This enzyme catalyzes the hydrogen sulfide production from sulfite. It is strictly anaerobic. It is regulated by electron acceptors rather than by cysteine. Binds 1 [2Fe-2S] cluster per subunit. Binds 1 FAD per subunit. Sulfur metabolism; sulfite reduction. The anaerobic sulfite reductase seems to consist of three subunits. By sulfite. Repressed by oxygen. Belongs to the PyrK family.