Role of cis-acting sites in stimulation of the phage λ Prm promoter by CI-mediated looping

Abstract

The lysogenic state of phage λ is maintained by the CI repressor. CI binds to three operators each in the right operator (OR)and left operator (OL)regions, which lie 2.4 kb apart. At moderate CI levels, the predominant binding pattern is two dimers of CI bound cooperatively at each regulatory region. The resulting tetramers can then interact, forming an octamer and a loop of the intervening DNA. CI is expressed from the PRMpromoter, which lies in the ORregion and is subjected to multiple regulatory controls. Of these, the most recently discovered is stimulation by loop formation. In this work, we have investigated the mechanism by which looping stimulates PRM.We find that two cis-acting sites lying in the OL region are involved. One site, an UP element, is required for stimulation. Based on the behavior of other promoters with UP elements located upstream of the-35 region, we suggest that a subunit of RNA polymerase (RNAP) bound at PRM binds to the UP element located in the OL region. In addition, adjacent to the UP element lies a binding site for integration host factor (IHF); this site plays a less critical role but is required for stimulation of the weak prm240 allele. A loop with CI at the OL2 and OL3 operators does not stimulate PRM,while one with CI only at OL2 provides some stimulation. We discuss possible mechanisms for stimulation. © 2013, American Society for Microbiology.