细菌RNA聚合酶通过σ因子来达到识别基因的目的，其中很多应激反应基因依赖于σ54因子，它们能和启动子DNA结合形成一个稳定的封闭复合物，这对于转录过程很重要。转录的开始需要结合于转录起始位点上游的激活蛋白驱动的ATP水解。在2008年11月7日出版的《分子细胞》（Molecular Cell）上，一组来自英国帝国理工学院的科学家提出了形成以上封闭复合物的结构基础，并分析了激活蛋白启动转录过程的机制。
转录过程的开始需要将一种包含RNA聚合酶（RNA polymerase RNAP）以及双链启动子DNA的封闭启动子复合物转变为开放型复合物，因为只有发生这一过程，生物酶才能与单链形式的DNA模板发生作用。而细菌RNAP和主要的σ因子σ54形成的复合物是封闭型的，只有通过依赖于ATP水解的激活蛋白重塑作用，它们才能变为开放复合物。以上重塑作用使DNA解链，促使封闭复合物发生转变。
在本期刊物的封面文章中，Bose等科学家提出了细菌RNAP与σ54结合产生的复合物的冷冻电子显微镜的重建结果，除此之外他们也得到了RNAP-σ54复合物与一个AAA+激活子结合的重建结果。结合光交联数据并找到启动子DNA与这一复合物结合的位点，研究小组解释了封闭型RNAP-σ54复合物为何无法与DNA模板结合，此外，科学家们还提出激活子的结合导致的结构变化是如何形成构象改变的，这最终将导致开放型复合物的形成。（生物谷Bioon.com）
生物谷推荐原始出处：
Molecular Cell，Volume 32, Issue 3, 337-346, 7 November 2008，Daniel Bose, Xiaodong Zhang
Organization of an Activator-Bound RNA Polymerase Holoenzyme
Daniel Bose1,4,Tillmann Pape1,4,Patricia C. Burrows2,Mathieu Rappas1,5,Siva R. Wigneshweraraj3,Martin Buck2andXiaodong Zhang1,,
1 Division of Molecular Biosciences, Centre for Structural Biology, Department of Life Sciences, Faculty of Natural Sciences, Imperial College London, London SW7 2AZ, UK
2 Division of Biology, Department of Life Sciences, Faculty of Natural Sciences, Imperial College London, London SW7 2AZ, UK
3 Division of Investigative Science, Department of Microbiology, Faculty of Medicine, Imperial College London, London SW7 2AZ, UK
Transcription initiation involves the conversion from closed promoter complexes, comprising RNA polymerase (RNAP) and double-stranded promoter DNA, to open complexes, in which the enzyme is able to access the DNA template in a single-stranded form. The complex between bacterial RNAP and its major variant sigma factor σ54 remains as a closed complex until ATP hydrolysis-dependent remodeling by activator proteins occurs. This remodeling facilitates DNA melting and allows the transition to the open complex. Here we present cryoelectron microscopy reconstructions of bacterial RNAP in complex with σ54 alone, and of RNAP-σ54 with an AAA+ activator. Together with photo-crosslinking data that establish the location of promoter DNA within the complexes, we explain why the RNAP-σ54 closed complex is unable to access the DNA template and propose how the structural changes induced by activator binding can initiate conformational changes that ultimately result in formation of the open complex.
