Component of the sulfite reductase complex that catalyzes the 6-electron reduction of sulfite to sulfide. This is one of several activities required for the biosynthesis of L-cysteine from sulfate (Probable). 3 H2O + hydrogen sulfide + 3 NADP(+) = 4 H(+) + 3 NADPH + sulfite Binds 1 siroheme per subunit. Binds 1 [4Fe-4S] cluster per subunit. Sulfur metabolism; hydrogen sulfide biosynthesis; hydrogen sulfide from sulfite (NADPH route): step 1/1. Alpha(8)-beta(8). The alpha component is a flavoprotein, the beta component is a hemoprotein (By similarity). Up-regulated by sulfate and the transcriptional regulator CysL. Cells lacking the cysIJ genes are unable to use sulfate, sulfite or butanesulfonate as sole sulfur source, grow poorly with sulfide, but can still grow with thiosulfate, cysteine or methionine. Belongs to the nitrite and sulfite reductase 4Fe-4S domain family.