This subfamily includes beta FeS subunit of selenate reductase (SER), a member of the DMSO reductase family. SER catalyzes the reduction of selenate to selenite in bacterial species that can obtain energy by respiring anaerobically with selenate as the terminal electron acceptor. The enzyme comprises three subunits SerABC, forming a heterotrimer, with the catalytic component (alpha-subunit), iron-sulfur protein (beta-subunit) and monomeric b-type heme-containing gamma subunit. Beta subunit contains coordinating one [3Fe-4S] cluster and three [4Fe-4S] clusters and functions as electron carrier.