Identification of an UP element within the IHF binding site at the PL1-PL2 tandem promoter of bacteriophage lambda.

An UP element defines a supplementary promoter element located upstream of the -35 region that stimulates transcription by interacting with the C-terminal domain of the RNA polymerase alpha subunit (alpha CTD). The alpha CTD also responds to various transcription activators, including the integration host factor protein, IHF, in the stimulation of the bacteriophage lambda PL promoter. PL consists of the tandem PL1-PL2 promoters where PL1 is stimulated and PL2 is repressed by IHF. We identified a functional UP element that binds the alpha subunit of RNA polymerase and is located in the region from -36 to -60 relative to the PL2 start site. PL2 expression requires the presence of the UP element and requires an intact alpha CTD. The UP element is nested within the DNA region protected by IHF against DNase I digestion. We used mutational analysis to identify the IHF recognition sequence which was found to be located downstream of the UP element, overlapping the -35 region of PL2. The possible function of the complex structure of the PL promoter is discussed.