Electron transfer subunit of the terminal reductase during anaerobic growth on chlorate. Binds 1 [3Fe-4S] cluster. Binds 3 [4Fe-4S] clusters. Heterotrimer of alpha, beta and gamma subunits. Probably translocated together with ClrA, which possesses a Tat-type signal. Has potential use in bioremediation of waste sites contaminated with chlorate, such as pulp and paper industry wastewater.