Fuller R S, Platt T
Nucleic Acids Res. 1978 Dec;5(12):4613-23.
In vivo, termination of transcription at the attenuator site of the tryptophan (trp) operon of E. coli is influenced by the protein termination factor rho. In vitro, termination does not depend on rho factor, and is very efficient in a purified system consisting only of RNA polymerase, the DNA template, nucleoside triphosphates, and buffer. The extent of termination in this system is unaffected over a wide range of salt and nucleoside triphosphate concentration. However, there is a 10-fold stimulation of trp leader mRNA synthesis if rho factor is present during the transcription reaction. This stimulation occurs only at low molar ratios of polymerase to template, and can be blocked by rifampicin. It is thus most likely due to the recycling of RNA polymerase molecules that have been released from the attenuator site by rho factor. In fact, transcription of the trp leader region in vitro results in the fomration of a stable termination complex which can be observed on sucrose gradients or by binding to nitrocellulose filters. These data indicate that a major function of rho at the trp attenuator is to release completed transcripts from a pre-formed termination complex, rather than to cause the cessation of elongation.
在体内，大肠杆菌色氨酸（trp）操纵子衰减位点处的转录终止受蛋白质终止因子ρ的影响。在体外，转录终止不依赖于ρ因子，并且在仅由RNA聚合酶、DNA模板、核苷三磷酸和缓冲液组成的纯化系统中效率很高。在该系统中，终止程度在很宽的盐和核苷三磷酸浓度范围内不受影响。然而，如果在转录反应过程中存在ρ因子，trp前导mRNA的合成会有10倍的刺激。这种刺激仅在聚合酶与模板的低摩尔比时发生，并且可被利福平阻断。因此，这很可能是由于被ρ因子从衰减位点释放的RNA聚合酶分子的再循环。事实上，体外trp前导区的转录会导致形成稳定的终止复合物，这可以在蔗糖梯度上观察到，或者通过与硝酸纤维素滤膜结合来观察。这些数据表明，ρ在trp衰减子处的主要功能是从预先形成的终止复合物中释放完整的转录本，而不是导致延伸的停止。
