第十章原核基因表达调控PPT.ppt

3. The repressor reduces transcription initiation by around 70-fold, which is much smaller than the binding of lac repressor. 阻遏蛋白的阻遏作用是转录起始降低70倍。 4. The repressor is a dimer of two subunits which has a structure with a central core and two flexible DNA-reading heads (carboxyl-terminal of each subunit ) trp operon 色氨酸操纵子属于一种负性调控的、可阻遏的操纵子（repressible operon），即这操纵子通常是开放转录的，有效应物（色氨酸为辅阻遏物）作用时则阻遏关闭转录。 细菌不少生物合成系统的操纵子都属于这种类型，其调控可使细菌处在生存繁殖最经济最节省的状态。? The attenuator （衰减子 ） 当色氨酸达到一定浓度、但还没有高到能够活化R使其起阻遏作用的程度时，产生色氨酸合成酶类的量已经明显降低，而且产生的酶量与色氨酸浓度呈负相关。 研究发现这种调控现象与色氨酸操纵子特殊的结构有关。 Repressor does not account for all the regulation； 阻遏蛋白 Deletion of a sequence between the operator and trpE gene coding region (attenuator) increase both the basal and the activated (derepressed) levels of transcription. Otrp和trpE之间序列的缺失导致基因基本转录水平和活化后的转录水平的上升。 Lies at the end of the transcribed leader sequence that precedes the trpE initiator codon. 弱化子位于trpE起始密码子前面162nt的转录前导序列的末端 2. Is a ρ-independent terminator site (GC-rich palindrome) f0llowed by eight successive U residues. 弱化子是一个不依赖于ρ因子的终止子，在一段富含GC的回文结构后是8个连续的U残基 3. Acts as a highly efficient transcription terminator if the hairpin structure is formed, and only a very short transcipt is synthesized. 如果这段序列能在RNA转录物中形成发夹结构，就可以作为一个高效的转录终止子。 Leader RNA structure （先导RNA的结构 ） Complementary 3:4 termination of transcription Complementary 2:3 Elongation of transcription The leader sequence contains four regions (sequence 1,2,3,4) of complementary sequence that can form different structures free leader RNA trp前导区的碱基序列已经全部测定， 发现完整的前导序列可分为1、2、3、4区域，这四个区域的片段能以两种不同的方式进行碱基配对。 有时以1-2和3-4配对，有时只以2-3方式互补配对。核糖体经过前导区继续翻译的能力控制着这两种结构的转换，它决定mRNA是否形成终止所需的结构。 前导序列的终止区与一般的转录终止位点特点相同，具有成串的 U和潜在的能形成茎环的二重对称结构。 The trp leader region can exist in alternative base-paired conformations. The center shows the four regions that can base pair. Region 1 is complementary to region 2, which is complementary to region 3, which is complementary to region 4. On the left is th