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). Cells lacking this gene display a blocked APS- (adenosine-5'-phosphosulfate) or PAPS- (3'-phosphoadenosine-5'-phosphosulfate) dependent cysteine synthesis but do retain AvrXa21 activity. Belongs to the nitrite and sulfite reductase 4Fe-4S domain family.