Fig. 2.

The Fe-S cluster of the NO-sensor NsrR modulates NsrR binding to DNA and subsequent NO stress response. (A) The [4Fe-4S]-NsrR from Streptomyces coelicolor (in green and white) complexed to 23-bp HmpA1 operator fragment (in grey) (PDB ID: 7B0C⁷⁰). (B) The [4Fe-4S] cluster is coordinated with three Cys residues (Cys93, Cys99, Cys105) from one monomer and Asp8 from the other. Disruption of the Arg12-Val36 and as well as the Asn97-Gly37 H-bonds in the absence of cluster results in the displacement of the DNA-recognition helix α3 by about 2.0 Å, which may be enough to prevent the binding of NsrR to DNA. (C) Holo-NsrR binds to the DNA upstream of the hmpA1 gene preventing its transcription. Upon NO stress, NsrR loses its cluster, which leads to the dissociation of NsrR from the DNA and subsequent lift of the transcription repression.