λ噬菌体nutL序列突变对N蛋白生物功能的影响

λ噬菌体 Ｎ 蛋白不仅是抗转录终止的正调控因子,也是在翻译水平上阻遏自身基因表达的负调控蛋白．ｎｕｔ Ｒ Ｎ Ａ 位点（包括 ｂｏｘ Ａ 和 ｂｏｘ Ｂ）参与了这两种生物效应．对位于左向操纵子（ｐ Ｌ）的 ｎｕｔ Ｌ 序列进行了突变后,证明 ｎｕｔ Ｌ 缺失及 ｂｏｘ Ｂ 第 ６ 位核苷酸突变使 Ｎ 丧失了正、负调控功能,提示 ｎｕｔ Ｌ 在 Ｎ 介导的调控反应中是必需的．ｎｕｔ Ｌ 序列中 ｂｏｘ Ａ 单碱基突变使 Ｎ 丧失了正调控功能,部分保留了负调控功能．这种负调控作用导至极性效应,使 ｌａｃ Ｚ基因转录水平明显下降．     

Analysis of mutations in the ninR region of bacteriophage lambda that bypass a requirement for lambda N antitermination.

Two mutations in the ninR region of bacteriophage lambda that bypass a requirement for antitermination have been studied. One mutation, byp, has been cloned and mapped by marker rescue to a 417-base-pair segment in the ninR region of the genome. Analysis of the byp mutation by using promoter detection vectors, DNA sequencing, and S1 nuclease analysis showed that the byp mutation created a new promoter that transcribed gene Q. The second mutation analyzed was the deletion nin3. Sequence analysis revealed that 2,485 base pairs of the ninR region were removed, beginning within the ren gene and ending in an open reading frame termed ninG. The tR2 and tR3 terminators, and probably others, were removed by the nin3 deletion, thereby allowing the phage to be N independent and to grow in hosts defective for Nus antitermination factors.     

nutR中的突变对λ噬菌体早期转录抗终止作用的影响

将λＤＮＡ的ｎｕｔＲ序列置于Ｌａｃ启动子的下游,在ｎｕｔＲ和β－半乳糖苷酶基因（ｇａｌＫ）之间插入λ噬茵体依赖ｒｈｏ的终止子ｔＲ１,使ｇａｌＫ的表达取决于Ｎ蛋白介导的转录抗终止作用。为研究ｎｕｔＲ对抗终止的影响,系统地进行了该序列中每个碱基的点突变（Ａ→Ｃ,Ｇ→Ｔ,Ｃ→Ａ及Ｔ→Ｇ）．结果表明ｂｏｘＡ中有两个碱基（位置２和５）的突变对抗终止作用是至关重要的,使抗终止效率降低了１０倍;而其他位置的改变影响甚微。ｂｏｘＡ的缺失在ｎｕｓ￣＋宿主中使抗终止数率降低了４０％,但在ｎｕｓＢ宿主中却恢复到野生型水平。ｂｏｘＢ茎环结构中,茎部顶端的碱基对及环中邻近茎部的两个碱基的突变对抗终止作用影响很大,被认为是Ｎ蛋白的识别序列．ｂｏｘＡ和ｂｏｘＢ之间的间隔序列中有两个碱基的突变几乎使抗终止作用丧失。