STRUCTURE, FUNCTION AND REGULATION OF NITRATE REDUCTASE IN HIGHER-PLANTS

Two types of nitrate reductase (NR) have been recognized in higher plants. Most plants contain a NADH-specific NR, a few plants contain only a NAD(P)H-bispecific NR and some plants contain both NR types. Most NRs are nitrate inducible. Many NRs are expressed in both roots and leaves, but in some species a root-specific NR has been identified. All NRs from higher plants are dimers of identical subunits with 881 (bean) to 926 (spinach) amino acid residues. The complete nucleotide sequence of NR genes or cDNAs from several higher plants have been obtained. The amino acid identity between the species analyzed ranges from 60 to 90%. NR is organized into 3 domains containing FAD, heme and a molybdenum cofactor. Electrons are transferred from NAD(P)H to nitrate via these cofactors. Nitrate triggers transcription of inducible genes for NR. NR activity and mRNA accumulation can he induced to a low level in etiolated and green dark-adapted plants by addition of nitrate. The accumulation of NR mRNA and activity is strongly stimulated by subsequent exposure to white light. Light also stimulates NR mRNA translation and/or the stability of the NR protein. In light/dark grown plants the mRNA level generally peaks at the end of the dark periods and the NR activity peaks a few hours later. These diurnal oscillations are controlled via transcription initiation.