Required for nitrite assimilation. 2 H2O + 3 NADP(+) + NH4(+) = 5 H(+) + 3 NADPH + nitrite 2 H2O + 3 NAD(+) + NH4(+) = 5 H(+) + 3 NADH + nitrite Binds 1 siroheme per subunit. Binds 1 [4Fe-4S] cluster per subunit. Nitrogen metabolism; nitrate reduction (assimilation). Homodimer. Positively regulated by TnrA under nitrogen-limited conditions. Induced by ArfM. Belongs to the nitrite and sulfite reductase 4Fe-4S domain family.