metabolism*; D001231:Aspergillus flavus / Q000201:enzymology* / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D004271:DNA, Fungal / Q000235:genetics; D005800:Genes, Fungal; D008954:Models, Biological; D008969:Molecular Sequence Data; D005810:Multigene Family; D016296:Mutagenesis; D009247:NADH, NADPH Oxidoreductases / Q000235:genetics* / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D013241:Sterigmatocystin / Q000031:analogs & derivatives* / Q000096:biosynthesis / Q000737:chemistry
chemical_list: D000880:Anthraquinones; D004271:DNA, Fungal; D013241:Sterigmatocystin; C029826:versicolorin A; D009247:NADH, NADPH Oxidoreductases