The Role of the [2Fe‐2S] Cluster of Escherichia coli IscR in Responding to Redox‐Cycling Agents

The mechanisms by which cells respond to growth inhibitory redox‐cycling agents is only partially understood. In Escherichia coli K12, the IscR regulon, which includes the ISC and SUF Fe‐S cluster biogenesis machineries, is differentially expressed in response to these agents. Here, we report how one redox‐cycling agent, phenazine methosulfate (PMS), regulates IscR activity via its [2Fe‐2S] cluster cofactor. A direct role for IscR in mediating the response to PMS was inferred from the PMS‐dependent weakening of [2Fe‐2S]‐IscR binding to an isc operon type 1 DNA site in vitro. This decrease in DNA binding was attributed to the accompanying oxidation of its [2Fe‐2S]1+ cluster. Exposure of anaerobic cultures to PMS leads to increased isc expression, as expected from IscR cluster oxidation and impaired binding to type 1 sites in the isc promoter. However, this same anaerobic PMS treatment did not change expression of type 2 site promoters, such as suf, which require IscR that lacks an Fe‐S cluster (apo‐IscR) for effective transcriptional regulation. In contrast, PMS exposure under aerobic conditions significantly increased both isc and suf expression, indicating the formation of both [2Fe‐2S]2+‐IscR and apo‐IscR. This effect was partially attributed to superoxide generation by PMS under aerobic conditions, as evidenced by a superoxide dismutase‐deficient mutant showing a modest impact on isc and suf expression. Together, these findings provide new insights into redox‐cycling dependent regulation of IscR activity and highlight the distinct activities of apo‐IscR, [2Fe‐2S]2+‐IscR and [2Fe‐2S]1+‐IscR in controlling the IscR regulon.