Role of DNA supercoiling and rpoS sigma factor in the osmotic and growth phase-dependent induction of the gene osmE of Escherichia coli K12.

Transcription of the gene osmE of Escherichia coli is osmotically inducible and regulated by the growth phase. In a medium of low osmotic pressure, expression of osmE is induced at the onset of stationary phase. At elevated osmotic pressure, a biphasic induction pattern is observed. The first step occurs during exponential phase, and this is followed by a strong induction at the onset of stationary phase. Both steps appear to result from stimulation of transcription at the same promoter, osmEp. In the absence of sigma s, the stationary phase sigma factor encoded by rpoS, osmEp stationary phase induction is abolished, while the osmotic effect is still observed. Mutations that compensate for the absence of sigma s mapped to the gene topA. The effect of such mutation and of novobiocin, an inhibitor of DNA gyrase, suggest that changes in DNA supercoiling are involved in the osmotic induction of osmEp. In addition, modulation of the supercoiling level of a reporter plasmid was observed during growth in rich media. The kinetics of osmEp transcription are discussed in light of the variations of DNA supercoiling.