Characterization and sequence of a novel nitrate reductase from barley

Barley (Hordeum vulgare L.) has both NADH-specific and NAD(P)H-bispecific nitrate reductases. Genomic and cDNA clones of the NADH nitrate reductase have been sequenced. In this study, a genomic clone (pMJ4.1) of a second type of nitrate reductase was isolated from barley by homology to a partial-length NADH nitrate reductase cDNA and the sequence determined. The open reading frame encodes a polypeptide of 891 amino acids and its interrupted by two small introns. The deduced amino acid sequence has 70% identity to the barley NADH-specific nitrate reductase. The non-coding regions of the pMJ4.1 gene have low homology (ca. 40%) to the corresponding regions of the NADH nitrate reductase gene. Expression of the pMJ4.1 nitrate reductase gene is induced by nitrate in root tissues which corresponds to the induction of NAD(P)H nitrate reductase activity. The pMJ4.1 nitrate reductase gene is sufficiently different from all previously reported higher plant nitrate reductase genes to suggest that it encodes the barley NAD(P)H-bispecific nitrate reductase.