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Quantitative analyses of the 5′ end of  gal  transcripts indicate that transcription from the galactose operon  P1  promoter is higher during cell division. When cells are no longer dividing, however, transcription is initiated more often from the  P2  promoter.  Escherichia coli  cells divide six times before the onset of the stationary phase when grown in LB containing 0.5% galactose at 37°C. Transcription from the two promoters increases, although at different rates, during early exponential phase (until the third cell division, OD 600  0.4), and then reaches a plateau. The steady-state transcription from  P1  continues in late exponential phase (the next three cell divisions, OD 600  3.0), after which transcription from this promoter decreases. However, steady-state transcription from  P2  continues 1 h longer into the stationary phase, before decreasing. This longer steady-state  P2  transcription constitutes the promoter transition from  P1  to  P2  at the onset of the stationary phase. The intracellular cAMP concentration dictates  P1  transcription dynamics; therefore, promoter transition may result from a lack of cAMP-CRP complex binding to the  gal  operon. The decay rate of  gal -specific transcripts is constant through the six consecutive cell divisions that comprise the exponential growth phase, increases at the onset of the stationary phase, and is too low to be measured during the stationary phase. These data suggest that a regulatory mechanism coordinates the synthesis and decay of  gal  mRNAs to maintain the observed  gal  transcription. Our analysis indicates that the increase in  P1  transcription is the result of cAMP-CRP binding to increasing  numbers  of galactose operons in the cell population.

In Vivo Transcription Dynamics of the Galactose Operon: A Study on the Promoter Transition from P1 to P2 at Onset of Stationary Phase