Part of the complex that catalyzes the reduction of sulfite to sulfide. The alpha and beta subunits may have arisen by gene duplication. They both bind 2 iron-sulfur clusters, but the alpha subunit seems to be catalytically inactive, due to substitutions along the putative substrate access channel, and because it binds sirohydrochlorin (the dematallated form of siroheme) instead of siroheme. Heterohexamer of two alpha, two beta and two gamma subunits.