The redox-regulated SoxR protein acts from a single DNA site as a repressor and an allosteric activator.

The SoxR protein of Escherichia coli responds to redox signals by activating the transcription of soxS, which encodes another transcription activator that directly stimulates oxidative stress genes. We show here that transcription of the soxR gene, which is positioned head-to-head with soxS in the chromosome, initiates in the intergenic region and is itself repressed by SoxR protein in in vitro transcription experiments. Analysis of single-copy operon fusions to soxR, combined with the results of Northern blotting experiments, verified this regulation and the transcription start site in vivo. The structure of the overlapping promoters is such that the single SoxR-binding site is located in the -10/-35 spacer of the soxS promoter, but just downstream of the -10 element of the soxR promoter. Activated and non-activated SoxR bind this site equally well, exerting nearly constant repression of soxR; activated SoxR simultaneously stimulates the soxS promoter >/=30-fold. The functional soxR promoter depresses soxS transcription when SoxR is not activated and enhances soxS transcription when SoxR is activated, as shown by comparing the expression of soxS'::lacZ fusions with and without the soxR -35 element (induction ratio only approximately 7-fold). SoxR thus represents a highly polar, redox-regulated transcriptional switch that maximizes the change in expression of soxS.