Promoter switch in the Escherichia coli pts operon

The ptsH operon of Escherichia coli is controlled by two promoters P0 and P1, each of which is regulated by cyclic AMP receptor protein (CRP) complexed with cAMP (CRP.cAMP). We have studied the in vitro as well as in vivo transcriptional regulation of these two promoters. Each promoter exhibits a switching mechanism in vitro, where, depending upon the presence or absence of CRP.cAMP, transcription is initiated from different start sites termed a and b. P0 (P0a) is affected by supercoiling: when the template is linear, transcription initiation is switched to a site 3 base pairs upstream (P0b) and becomes more CRP.cAMP dependent. Transcription from the P1 promoter (P1a) switches initiation sites to 7 base pairs downstream (P1b) in the presence of CRP.cAMP. Most transcription in vivo was from P1a, and P0b could not be detected in vivo. Glucose has independent positive effects on pts expression in vivo. The results indicate that the two different regulatory mechanisms (one through CRP.cAMP, the other through glucose) are working together for fine control of pts expression.