Domain-wide, locus-specific suppression of nitrogen metabolite repressed mutations in Aspergillus nidulans.

Previous work has shown that loss of function mutations (designated are A(r)) in areA, a positive acting regulatory gene mediating nitrogen metabolite repression in Aspergillus nidulans, lead to inability to utilise nitrogen sources other than ammonium. This work establishes the existence of a gene designated areB where mutations can suppress areA(r) mutations in a locus-specific manner for expression of apparently all of the genes under areA control. areB mutations are partially dominant in diploids, extremely rare and, to varying degrees, deleterious. In agreement with their ability to suppress areA(r) mutations, areB mutations lead to nitrogen metabolite derepressed enzyme levels. areB is located about 5 cM from creA on the left arm of linkage group I. Four of the five areB mutations selected are accompanied by translocations. In at least two of these cases the areB mutations are extremely tightly linked to and probably identical with the translocation breakpoints. These two translocations, although induced in different strains in separate experiments, apparently have almost identical breakpoints. This suggests that chromosomal rearrangements of a highly specific nature can give rise to an areB mutant phenotype.