Linderoth N A, Tang G, Calendar R
Department of Molecular and Cell Biology, University of California, Berkeley, California, 94720-3202, USA.
Virology. 1997 Jan 6;227(1):131-41. doi: 10.1006/viro.1996.8325.
The polarity suppression (Psu) protein of bacteriophage P4 causes suppression of transcriptional polarity in Escherichia coli by overcoming Rho termination factor activity. Two new psu mutants defective in polarity suppression are described. The psu5 mutation deletes codons 95-98 from about the middle of the gene, and the mutant protein is inactive. The psu6 mutation changes Phe169 to Val and encodes a temperature-sensitive protein. Constitutive overexpression of psu+ from a plasmid prevents colony formation, but overexpression of mutant genes (psu5, psu6) does not, suggesting that Psu disturbs essential host function(s). Rho protein synthesis is enhanced several-fold in cells containing wild-type Psu, due to readthrough at the rho attenuator, while the physical stability of Rho is maintained. As a consequence, Psu-producing cells accumulate significantly more Rho than normal cells, reminiscent of termination-defective rho mutants. The polarity suppression activity induced by Psu is demonstrated in vitro by the efficient readthrough of Rho-dependent terminators lambda tR1 and TIS2 during coupled transcription-translation. Purified Rho protein restores termination at TIS2 when added to Psu-containing reactions but NusG does not. The data support the hypothesis that Psu has or elicits an anti-Rho function.
噬菌体P4的极性抑制（Psu）蛋白通过克服Rho终止因子活性来抑制大肠杆菌中的转录极性。本文描述了两个在极性抑制方面存在缺陷的新psu突变体。psu5突变从基因中部附近缺失了第95 - 98位密码子，突变蛋白无活性。psu6突变将Phe169突变为Val，编码一种温度敏感型蛋白。从质粒组成型过表达psu +会阻止菌落形成，但过表达突变基因（psu5、psu6）则不会，这表明Psu干扰了宿主的基本功能。由于在rho弱化子处的通读，含有野生型Psu的细胞中Rho蛋白的合成增强了几倍，同时Rho的物理稳定性得以维持。因此，产生Psu的细胞比正常细胞积累的Rho明显更多，这让人联想到终止缺陷型rho突变体。在体外，通过在转录 - 翻译偶联过程中对Rho依赖性终止子λtR1和TIS2的有效通读，证明了Psu诱导的极性抑制活性。当将纯化的Rho蛋白添加到含有Psu的反应中时，它能恢复TIS2处的终止，但NusG则不能。这些数据支持了Psu具有或引发抗Rho功能的假说。
