*4.1.1 SoxR: [Fe-S]-cluster based, superoxide/nitric oxide stress sensor*

The *E. coli* SoxRS system enhances the production of ~45 proteins in response to superoxide exposure, including those in detoxification (*sodA*, manganese superoxide dismutase), DNA repair (*nfo*, endonuclease IV), maintaining cellular reducing power (*zwf*, glucose-6-phosphate dehydrogenase) and central metabolism (*fumC*, superoxide-stable fumarase C and *acnA*, aconitase A). The *E. coli* SoxR protein exists as a homodimer that contains one [2Fe–2S] cluster per subunit. During aerobic growth, up to 95% of SoxR are held in the reduced ([2Fe–2S]1+) state. Upon sensing conditions that promote the production of superoxide, SoxR is oxidized to ([2Fe–2S]2+) clusters and it leaves the *SoxR/S* promoter region (*psoxRS*) to activate the expression of transcription factor SoxS. SoxS, unlike SoxR, when bound to *psoxRS* initiates the expression of the proteins listed above located downstream of the promoter [78, 79].
