A novel nitrite reductase gene from the cyanobacterium Plectonema boryanum

The gene (nirA) for nitrite reductase was cloned from the nonheterocystous, filamentous cyanobacterium Plectonema boryanum. The predicted protein consists of 654 amino acids and has a calculated molecular weight of 72,135. The deduced amino acid sequence from positions 1 to 511 is strongly similar to the entire sequence of the ferredoxin-dependent nitrite reductases from other phototrophs, while the remainder of the protein is unique to the Plectonema nitrite reductase. The C-terminal portion of the protein (amino acids 584 to 654) is 30 to 35% identical to [2Fe-2S] ferredoxins from higher plants and cyanobacteria, with all of the four Cys residues involved in binding of the [2Fe-2S] cluster in the ferredoxins being conserved. Immunoblotting analysis of the extracts of P. boryanum cells showed that the NirA polypeptide has an apparent molecular mass of 75 kDa. An insertional mutant of nirA lacked the 75-kDa polypeptide, had no nitrite reductase activity, and failed to grow on nitrate and nitrite, indicating that the novel nirA is the sole nitrite reductase gene in P. boryanum and that the NirA polypeptide with the ferredoxin-like domain is the apoprotein of the functional nitrite reductase. As in Synechococcus sp. strain PCC7942, nirA is the first gene of a large transcription unit (> 7 kb in size) and is repressed by ammonium and derepressed simply by deprivation of ammonium from the medium. The development of nitrite reductase activity was, however, found to require the presence of nitrate in the medium.