Electron transfer subunit of the terminal reductase during anaerobic growth on selenate and nitrate. A + H2O + selenite = AH2 + selenate Binds 1 [3Fe-4S] cluster. Binds 3 [4Fe-4S] clusters. Heterotrimer of alpha, beta and gamma subunits. Probably translocated together with SerA, which possesses a Tat-type signal. Has potential use in bioremediation of waste sites contaminated with selenate, such as agricultural drainage waters.