The C-terminal domain of the Escherichia coli RNA polymerase α subunit plays a role in the CI-dependent activation of the bacteriophage λ p(M) promoter

The bacteriophage λ p(M) promoter is required for maintenance of the λ prophage in Escherichia coli, as it facilitates transcription of the cI gene, encoding the λ repressor (CI). CI levels are maintained through a transcriptional feedback mechanism whereby CI can serve as an activator or a repressor of p(M). CI activates p(M) through cooperative binding to the O(R)1 and O(R)2 sites within the O(R) operator, with the O(R)2-bound CI dimer making contact with domain 4 of the RNA polymerase σ subunit (σ(4)). Here we demonstrate that the 261 and 287 determinants of the C-terminal domain of the RNA polymerase α subunit (αCTD), as well as the DNA-binding determinant, are important for CI-dependent activation of p(M). We also show that the location of αCTD at the p(M) promoter changes in the presence of CI. Thus, in the absence of CI, one αCTD is located on the DNA at position −44 relative to the transcription start site, whereas in the presence of CI, αCTD is located at position −54, between the CI-binding sites at O(R)1 and O(R)2. These results suggest that contacts between CI and both αCTD and σ are required for efficient CI-dependent activation of p(M).