PARTICIPATION OF IHF AND A DISTANT UP ELEMENT IN THE STIMULATION OF THE PHAGE-LAMBDA P-L PROMOTER

We have previously identified a UP element in the phage lambda P-L pro moter, centred at position -90 from the transcription start site. Inte gration host factor (IHF), a heterodimeric DNA-binding and -bending pr otein, binds upstream of the lambda P-L promoter in a region overlappi ng the UP element. Stimulation of transcription by IHF requires an int act alpha CTD and affects the initial binding of RNA polymerase to the promoter. We propose a model for the stimulation of P-L by IHF in whi ch IHF bends the DNA to bring the distal UP sequence in closer proximi ty to the promoter core sequences to allow the docking of the alpha CT D of RNA polymerase. Furthermore, IHF may also participate in protein- protein interactions with the alpha CTD. In support of this model, we found that alanine substitutions in alpha CTD at positions 265, 268, 2 70 and 275 reduced P-L promoter activity. Mutations in the IHF DNA bin ding site, as well as IHF mutant proteins exhibiting a decreased abili ty to bend the DNA, were both defective in stimulating the P-L promote r. In addition, some of the mutated IHF residues are clustered at a pr otein surface that interacts with the UP DNA sequence. These residues may also participate in protein-protein interactions with the alpha CT D.