The role of the essential sulfhydryl group in assimilatory NADH: nitrate reductase of Chlorella

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.

Sequence of a cDNA encoding the bi-specific NAD(P)H-nitrate reductase from the tree Betula pendula and identification of conserved protein regions

Nitrate reductase (NR) assays revealed a bispecific NAD(P)H-NR (EC 1.6.6.2.) to be the only nitrate-reducing enzyme in leaves of hydroponically grown birches. To obtain the primary structure of the NAD(P)H-NR, leaf poly(A)+ mRNA was used to construct a cDNA library in the lambda gt11 phage. Recombinant clones were screened with heterologous gene probes encoding NADH-NR from tobacco and squash. A 3.0 kb cDNA was isolated which hybridized to a 3.2 kb mRNA whose level was significantly higher in plants grown on nitrate than in those grown on ammonia. The nucleotide sequence of the cDNA comprises a reading frame encoding a protein of 898 amino acids which reveals 67%-77% identity with NADH-nitrate reductase sequences from higher plants. To identify conserved and variable regions of the multicentre electron-transfer protein a graphical evaluation of identities found in NR sequence alignments was carried out. Thirteen well-conserved sections exceeding a size of 10 amino acids were found in higher plant nitrate reductases. Sequence comparisons with related redox proteins indicate that about half of the conserved NR regions are involved in cofactor binding. The most striking difference in the birch NAD(P)H-NR sequence in comparison to NADH-NR sequences was found at the putative pyridine nucleotide binding site. Southern analysis indicates that the bi-specific NR is encoded by a single copy gene in birch.

Cloning and analysis of the tomato nitrate reductase-encoding gene: protein domain structure and amino acid homologies in higher plants

We have cloned and sequenced the nitrate reductase (NR)-encoding gene (nia) from tomato. When compared to the two Nicotiana tabacum nia structural genes, this 5-kb tomato gene shows a highly conserved structure, the coding sequence being interspersed with three introns at the same positions. Nucleotide sequences of the 5' promoter regions are not homologous, except for a 250-bp fragment. This small region might be involved in the similar regulation of the nia expression in tomato and tobacco plant species. The tomato gene codes for a 911 amino acid (aa) polypeptide chain. This sequence was aligned with and compared to other higher plant NR sequences. This alignment clearly identifies the three catalytic domains of NR, namely, a molybdopterin cofactor-binding domain, a heme domain and a FAD/NADH domain. On the other hand, it suggests that the less conserved 80-aa N-terminal region, containing a striking acidic aa cluster, is an additional domain bearing regulatory or structural function.