Catalyzes the reduction of nitrite to ammonia, consuming six electrons in the process. Has very low activity toward hydroxylamine, and even lower activity toward sulfite. Sulfite reductase activity is maximal at neutral pH. 6 Fe(III)-[cytochrome c] + 2 H2O + NH4(+) = 6 Fe(II)-[cytochrome c] + 8 H(+) + nitrite Binds 1 Ca(2+) ion per monomer. Binds 5 heme c groups covalently per monomer. Nitrogen metabolism; nitrate reduction (assimilation). Homodimer (PubMed:10984487, PubMed:12296741). Interacts with NrfH (PubMed:10672190). May form a heterotetramer with NrfH (PubMed:11807271). Belongs to the cytochrome c-552 family.