第16章基因表达的调控.ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Introduction,Activation,of gene structure(chromosome),Initiation,and,termination,of transcription,Processing,the transcript,Transport,to cytoplasm,Translation,of mRNA,Stages of regulation,调控位点,:,Prokaryotic cell,原核,Eukaryotic cell,真核,第一节 转录水平的调控,一、操纵子模型,由法国科学家,Jacob,和,Monod,于,1960,年提出的一个有关原核,基因表达调控的模型。获得,1965,年诺贝尔生理学奖。,The Nobel Prize in Physiology or Medicine 1965.,for their discoveries concerning genetic control of enzyme,（二）乳糖操纵子的结构,Figure 10.4 The,lac,operon,occupies 6000 bp of DNA.At the left the,lacI,gene has its own promoter and terminator.The end of the,lacI,region is adjacent to the,promoter,P,.The,operator,O,occupies the first 26 bp of the long,lacZ,gene,followed by the,lacY,and,lacA,genes and a terminator.,lacZ,codes for the enzyme-galactosidase(,-,半乳糖苷酶,),whose active form is a tetramer of 500kDa.The enzyme breaks a-galactoside into its component sugars.For example,lactose is cleaved into glucose and galactose.,lacY,codes for the-galactoside permease,（,-,半乳糖苷透过酶,）,a 30kDa membrane-bound protein constituent of the transport system.This transports-galactosides into the cell.,lacA,codes for-galactoside transacetylase,（,-,半乳糖苷乙酰转移酶,）,an enzyme that transfers an acetyl group from acetyl-CoA to-galactosides.,（三）乳糖操纵子学说（,Lac Operon Theory,）,一个或几个结构基因与一个调节基因和一个操纵基因组成,一个操纵子。调节基因编码调节蛋白，调节蛋白与操纵基因结,合，从而调控结构基因的表达。,乳糖为诱导物。当有乳糖存在时，乳糖与有活性的调节蛋,白（阻遏物）结合时，阻遏物失活，则不能与操纵基因结合，,解除对,-,半乳糖苷酶基因（结构基因）的抑制，开始表达。,如果去掉乳糖时，阻遏物又恢复其活力，与操纵基因,DNA,结合，将,-,半乳糖苷酶基因关闭。但这种关闭或开启并不是,100%,。,Figure 10.7 Repressor maintains the lac operon in the inactive condition by binding to the operator;addition of inducer releases the repressor,and thereby allows RNA polymerase to initiate transcription.,（四）调控类型,正调控（,positive control,）：,在操纵子中，结构基因本来,不表达，可当加入调节蛋白（,无辅基诱导蛋白,）时，使该结构,基因进行表达。这样的调控叫正调控。,负调控（,negative control,）：,在操纵子中，结构基因本来,是表达的，当加入调节蛋白（,阻遏蛋白,）时，使该结构基因不,表达。这样的调控叫负调控。,1.,可诱导的调控：,在可诱导的操纵子中，加入 对基因表,达有调节作用的小分子物质（,诱导物,），则开启基因的转录活,性。,2.,可阻遏的调控：,在可阻遏的操纵子中，加入 对基因表,达有调节作用的小分子物质（,辅阻遏物,），则关闭基因的转录,活性。,Figure 10.3 In,positive control,trans-acting factors must bind to cis-acting sites in order for RNA polymerase to initiate transcription at the promoter.In a eukaryotic system,a structural gene is controlled individually.,Figure 10.2 In,negative control,a trans-acting repressor binds to the,cis,-acting operator to turn off transcription.In prokaryotes,multiple genes are controlled coordinately.,Figure 10.21 Control circuits are versatile and can be designed to allow positive or negative control of induction or repression.,（五）操纵基因的结构,Figure 10.11 The,lac,operator has a symmetrical sequence.The sequence is numbered relative to the startpoint for transcription at+1.The regions of dyad symmetry are indicated by the shaded blocks.,Figure 10.20 Operators may lie at various positions relative to the promoter.,（六）操纵基因的位置,Figure 9.10 RNA polymerase initially contacts the region from-55 to+20.When sigma dissociates,the core enzyme contracts to-30;when the enzyme moves a few base pairs,it becomes more compactly organized into the general elongation complex.,第二节 操纵子的其他调控形式,阿拉伯糖操纵子,半乳糖操纵子,氨基酸操纵子（,色氨酸,、苏氨酸、组氨酸、苯丙氨,酸、亮氨酸、异亮氨酸等十几种）,核苷酸操纵子,色氨酸（,Trp,）操纵子,:,目前，有多种氨基酸操纵子的结构已研究清楚，如色氨酸、苏氨酸、组氨酸、苯丙氨酸、亮氨酸、异亮氨酸等,十余种,。