分子生物学（张海红）CHAPTER 16.pptVIP

* Figure 11.10 shows that there are two immediate early genes, N and cro, which are transcribed by host RNA polymerase. N is transcribed toward the left, and cro toward the right. Each transcript is terminated at the end of the gene. pN is the regulator that allows transcription to continue into the delayed early genes. It is an antitermination factor that suppresses use of the terminators tL and tR. (Its mechanism is discussed in 9 Transcription.) In the presence of pN, transcription continues to the left of N into the recombination genes, and to the right of cro into the replication genes. 图13.9（入噬菌体有两个早期转录的单元。在左端的单元中，上部向左端转录；在右端的单元中，下部向右端转录。启动子通过红或蓝箭头来表示。终止子通过绿色阴影来表示。两个立即早期基因，N和Cro，通过终止子与晚期基因分离。N蛋白的合成允许RNA聚合酶越过TL1左端和TR1右端的终止子）表明两个立即早期基因，N和Cro, 由宿主RNA聚合酶表达。N基因向左边转录，Cro基因向右边转录。每个转录过程都在基因的末端结束。PN是一个调节基因，作用是允许转录向晚早期基因延续。而压制终止子tL和tR的工作则是由一个抗终止子完成的（机理见第１１章）。在PN存在的情况下，转录过程继续进行越过N基因左端而进入重组基因，进行到Cro基因右端后进入复制基因。 * The map in Figure 11.9 gives the organization of the lambda DNA as it exists in the phage particle. But shortly after infection, the ends of the DNA join to form a circle. Figure 11.11 shows the true state of lambda DNA during infection. The late genes are welded into a single group, containing the lysis genes S-R from the right end of the linear DNA, and the head and tail genes A-J from the left end. 图13.8中的图谱是存在于噬菌体颗粒中的入DNA 组织方式。但刚刚发生感染后，该DNA 的两端就接在一起成为一个环。图13.10（入DNA在感染时环化，所以晚期的基因在转录时是完整的）画出了感染后入DNA 的真实状态。晚期基因接成了一个单一的组，包括以线性DNA右端开始的裂解基因s-R和右端开始的头部和尾部的基因。 后期基因作为单一的转录单位，从一个排在Q与S间的 PR 启动子开始表达。后期的启动子是必不可缺的。然而，缺少了Q基因的表达产物（晚早期基因右端的最后一个基因），晚期转录将在 tR3 位点终止。由于这个终止事件而得到的转录产物长194个碱基，为6SRNA。而PQ出现后，它将抑制tR3的终止，于是6SRNA得以延伸，结果使晚期基因得到表达。 后期基因的转录似乎不在任何特定点终止，而是持续着起动所有后期基因乃至走出这个区域。在晚早期基因的左端进行转录时也发生相似的情形，在经过重组功能区后仍在继续。而在每个方向上进行的转录可能在聚合酶撞到一块前就终止了。 Strategies to meet the challenges (cont): For each operon, one (or two) ribosomal proteins binds the mRNA near the translation initiation sequence, preventing the ribosome from binding and initiating translation. Repressing tran