Respiratory sulfite reductase that catalyzes the reduction of sulfite to sulfide in a single step, consuming six electrons in the process (PubMed:22040142, PubMed:25642962). Required for sulfite respiration under anaerobic growth conditions (PubMed:22040142). Has only marginal activity with nitrite. [protein]-disulfide + 2 A + 3 H2O + hydrogen sulfide = [protein]-dithiol + 2 AH2 + H(+) + sulfite Exposure to oxygen reduces copper binding and leads to the formation of a disulfide bond between the two Cys residues that bind the copper ion. Binds 8 heme c groups covalently per monomer. Sulfur metabolism; sulfite reduction. Homotrimer. Repressed by fumarate and nitrate. Up-regulated by sulfite, but only in the absence of fumarate and nitrate (at protein level). Loss of growth based on sulfite respiration. The eighth heme binding site has an unusual CXXXXXCH motif. Belongs to the multiheme cytochrome c family.