van Ulsen P, Hillebrand M, Zulianello L, van de Putte P, Goosen N
Laboratory of Molecular Genetics, Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, The Netherlands.
Mol Microbiol. 1996 Aug;21(3):567-78. doi: 10.1111/j.1365-2958.1996.tb02565.x.
Integration host factor (IHF), which is a histone-like protein, has been shown to positively regulate transcription in two different ways. It can either help the formation of a complex between a transcription factor and RNA polymerase or it can itself activate RNA polymerase without the involvement of other transcription factors. In this study, we present a third mechanism for IHF-stimulated gene expression, by counteracting the repression by another histone-like protein, H-NS. The early (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 large DNA region overlapping the Pe promoter. Binding of IHF to a binding site just upstream of Pe alleviates the H-NS-mediated repression of transcription. This same ihf site is also involved in the direct activation 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 relevant 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 ihf 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 site. These results suggest that IHF disrupts a higher-order nucleoprotein complex that is formed by H-NS and the DNA.
整合宿主因子（IHF）是一种类组蛋白，已被证明以两种不同方式正向调节转录。它既可以帮助转录因子与RNA聚合酶之间形成复合物，也可以在没有其他转录因子参与的情况下自身激活RNA聚合酶。在本研究中，我们提出了IHF刺激基因表达的第三种机制，即通过对抗另一种类组蛋白H-NS的抑制作用。噬菌体Mu的早期（Pe）启动子在体内和体外均受到H-NS的特异性抑制。对于这种抑制作用，H-NS结合到与Pe启动子重叠的大片段DNA区域。IHF与Pe上游的一个结合位点结合可减轻H-NS介导的转录抑制。同一个ihf位点也参与了IHF对Pe的直接激活。然而，与IHF的直接激活作用不同，IHF的缓解作用似乎不依赖于ihf位点在DNA螺旋上的相关位置，也不需要RNA聚合酶α亚基的C末端结构域的存在。足迹分析表明，IHF与ihf位点的结合不仅使H-NS与IHF结合区域的DNA相互作用不稳定，也使ihf位点两侧的DNA区域的相互作用不稳定。这些结果表明，IHF破坏了由H-NS和DNA形成的高阶核蛋白复合物。
