Catalyzes the reduction of sulfite to sulfide, one of several activities required for the biosynthesis of L-cysteine from sulfate. 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(2)-beta(2). The alpha component is a flavoprotein, the beta component is a hemoprotein (By similarity). Belongs to the nitrite and sulfite reductase 4Fe-4S domain family.