Bacteriophage Mu utilizes two converging promoters to regulate the lytic and lysogenic pathways. Messenger RNA encoding the repressor gene is synthesized leftward from a promoter (P(cM)) located 1066 base pairs from the Mu left end. This transcript overlaps and is complementary to RNA synthesized rightward from the early promoter (P(E)) for transposase and replication proteins which initiates transcription at base pair 1028. Purified Mu repressor binds to three distinct operator sites (O1, O2, and O3); repressor binding at O2 blocks RNA polymerase binding at P(E) and repressor binding at O3 blocks RNA polymerase binding at P(cM). O1 and O2 have higher affinity for repressor than O3, and transcription from P(E) is blocked at repressor concentrations that do not affect P(cM). Thus, maintenance of the lysogenic state and autoregulation of the repressor gene is achieved by RNA polymerase transcription through DNA-repressor ensembles at O1 and O2. Integration host factor (IHF) encoded by the Escherichia coli him A and him D genes binds to Mu operator DNA between sites O1 and O2. IHF enhances transcription from P(E) 3-5-fold on supercoiled plasmid substrates in vitro and in Mu monolysogens in vivo. In vitro, IHF simultaneously decreases transcription from P(cM) 5-10-fold which results in a 25-fold change in lytic transcription relative to repressor transcription. A model for regulating the Mu lysis-lysogeny decision is presented.