Dual regulation of the paraquat-inducible gene pqi-5 by SoxS and RpoS in Escherichia coli.

The pqi-5 gene, producing a probable membrane protein of unknown function, has been reported to be a member of the soxRS regulon. The SoxRS-dependent induction of pqi-5 by paraquat occurs only during the exponential phase. The expression of pqi-5 increased in the absence of paraquat during the stationary phase or under conditions of carbon or phosphate starvation. This increase was regulated at the transcriptional level by RpoS (sigma S), which recognized the second promoter (P2) approx. 5 nucleotides upstream from the promoter (P1) used at the exponential phase. Studies with a series of 5' deletions revealed that the paraquat-responsive element resides between -52 and -42 nucleotides upstream from the P1 start site, whose nucleotide sequence matches closely to other SoxS-binding sequences. The stationary-phase induction required sequences up to position -42, which correspond to the 5' border of the putative -35 hexamer for the P2 promoter. The binding of the purified SoxS protein to the pqi-5 promoter upstream sequences was demonstrated by gel mobility-shift and DNase I protection assays. The transcription from P1 promoter by E sigma D was activated by purified SoxS in vitro, as was observed in vivo. The dual regulation of pqi-5 by SoxS at the exponential phase and RpoS at the stationary phase is the first to be reported among the members of the soxRS regulon, suggesting that this gene might indeed play some role under stressful conditions.