Crystal Structure of the Transcription Regulator RsrR Reveals a [2Fe–2S] Cluster Coordinated by Cys, Glu, and His Residues

The recently discovered Rrf2 family transcriptional regulator RsrR coordinates a [2Fe–2S] cluster. Remarkably, binding of the protein to RsrR-regulated promoter DNA sequences is switched on and off through the facile cycling of the [2Fe–2S] cluster between +2 and +1 states. Here, we report high resolution crystal structures of the RsrR dimer, revealing that the [2Fe–2S] cluster is asymmetrically coordinated across the RsrR monomer–monomer interface by two Cys residues from one subunit and His and Glu residues from the other. To our knowledge, this is the first example of a protein bound [Fe–S] cluster with three different amino acid side chains as ligands, and of Glu acting as ligand to a [2Fe–2S] cluster. Analyses of RsrR structures revealed a conformational change, centered on Trp9, which results in a significant shift in the DNA-binding helix-turn-helix region