Nitrate reductase is a key enzyme involved in the first step of nitrate assimilation. Catalyzes the reduction of nitrate to nitrite, using ferredoxin as the electron donor. Can use reduced methyl viologen but neither NADPH nor NADH as electron donors. H2O + nitrite + 2 oxidized [2Fe-2S]-[ferredoxin] = 2 H(+) + nitrate + 2 reduced [2Fe-2S]-[ferredoxin] Binds 1 molybdenum-bis(molybdopterin guanine dinucleotide) (Mo-bis-MGD) cofactor per subunit. Inhibited by cyanide and azide. Optimum pH is 9. Optimum temperature is 80 degrees Celsius at 2.2 and 3.1 M NaCl, and 60 degrees Celsius at 0.9 and 1.3 M NaCl. Nitrogen metabolism; nitrate reduction (assimilation). Is probably a monomer (PubMed:16182473). Initially characterized as a dimer of proteins with a MW of 105 and 50 kDa (PubMed:11731152). Repressed by the presence of ammonium as nitrogen source. Enzyme stability and activity depend upon the salt concentration. Belongs to the prokaryotic molybdopterin-containing oxidoreductase family. NasA/NapA/NarB subfamily.