<p>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, <db_xref db="SWISSPROT" dbkey="P13655"/>). The function of BFD is not known, but it may be a general redox and/or regulatory component involved in the iron storage or mobilisation functions of bacterioferritin in bacteria [<cite idref="PUB00009950"/>]. This domain is also found in nitrate reductase proteins in association with the nitrite and sulphite reductase 4Fe-4S domain (<db_xref db="INTERPRO" dbkey="IPR006067"/>), nitrite/sulphite reductase ferredoxin-like half domain (<db_xref db="INTERPRO" dbkey="IPR005117"/>) and pyridine nucleotide-disulphide oxidoreductase (<db_xref db="INTERPRO" dbkey="IPR001327"/>). It is also found in NifU nitrogen fixation proteins, in association with NifU-like N-terminal domain (<db_xref db="INTERPRO" dbkey="IPR002871"/>) and C-terminal domain (<db_xref db="INTERPRO" dbkey="IPR001075"/>).</p>