Crystal structure of FNR from A. fischeri. (A) AfFNR is a homodimer composed of monomeric subunits (blue and brown). Each subunit binds an all cysteine-ligated [4Fe-4S] cluster (shown as space filled) and contains an N-terminal sensory domain, a C-terminal DNA binding domain, and a large dimerization helix that forms the dimer interface. (B) The [4Fe-4S] cluster binding loop of AfFNR (Protein Bata Dank ID code 5E44). The location of Ser24 is shown relative to the positions of the cluster ligands (Cys20, -23, -29, and -122).