Mutations in nirA gene of Aspergillus nidulans and nitrogen metabolism

Abstract

IN both prokaryotes and eukaryotes many structural genes are subject to more than one form of control. The ways in which these various forms of regulation interact with each other are therefore of considerable importance for an understanding of the control of gene expression. In the ascomycete fungus Aspergillus nidulans, the structural genes for nitrate reductase and nitrite reductase, the enzymes of nitrate assimilation, are under the control of two positive regulatory genes, nirA (refs 1–3) and areA (ref. 4). The nirA gene product is required for the synthesis of nitrate and nitrite reductases and mediates nitrate induction of these enzymes. The areA gene product is necessary for the synthesis of a wide range of permeases and enzymes involved in nitrogen metabolism, including nitrate and nitrite reductases, and mediates ammonium repression of these permeases and enzymes. Previously available data have indicated that both nirA and areA products would be required for the synthesis of nitrate and nitrite reductases, the role of nirA being largely confined to nitrate induction and that of areA being confined to ammonium repression. Here we show that a more complex situation exists. A new allele of the nirA gene, designated nirA^d–101 and derived from the wild-type allele in two mutational steps, results in both constitutivity and derepression and eliminates the need for a functional areA product.