Required for formate dehydrogenase (FDH) activity (PubMed:3077634, PubMed:8522521, PubMed:22194618, PubMed:25649206). Acts as a sulfur carrier protein that transfers sulfur from IscS to the molybdenum cofactor prior to its insertion into FDH. Specifically interacts with IscS and stimulates its cysteine desulfurase activity. Also binds the molybdenum cofactor (PubMed:22194618, PubMed:25649206). Required for activity of formate dehydrogenase N (FDH-N), formate dehydrogenase O (FDH-O) and formate dehydrogenase H (FDH-H) (PubMed:3077634, PubMed:8522521, PubMed:22194618). Homodimer (PubMed:25649206). Interacts with IscS (PubMed:22194618). Repressed by aerobiosis. Mutants lack FDH-N activity and exhibit defects in FDH-H activity. Belongs to the FdhD family.