Establishment and maintenance of a lysogen of the lambdoid bacteriophage 434 require that the 434 repressor both activate transcription from the P_RM promoter and repress transcription from the divergent P_R promoter. Several lines of evidence indicate that the 434 repressor activates initiation of P_RM transcription by occupying a binding site adjacent to the P_RM promoter and directly contacting RNA polymerase. The overlapping architecture of the P_RM and P_R promoters suggests that an RNA polymerase bound at P_R may repress P_RM transcription initiation. Hence, part of the stimulatory effect of the 434 repressor may be relief of interference between RNA polymerase binding to the P_RM promoter and to the P_R promoter. Consistent with this proposal, we show that the repressor cannot activate P_RM transcription if RNA polymerase binds at P_R prior to addition of the 434 repressor. However, unlike the findings with the related  phage, formation of RNA polymerase promoter complexes at P_RM and at P_R apparently are mutually exclusive. We find that the RNA polymerase-mediated inhibition of repressor-stimulated P_RM transcription requires the presence of an open complex at P_R. Taken together, these results indicate that establishment of an open complex at P_R directly prevents formation of an RNA polymerase-P_RM complex.