Nitric oxide (NO) has diverse roles in intercellular communication and (at higher levels) in immune-mediated cell killing. NO reacts with many cellular targets, with cell-killing effects correlated to inactivation of key enzymes through nitrosylation of their iron-sulfur centers. SoxR protein, a redox-sensitive transcription activator dependent on the oxidation state of its binuclear iron-sulfur ([2Fe-2S]) centers, is also activated in Escherichia coli on exposure to macrophage-generated NO. We show here that SoxR activation by NO occurs through direct modification of the [2Fe-2S] centers to form protein-bound dinitrosyl-iron-dithiol adducts, which we have observed both in intact bacterial cells and in purified SoxR after NO treatment. Functional activation through nitrosylation of iron-sulfur centers contrasts with the inactivation typically caused by this modification. Purified, nitrosylated SoxR has transcriptional activity similar to that of oxidized SoxR and is relatively stable. In contrast, nitrosylated SoxR is short-lived in intact cells, indicative of mechanisms that actively dispose of nitrosylated iron-sulfur centers.

SoxR protein is a redox-responsive transcription factor that governs a regulon of oxidative stress and antibiotic resistance genes in Escherichia coli. Purified SoxR contains oxidized [2Fe-2S] clusters and stimulates in vitro transcription of its target gene soxS up to 100-fold. SoxR transcriptional activity, but not DNA binding, is completely dependent on the [2Fe-2S] clusters; apo-SoxR prepared in vitro binds the soxS promoter with unchanged affinity but does not have transcription activity. Thus, modulation of the SoxR [2Fe-2S] clusters was proposed to control the protein's function in transcription. Here, we provide evidence that SoxR with reduced [2Fe-2S] clusters is inactive. Redox titration of purified SoxR revealed a midpoint potential of −285 ± 10 mV (pH 7.6). In vitro transcription assays showed that SoxR was inactivated when the [2Fe-2S] cluster was reduced (−380 mV), and full activity was restored upon reoxidation (+100 mV). The results suggest that one-electron oxidation and reduction of the [2Fe-2S] cluster regulate SoxR transcriptional activity.

SoxR is a transcription activator governing a cellular response to superoxide and nitric oxide in Escherichia coli . SoxR protein is a homodimer, and each monomer has a redox-active [2Fe–2S] cluster. Oxidation and reduction of the [2Fe–2S] clusters can reversibly activate and inactivate SoxR transcriptional activity. Here, we use electron paramagnetic resonance spectroscopy to follow the redox-switching process of SoxR protein in vivo . SoxR [2Fe–2S] clusters were in the fully reduced state during normal aerobic growth, but were completely oxidized after only 2-min aerobic exposure of the cells to superoxide-generating agents such as paraquat. The oxidized SoxR [2Fe–2S] clusters were rapidly re-reduced in vivo once the oxidative stress was removed. The in vivo kinetics of SoxR [2Fe–2S] cluster oxidation and reduction exactly paralleled the increase and decrease of transcription of soxS , the target gene for SoxR. The kinetic analysis also revealed that an oxidative stress-linked decrease in soxS mRNA stability contributes to the rapid attainment of a new steady state after SoxR activation. Such a redox stress-related change in soxS mRNA stability may represent a new level of biological control.