我们仅讨论,Trp,操纵子的基因表达调控。,Trp,操纵子是合成代谢途径中最具代表性的例子。它控制着,5,个结构基因（编码,3,种酶）的表达，分别为,trpE,，,D,（邻氨基苯甲酸合成酶）,，,C,（吲哚甘油硼酸合成酶）,，,B,、,A,（色氨酸合成酶,B,链和,A,链）,。当色氨酸少或缺乏时，这,5,个邻近的基因才开始转录成,mRNA,。,由于,Trp,操纵子是一个合成体系，与糖的分解代谢无关，所以，与,Glu,的存在与否没有关系。,Figure 10.41 The,trp,operon consists of five contiguous structural genes preceded by a control region that includes a promoter,operator,leader peptide coding region,and attenuator.,1.,结构与功能,结构特点：,（,1,）阻遏物基因,trpR,和结构基因,trpEDCBA,不紧密连锁；,（,2,）操纵基因在启动子区域内；,（,3,）启动子，操纵基因不直接和结构基因毗邻，而和前导,序列直接相连；,（,4,）有衰减子结构。在合成代谢的操纵子的前导区内，存,在着类似终止子结构的一段,DNA,序列，该序列可以辅助阻遏作,用，进行转录调控，故称为,衰减子,（,attenuator,）。,2.,调控方式,trpR,mRNA,A:,B:,无活性,mRNAs,Trp,trpR,mRNA,RNA pol,经测序后发现，在第一个结构基因（,trpE,）的,5-,端有一个长,160bp,的前导序列（,leader sequence,）。当此序列缺失,130160bp,时，,mRNA,总是最高水平的。而当,trp,存在时，,mRNA,的表达水平降低。因此，此序列称为,衰减子（,attenuator,）。,3.,衰减作用,表,.,衰减子控制的氨基酸合成的操纵子中前导肽的氨基酸序列,操纵子 前导肽的氨基酸序列,色氨酸,MetLysAlaIlePheValLysGly,TrpTrp,ArgThrSer(14,肽,),苏氨酸,MetLysArgIleSer,ThrThr,Ile,ThrThrThr,Ile,Thr,Ile,ThrThr,GlyAsnGlyAlaGly,组氨酸,MetThrArgValGlnPheLys,HisHisHisHisHisHisHis,ProAsp,异亮氨酸,MetThrAla,LeuLeu,Arg,ValIle,Ser,LeuValVal,Ile,Ser,ValValVaLIle,缬氨酸,GEDA,IleIleProProCysGlyAlaAlaLeuGlyArgGlyLysAla,亮氨酸,MetSerHisIleValArgPheThrGly,LeuLeuLeuLeu,AsnAlaPheIleVei,ArgGlyArgProValGlyGlyIleGlnHis,苯丙氨酸,MetLysHisIlePro,PhePhePhe,Ala,PhePhePheThrPhe,Pro,异亮氨酸,-,缬氨酸,B,MetThrThrSerMetLeuAsnAlaLysLeuLeuProThrAlaProSerAla,Ala,ValValValValArgValValValVaLVal,GlyAsnAlaPro,Figure 10.42 An attenuator controls the progression of RNA pol.into the,trp,genes.RNA pol.initiates at the P and then proceeds to position 90,where it pauses before proceeding to the attenuator at 140.,In the absence of Trp,the RNA pol.continues into the,trpE,genes(starts at+163).,In the presence of Trp,90%probability of termination to release the 140b leader RNA.,Figure 10.43 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 the conformation produced when region 1 pairs with region 2,and region 3 pairs with region 4.On the right is the conformation when region 2 pairs with region 3,leaving regions 1 and 4 unpaired.,Figure 9.26 Intrinsic terminators include palindromic regions that form hairpins varying in length from 7-20 bp.The stem-loop structure includes a G-C-rich region and is followed by a run of U residues.,Figure 11.29 The alternatives for RNA polymerase at the attenuator depend on the location of the ribosome,which determines whether regions 3 and 4 can pair to form the terminator hairpin.,第,3,节 真核基因表达调控的要点,真核生物与原核生物基因表达调控的特点比较：,1.,真核生物的基因数目比原核生物的多，而且多数基因组基因含有内含子以及功能不清的重复序列等；,原核的染色质是裸露的,DNA,，而真核的染色质则是由,DNA,与组蛋白紧密结合形成为核小体；,在原核细胞中，染色质的结构对基因的表达没有明显的调控作用，而在真核中，这种作用是明显的。,2.,在原核基因转录的调控中，既有激活物的调控（,正调控,），也有阻遏物的调控（,负调控,），二者同等重要。在真核中虽然也有正调控成分和负调控成分，但迄今已知的主要是正调控。而且一个真核基因通常有多个调控序列，必须有多个激活物同时特异地结合上，才能启动基因的转录；,3.,原核基因的转录和翻译通常偶联在一起，而真核基因的转录是在细胞核中进行，翻译在胞质中进行；,4.,生成的初级转录物需在核中进行转录后的加工和运输，所以，真核基因的表达有多种转录后的调控机制；,5.,真核生物大都为多细胞生物，在个体发育过程中逐步分化形成各种组织和细胞类型。分化是不同基因表达的结果。不同类型的细胞，功能不同，基因表达的情况也不一样。某些基因仅特异地在某种细胞中表达，称为细胞特异性或组织特异性表达，因而具有调控这种特异性表达的机制。,6.,真核生物对外界环境条件变化的反应和 原核生物十分不同。同一群原核生物细胞处在相同的环境条件中，对环境条件的变化会作出基本一致的反应；而真核生物常常只有少部分细胞基因的表达直接受到环境条件变化的影响和调控，其他大部分间接或不受影响。,