Catalyzes the reduction of sulfite to sulfide, a step in the biosynthesis of sulfur-containing amino acids and cofactors. 3 H2O + hydrogen sulfide + 6 oxidized [2Fe-2S]-[ferredoxin] = 7 H(+) + 6 reduced [2Fe-2S]-[ferredoxin] + sulfite Binds 1 siroheme per subunit. Binds 1 [4Fe-4S] cluster per subunit. Monomer. Belongs to the nitrite and sulfite reductase 4Fe-4S domain family. Extended N-terminus.