Regulation of phage Mu repressor transcription by IHF depends on the level of the early transcription.

Integration Host Factor (IHF) of E. coli can stimulate both early and repressor transcription of bacteriophage Mu. We introduced several mutations in the early promoter (Pe) and studied the effect of these mutations on the stimulation of early and repressor transcription by IHF. All mutant promoters are still positive regulated by IHF, but the level of stimulation is dependent on the strength of the promoter. The strength of the early promoter has an even greater impact on the regulation of the repressor promoter by IHF: stimulation is observed in the presence of a relatively weak Pe, whereas with a strong Pe the repressor promoter Pc is inhibited by IHF. This inhibition is most probably due to an interference of the early transcription with the opposing repressor transcription. The implication of this type of regulation for the Mu life cycle is discussed.