Induction of the soxRS regulon of Escherichia coli by superoxide

The soxRS regulon orchestrates a multifaceted defense against oxidative str ess, by inducing the transcription of similar to 15 genes. The induction of this regulon by redox agents, known to mediate O radical anion production, led to the view that O2 radical anion is one signal to which it responds. However, redox cycling agents deplete cellular reductants while producing O 2 radical anion, and one may question whether the regulon responds to the d epletion of some cytoplasmic reductant or to O2 radical anion, or both. We demonstrate that raising [O2 radical anion] by mutational deletion of super oxide dismutases and/or by addition of paraquat, both under aerobic conditi ons, causes induction of a member of the soxRS regulon and that a mutationa l defect in soxRS eliminates that induction. This establishes that O2 radic al anion, directly or indirectly, can cause induction of this defensive reg ulon.