Identification of an UP element within the IHF binding site at the P(L)1-P(L)2 tandem promoter of bacteriophage λ

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 α subunit (αCTD). The αCTD also responds to various transcription activators, including the integration host factor protein, IHF, in the stimulation of the bacteriophage λ P(L) promoter. P(L) consists of the tandem P(L)1-P(L)2 promoters where P(L)1 is stimulated and P(L)2 is repressed by IHF. We identified a functional UP element that binds the a subunit of RNA polymerase and is located in the region from -36 to -60 relative to the P(L)2 start site. P(L)2 expression requires the presence of the UP element and requires an intact αCTD. 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 P(L)2. The possible function of the complex structure of the P(L) promoter is discussed.