Marshall N F, Price D H
Department of Biochemistry, University of Iowa, Iowa City 52242.
Mol Cell Biol. 1992 May;12(5):2078-90. doi: 10.1128/mcb.12.5.2078-2090.1992.
We have examined elongation by RNA polymerase II initiated at a promoter and have identified two classes of elongation complexes. Following initiation at a promoter, all polymerase molecules enter an abortive mode of elongation. Abortive elongation is characterized by the rapid generation of short transcripts due to pausing of the polymerase followed by termination of transcription. Termination of the early elongation complexes can be suppressed by the addition of 250 mM KCl or 1 mg of heparin per ml soon after initiation. Elongation complexes of the second class carry out productive elongation in which long transcripts can be synthesized. Productive elongation complexes are derived from early paused elongation complexes by the action of a factor which we call P-TEF (positive transcription elongation factor). P-TEF is inhibited by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole at concentrations which have no effect on the initiation of transcription. By using templates immobilized on paramagnetic particles, we show that isolated preinitiation complexes lack P-TEF and give rise to transcription complexes which can carry out only abortive elongation. The ability to carry out productive elongation can be restored to isolated transcription complexes by the addition of P-TEF after initiation. A model is presented which describes the role of elongation factors in the formation and maintenance of elongation complexes. The model is consistent with the available in vivo data concerning control of elongation and is used to predict the outcome of other potential in vitro and in vivo experiments.
我们研究了在启动子处由RNA聚合酶II起始的延伸过程，并鉴定出两类延伸复合物。在启动子处起始后，所有聚合酶分子进入延伸的无效模式。无效延伸的特征是由于聚合酶暂停随后转录终止而快速产生短转录本。在起始后不久加入250 mM KCl或每毫升1 mg肝素可抑制早期延伸复合物的终止。第二类延伸复合物进行有效的延伸，其中可以合成长转录本。有效的延伸复合物是由一种我们称为P-TEF（正转录延伸因子）的因子作用于早期暂停的延伸复合物而产生的。5,6-二氯-1-β-D-呋喃核糖基苯并咪唑在对转录起始无影响的浓度下可抑制P-TEF。通过使用固定在顺磁性颗粒上的模板，我们表明分离的起始前复合物缺乏P-TEF，并产生仅能进行无效延伸的转录复合物。在起始后加入P-TEF可使分离的转录复合物恢复进行有效延伸的能力。提出了一个模型，该模型描述了延伸因子在延伸复合物形成和维持中的作用。该模型与关于延伸控制的现有体内数据一致，并用于预测其他潜在的体外和体内实验的结果。
