Site-directed mutagenesis of nitrate reductase from Aspergillus nidulans. Identification of some essential and some nonessential amino acids among conserved residues.

Nitrate reductase is a multiredox enzyme possessing three functional domains associated with the prosthetic groups FAD, heme iron, and molybdopterin. In Aspergillus nidulans, it is encoded by the niaD gene. A homologous transformation system has been used whereby a major deletion at the niiAniaD locus of the host was repaired by gene replacement. Employing site-directed mutagenesis and this transformation system, nine niaD mutants were generated carrying specific amino acid substitutions. Mutants in which alanine replaced cysteine 150, which is thought to bind the molybdenum atom of the molybdenum-pterin, and in which alanine replaced histidine 547, which putatively binds heme iron, had no detectable nitrate reductase (NAR) activity. This clearly establishes an essential catalytic role for these residues. Of the remaining mutants, all altered in the NADPH/FAD domain, two were temperature-sensitive for NAR activity, two had reduced NAR activity levels, and three had normal levels. Since some of these mutants change residues conserved between homologous nitrate reductases from a wide range of species, it is clear that such amino acid identities do not necessarily signify essential roles for the activity of the enzyme. These findings are considered in the light of predicted structural/functional roles for the altered amino acids.