Growth-phase-dependent expression of the osmotically inducible gene osmC of Escherichia coli K-12.

The transcription of the osmotically inducible gene osmC of Escherichia coli is initiated by two overlapping promoters, osmCp1 and osmCp2. The existence of these two promoters was confirmed by site-directed mutagenesis. osmC transcription is regulated by the growth phase. In a medium of low osmotic pressure, expression of osmC is induced at the onset of decelerating phase and continues during the beginning of stationary phase. At elevated osmotic pressure, the induction occurs somewhat earlier during growth. Both promoters are repressed during early exponential phase. osmCp2 is induced during entry into stationary phase. Transcription from osmCp1, which is approximately 10-fold lower than that of osmCp2 in rich medium, starts during the mid-log phase and stops in early stationary phase. In the absence of sigmaS, the stationary-phase sigma factor encoded by rpoS, osmCp2 expression is much reduced while expression of osmCp1 is unaffected. As a consequence, the regulation of osmC as a function of growth is at least partially independent of sigmaS.