INTERFERENCE BY P(R)-BOUND RNA-POLYMERASE WITH P(RM) FUNCTION-INVITRO- MODULATION BY THE BACTERIOPHAGE-LAMBDA CI PROTEIN

Activation of the weak P(RM) promoter by cI protein is an essential pr ocess in the establishment of lysogeny. Much evidence has accumulated that cI protein binds cooperatively to the operators O(R)1 and O(R)2 a nd that protein at the O(R)2 site contacts RNA polymerase to facilitat e open complex formation at the P(RM) promoter. We had shown previousl y in vitro that RNA polymerase situated at the nearby P(R) promoter co uld interfere with open complex formation at P(RM) and that an additio nal mechanism Of P(RM) activation in vitro involved cI-mediated RNA po lymerase exclusion from P(R). Here we further characterize this second indirect mode of activation. We demonstrate that addition of cI and i nactivation of the P(R) Promoter activate open complex formation at P( RM) similarly over the temperature range from 37 to 20-degrees-C in wh ich the extent of activation decreases from 8- to 2-fold. We also show that the binding of cI protein to O(R)1 is sufficient to effect an in crease in the rate of synthesis of abortive RNA products at P(RM). Thi s result is difficult to explain based on direct cI-RNA polymerase con tacts alone but is readily interpreted in terms of our previously prop osed model involving the exclusion of an interfering RNA polymerase fr om binding at P(R).