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Applied and Environmental Microbiology, February 2006, p. 1096-1101, Vol. 72, No. 2
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.2.1096-1101.2006

The Aflatoxin Biosynthesis Cluster Gene, aflX, Encodes an Oxidoreductase Involved in Conversion of Versicolorin A to Demethylsterigmatocystin

Jeffrey W. Cary,^* Kenneth C. Ehrlich, John M. Bland, and Beverly G. Montalbano

Southern Regional Research Center/ARS/USDA, P.O. Box 19687, New Orleans, Louisiana 70124

Received 2 August 2005/ Accepted 15 November 2005

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 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.

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* Corresponding author. Mailing address: USDA, ARS, Southern Regional Research Center, 1100 Robert E. Lee Blvd., P.O. Box 19687, New Orleans, LA 70179. Phone: (504) 286-4264. Fax: (504) 286-4419. E-mail: jcary{at}srrc.ars.usda.gov.

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Applied and Environmental Microbiology, February 2006, p. 1096-1101, Vol. 72, No. 2
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.2.1096-1101.2006

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