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 indu cible and regulated by the growth phase. Ln a medium of low osmotic pr essure, expression of osmE is induced at the onset of stationary phase . At elevated osmotic pressure, a biphasic induction pattern is observ ed. The first step occurs during exponential phase, and this is follow ed by a strong induction at the onset of stationary phase. Both steps appear to result from stimulation of transcription at the same promote r, osmE(p). Ln the absence of sigma(s), the stationary phase sigma fac tor encoded by rpoS, osmE(p) 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 suc h mutation and of novobiocin, an inhibitor of DNA gyrase, suggest that changes in DNA supercoiling are involved in the osmotic induction of osmE(p). Ln addition, modulation of the supercoiling level of a report er plasmid was observed during growth in rich media. The kinetics of o smE, transcription are discussed in light of the variations of DNA sup ercoiling. (C) 1997 Academic Press Limited.