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, Swiss:P13655). 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 [1]. This domain is also found in nitrate reductase proteins in association with Nitrite and sulphite reductase 4Fe-4S domain [2] (Pfam:PF01077), Nitrite/Sulfite reductase ferredoxin-like half domain (Pfam:PF03460) and Pyridine nucleotide-disulphide oxidoreductase (Pfam:PF00070). It is also found in NifU nitrogen fixation proteins, in association with NifU-like N terminal domain (Pfam:PF01592) and NifU-like domain [3] (Pfam:PF01106).

1: A [2Fe-2S] protein encoded by an open reading frame upstream of the Escherichia coli bacterioferritin gene. Garg RP, Vargo CJ, Cui X, Kurtz DM Jr; Biochemistry 1996;35:6297-6301. PMID:8639572

2: Spectroscopic and voltammetric characterisation of the bacterioferritin-associated ferredoxin of Escherichia coli. Quail MA, Jordan P, Grogan JM, Butt JN, Lutz M, Thomson AJ, Andrews SC, Guest JR; Biochem Biophys Res Commun 1996;229:635-642. PMID:8954950

3: Iron storage in bacteria. Andrews SC; Adv Microb Physiol 1998;40:281-351. PMID:9889981