Divergent regulation of aflatoxin production at acidic pH by two <Emphasis Type="Italic">Aspergillus</Emphasis> strains

Production of aflatoxins (AF) by Aspergillus flavus and A. parasiticus is known to occur only at acidic pH. Although typical A. flavus isolates produced more AF as the external pH became increasingly acidic, an atypical strain from West Africa produced less. The lower AF production was not well correlated with decreases in expression of the aflatoxin pathway regulatory gene, aflR, or of two other biosynthesis genes.     

Alteration of Different Domains in AFLR Affects Aflatoxin Pathway Metabolism inAspergillus parasiticusTransformants

AFLR, a zinc binuclear cluster DNA-binding protein, is required for activation of genes comprising the aflatoxin biosynthetic pathway inAspergillusspp. Transformation ofAspergillus parasiticuswith plasmids containing the intactaflRgene gave clones that produced fivefold more aflatoxin pathway metabolites than did the untransformed strain. When a 13-bp region in theaflRpromoter (positions −102 to −115 with respect to the ATG) was deleted, including a portion of a palindromic site previously shown to bind recombinant AFLR, metabolite production was 40% that of transformants with intactaflR.This result provides further evidence that this site may be involved in the autoregulation ofaflR.Overexpression of pathway genes could also result from increased quantities of AFLR titrating out a putative repressor protein. In AFLR, a 20-amino-acid acidic region near its carboxy-terminus resembles the region in yeast GAL4 required for GAL80 repressor binding. When 3 of the acidic amino acids in this region were deleted, levels of metabolites were even higher than those produced by transformants with intactaflR,as would be expected if repressor binding was suppressed in transformants containing this altered protein. Transformation with plasmids mutated at the AFLR zinc cluster (Cys to Trp at amino acid position 49) or at a putative nuclear localization signal region (RRARK deleted) gave clones with one-fifth the metabolite production of the untransformed fungus in spite of the transformants making the same or moreaflRmRNA. Since these transformants retained a copy of intactaflR,the latter results can be explained best by assuming that AFLR activates genes involved in aflatoxin production as a dimeric protein and that heterodimers containing both mutant and intact AFLR strands are inactive.