Mutations to constitutivity and derepression are separate and separable in a regulatory gene of Aspergillus nidulans

Previous work has shown that expression of the structural genes for the enzymes of nitrate and nitrite assimilation in Aspergillus nidulans requires the products of two positively acting regulatory genes - nirA, mediating induction, and areA, mediating nitrogen metabolite repression. Here we show that, in addition to previously described mutations in nirA leading to constitutivity, other mutations can be selected in nirA leading to nitrogen metabolite derepression. These constitutivity and depression mutations in nirA are additive and separable by intragenic recombination. This suggests that the nirA gene product contains two separate domains, a co-inducer binding region, defined by constitutivity mutations, and a region interacting with the areA gene product or with initiator sites adjacent to structural genes under areA and nirA control, defined by derepression mutations. These findings might indicate a striking similarity of action between the eukaryotic regulatory gene nirA and a comparable prokaryotic regulatory gene.