Required for nitrite assimilation. Required for activity of the reductase (By similarity). 2 H2O + 3 NADP(+) + NH4(+) = 5 H(+) + 3 NADPH + nitrite 2 H2O + 3 NAD(+) + NH4(+) = 5 H(+) + 3 NADH + nitrite Binds 1 [2Fe-2S] cluster. Associates with NasD. Positively regulated by TnrA under nitrogen-limited conditions. Induced by ArfM.