The lysogenic state of phage lambda is maintained by the CI repressor. CI binds to three operators each in the right operator (O-R) and left operator (O-L) 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 P-RM promoter, which lies in the O-R region 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 P-RM. We find that two cis-acting sites lying in the O-L 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 P-RM binds to the UP element located in the O-L 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 O(L)2 and O(L)3 operators does not stimulate P-RM, while one with CI only at O(L)2 provides some stimulation. We discuss possible mechanisms for stimulation.
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