INTEGRATION HOST FACTOR ALLEVIATES THE H-NS-MEDIATED REPRESSION OF THE EARLY PROMOTER OF BACTERIOPHAGE-MU

Integration host factor (IHF), which is a histone-like protein, has be en shown to positively regulate transcription in two different ways, I t can either help the formation of a complex between a transcription f actor and RNA polymerase or it can itself activate RNA polymerase with out the involvement of other transcription factors. In this study, we present a third mechanism for IHF-stimulated gene expression, by count eracting the repression by another histone-like protein, H-NS. The ear ly (Pe) promoter of bacteriophage Mu is specifically inhibited by H-NS , both in vivo and in vitro, For this inhibition, H-NS binds to a larg e DNA region overlapping the Pe promoter. Binding of IHF to a binding site just upstream of Pe alleviates the H-NS-mediated repression of tr anscription. This same ihf site is also involved in the direct activat ion of Pe by IHF. In contrast to the direct activation by IHF, however , the alleviating effect of IHF appears not to be dependent on the rel evant position of the ihf site on the DNA helix, and it also does not require the presence of the C-terminal domain of the alpha subunit of RNA polymerase. Footprint analysis shows that binding of IHF to the ih f site destabilizes the interaction of H-NS with the DNA, not only in the IHF-binding region but also in the DNA regions flanking the ihf si te, These results suggest that IHF disrupts a higher-order nucleoprote in complex that is formed by H-NS and the DNA.