The two Fe ions are each coordinated by two conserved cysteine residues. This domain occurs alone in small proteins such as Bacterioferritin-associated ferredoxin (BFD). The function of BFD is not known, but it may may be a general redox and/or regulatory component involved in the iron storage or mobilisation functions of bacterioferritin in bacteria. This domain is also found in nitrate reductase proteins in association with Nitrite and sulphite reductase 4Fe-4S domain (pfam01077), Nitrite/Sulfite reductase ferredoxin-like half domain (pfam03460) and Pyridine nucleotide-disulphide oxidoreductase (pfam00070). It is also found in NifU nitrogen fixation proteins, in association with NifU-like N terminal domain (pfam01592) and NifU-like domain (pfam01106).