NikR is a transcription factor that regulates nickel uptake. It consists of two dimeric DNA binding domains separated by a tetrameric regulatory domain that binds nickel. This domain corresponds to the C terminal regulatory domain which contains four nickel binding sites at the tetramer interface [1].

1: Crystal structure of the nickel-responsive transcription factor NikR. Schreiter ER, Sintchak MD, Guo Y, Chivers PT, Sauer RT, Drennan CL; Nat Struct Biol. 2003;10:794-799. PMID:12970756