Nucleotide sequence analysis of the gene (nirA) for ferredoxin-nitrite reductase (NiR; EC 1.7.7.1) from the cyanobacterium Synechococcus PCC 7942 revealed that the gene encodes a protein of 512 amino acid residues. In the predicted protein, about 50% of the amino acid residues are identical to those of NiR proteins from higher plants. S1 nuclease mapping and Northern hybridization analysis showed that the nirA gene is cotranscribed with the nrtABCD and narB genes, which are located downstream of nirA and are involved in the uptake and reduction of nitrate. The transcript from the nirA-nrtABCD-narB gene cluster (designated the nirA operon) was unstable and turned over rapidly. The level of the transcript of the operon was negligible in cells utilizing ammonium but was increased sharply upon removal of ammonium from the growth medium or upon inhibition of the fixation of ammonium by L-methionine-DL-sulfoximine (MSX), an inhibitor of glutamine synthetase. Preincubation of the cells with 10 mM glutamine (Gln) prevented MSX from inducing accumulation of the transcript of the nirA operon, whereas 5-diazo-6-oxo-L-norleucine (DON), an inhibitor of glutamine amidotransferases, induced the accumulation of the transcript in cells that had been preincubated with Gln. These results suggest that the accumulation of a compound, synthesized by transfer of the amide nitrogen from Gln, or of a derivative of this compound, acts as a regulatory signal for repression of the nirA operon.
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Suzuki et al. (1993) studied this question.