二、转录水平的调 控（一）原核生物的操纵子模型 1.乳糖操纵子1.乳糖操纵 子大肠杆菌生长需要碳源, 常见的是糖类 , 最方便利用的是葡萄糖, 但有些条件下 培养基中并无葡萄糖，仅有半乳糖等糖类 ，这时分解它们的酶类必须合成，才能利 用半乳糖当葡萄糖与乳糖共存，情况比 较复杂，分解它们的酶类必须不 合成降解物基因活化蛋白CAP（catabolic gene activate protein, CAP), 与cAMP形成 复合物结合于启动子部位，引起DNA构象 的变化，促进RNA聚合酶与启动子结合， 使转录的起始更加频繁，是一种正调控当 有葡萄糖存在时，其分解代谢产物可抑制腺 苷酸环化酶活性，激活磷酸二酯酶活性， cAMP含量下降，使CAP失活cAMP-CAP-DNAEver since the function of CRP as an activator of transcription was established, a great deal of work has been done to try and understand exactly how this occurs. •Does CRP activate transcription directly? In other words, does it assist RNA polymerase to recognize the promoter by means of direct protein-protein contacts between CRP and RNA polymerase?OR Does CRP activate transcription indirectly? Since CRP bends and thus distorts the DNA double helix when it binds, is it possible that increased RNA polymerase binding is a result of structural changes in the DNA?•A third possibility is that some combination of both direct and indirect effects might be responsible for activation at individual promoters.We can now distinguish 3 classes of CRP-activated promoter: CLASS IClass I CRP-activated promoters require only CRP for activation and the CRP binding site is located upstream of the promoter.The prototype CLASS I CRP-activated promoter is the lac operon promoter (lacP1). This CRP-binding site is centred 61.5 bp upstream of the startpoint of transcription.CLASS IICLASS II CRP-activated promoters require only CRP for activation and the CRP binding site overlaps the promoter. In these promoters, the CRP binding site appears to replace the usual -35 region of the promoter.The prototype CLASS II CRP-activated promoter is the gal operon promoter (galP1). This CRP-binding site is centred 41.5 bp upstream of the startpoint of transcription.CLASS IIICLASS III CRP-activated promoters require additional regulator proteins as well as CRP for activation. The location of the CRP binding site can be quite variable though it is typically more than 90 bp upstream of the startpoint of transcription.There is no prototype CLASS III CRP-activated promoter since they all have different requirements by way of additional regulator proteins. Examples of CLASS III CRP-activated promoters are the araBAD promoter and the malK promoter2.色氨酸操纵子大肠肝菌培养在只含无机盐及单一 碳源的培养基中，大肠杆菌细胞内可以 测出色氨酸合成的酶系, 如果在培养基 中加入色氨酸, 大肠杆菌中色氨酸合成 的酶系就明显降低。
色氨酸存在时, 阻 止了色氨酸合成酶系的形成, 细菌可直 接利用色氨酸, 而不用自己合成, 这种减 少酶量的现象称为酶合成的阻遏翻译过程对转录的调节 衰减作用(弱化作用)E. Coli 的色氨 酸合成翻译过程对转录的调节 衰减作用(弱化作用) 色氨酸mRNA的5’端有162核苷酸 的前导序列，当RNA的合成启动后除 非缺乏色氨酸，否则大部分mRNA仅 合成140核苷酸即停止前导肽能编码 一小段14肽，其终止区具有潜在的茎 环构象和成串的U，表现出转录终止位 点的特征前导RNA链有4个区域彼此 互补，可形成奇特的二级结构，有些 情况下出现终止子结构 转录与翻译偶联是原核生物的特 征原核生物翻译结束mRNA上有特殊的 终止子结构 衰减作用(弱化作用) 的意义E. ColiE. coli Bacterium弱化子系统主要是对外源色氨酸 浓度作出反应，维持浓度的恒定色氨酸浓度tAUG-AAA-CGC-GUU-CAA-UUU-AAA-CAC-CAC-CAU-CAU-CAC -CAU-CAU-CCU-GAC Met-Thr-Arg-Val-Gln-Phe-Lys-His-His-His-His-His-His-His-Pro-Asp-a leader coding region3.The Histidine Operon: An Attenuator2． 阿拉伯糖操纵子阿拉伯糖(arabinose)是另一个可以 为代谢提供碳源的五碳糖。
在大肠杆 菌中阿拉伯糖的降解需要3个基因： araB、araA和araD，分别编码3个酶 ：araB基因编码核酮糖激酶 (ribulokinase)，araA编码L-阿拉伯糖 异构酶(L-arabinose isomerase)，araD 编码L-核酮糖-5-磷酸-4-差向异构酶(L- ribulose-5phosphate-4epimerase)与araBAD相邻的是一个复合的 启动子区域和一个调节基因araC，这 个AraC蛋白同时显示正、负调节因子 的功能AraBAD和araC基因的转录 是分别在两条链上以相反的方向进行 的在标准的遗传学图谱上， araBAD基因簇从启动子PBAD开始向 左进行转录，而araC基因则是从Pc向 右转录AraC蛋白以Crep与araO1的 结合阻遏自身基因的调节并 控制着自身的合成有葡萄糖无阿拉伯糖无葡萄糖有阿拉伯糖阿拉伯糖和葡萄糖都很低或 都很高的情况下，araBCD的 转录都被阻遏5.Quorum Sensing: An Activator (群体意识）There exists a species of squid （鱿鱼 ）the squid has evolved a light organ in which it cultures a very pure, very dense population of a bacteria called Vibrio fischeri. This bacteria produces a substance called luciferase, which glows with the same intensity as the moon.Euprymna scolopes swimming Euprymna scolopes hatching Euprymna scolopes ventral view of light organIn this ventral cross- section of Euprymna Scolopes, the symbiotic light- emitting “photophore“ is the black and silver, two -lobed structure in the center. Courtesy of Margaret McFall- Ngai.bobtail squid 4This Hawaiian bobtail squid (Euprymna scolopes) has silvery tissue around and across its eye. These reflective, silvery tissues of the eye and skin are made of the same unusual proteins that help make the squid's flashlight work. 4This Hawaiian bobtail squid (Euprymna scolopes) is a night active predator that buries in the sand during the day and comes out at night to forage. This adult specimen, which is preparing to bury for the day, has silvery tissues around its eyes and blue- silver tissues in its skin due to proteins called reflectins. 4Protein-based reflectors appear to be unique to squid, octopus and other members of the cephalopod class of marine animals. Most cephalopods have big heads, large eyes, grabbing tentacles and sacs filled with ink. 4Close-up of the eye of a Hawaiian bobtail squid (Euprymna scolopes). When Vibrio fischeri is not in the squi。