Rojo F, Mencía M, Monsalve M, Salas M
Centro Nacional de Biotecnología (CSIC), Universidad Autónoma, Madrid, Spain.
Prog Nucleic Acid Res Mol Biol. 1998;60:29-46. doi: 10.1016/s0079-6603(08)60888-0.
Regulatory protein p4, encoded by Bacillus subtilis phage phi 29, has proved to be a very useful model to analyze the molecular mechanisms of transcription regulation. Protein p4 modulates the transcription of phage phi 29 genome by activating the late A3 promoter (PA3) and simultaneously repressing the two main early promoters, A2b and A2c (or PA2b and PA2c). This review describes in detail the regulatory mechanism leading to activation or repression, and discusses them in the context of the recent findings on the role of the RNA polymerase alpha subunit in transcription regulation. Activation of PA3 implies the p4-mediated stabilization of RNA polymerase at the promoter as a closed complex. Repression of the early A2b promoter occurs by binding of protein p4 to a site that partially overlaps the -35 consensus region of the promoter, therefore preventing the binding of RNA polymerase to the promoter. Repression of the A2c promoter, located 96 bp downstream from PA2b, occurs by a different mechanism that implies the simultaneous binding of protein p4 and RNA polymerase to the promoter in such a way that promoter clearance is inhibited. Interestingly, activation of PA3 and repression of PA2c require an interaction between protein p4 and RNA polymerase, and in both cases this interaction occurs between the same surface of protein p4 and the C-terminal domain of the alpha subunit of RNA polymerase, which provides new insights into how a protein can activate or repress transcription by subtle variations in the protein-DNA complexes formed at promoters.
枯草芽孢杆菌噬菌体φ29编码的调控蛋白p4已被证明是分析转录调控分子机制的非常有用的模型。蛋白p4通过激活晚期A3启动子（PA3）并同时抑制两个主要的早期启动子A2b和A2c（或PA2b和PA2c）来调节噬菌体φ29基因组的转录。本综述详细描述了导致激活或抑制的调控机制，并结合最近关于RNA聚合酶α亚基在转录调控中作用的研究结果进行了讨论。PA3的激活意味着p4介导RNA聚合酶在启动子处稳定为封闭复合物。早期A2b启动子的抑制是通过蛋白p4与启动子-35共有区域部分重叠的位点结合，从而阻止RNA聚合酶与启动子结合来实现的。位于PA2b下游96 bp处的A2c启动子的抑制是通过一种不同的机制实现的，即蛋白p4和RNA聚合酶同时与启动子结合，从而抑制启动子清除。有趣的是，PA3的激活和PA2c的抑制需要蛋白p4与RNA聚合酶之间的相互作用，并且在这两种情况下，这种相互作用都发生在蛋白p4的同一表面与RNA聚合酶α亚基的C末端结构域之间，这为蛋白质如何通过在启动子处形成的蛋白质-DNA复合物的细微变化来激活或抑制转录提供了新的见解。
