Genes Differentially Expressed by Aspergillus flavus Strains After Loss of Aflatoxin Production by Serial Transfers

Aflatoxins are carcinogenic fungal secondary metabolites. Levels of aflatoxins in agricultural commodities are stringently regulated by many countries. A cluster of genes is responsible for aflatoxin biosynthesis by Aspergillus flavus and other closely related species. Expression of the clustered aflatoxin genes is governed by a complex network of regulatory mechanisms. To better understand the molecular events that are associated with aflatoxin production, transcription profiling by microarray analyses which compared three independent aflatoxigenic A. flavus strains to individual isogenic progenies that no longer produced aflatoxins after serial transfers was carried out. Twenty-two significantly differentially expressed features were identified. After physical mapping using the A. oryzae genome sequence as the reference, the number of unique genes was reduced to 16. Compared to the parental strains, changes in the aflatoxin gene expression levels in the progenies were not significant, which suggests that the inability to produce aflatoxins is not caused by decreased expression. The only gene showing higher expression levels in the progenies is homologous to glutathione S-transferease genes. Overexpression of this gene, named hcc, at six- to nine-fold in an aflatoxigenic A. flavus did not cause discernible changes in colony morphology or aflatoxin production. Loss of aflatoxin production after serial transfers may not result from a single event but caused by multiple factors. Keywords: Compartiave hybridization toxigenic and atoxigenic lines of Aspergillus Overall design: Aspergillus flavus NRRL 29459, NRRL 29474, and NRRL 29490 are aflatoxigenic strains originated from soil collection in a peanut field (Terrell Co., Georgia, USA). Strains 459B-20-2, 474A-20, and 499A-20 were nonaflatoxigenic isolates obtained after 20 serial transfers of the parental strains on potato dextrose agar slants (Horn and Dorner 2002). Comparsions in each experiment consisted of one aflatoxigenic parental strain and one nonaflatoxigenic progeny, compared after 48- or 72-hr growth. Each comparison was repeated with duplicate dye-flip.