2011 分子生物学 第四章(3).ppt

l非进行型复合体 Non processivecomplex 未被激活的基因的表达方式 RNApolIIpausingat5 endofgene 并表现为DRBR 是对基因非激活转录的中止机制 维持正常生理代谢进行的机制 与转录启动复合体的非完整性有关 abortiveTIC Terminator AsequenceofDNA presentedattheendofthetranscript thatcausesRNApolymerasetoterminatetranscription 4 4Transcriptiontermination 4 4 1Terminationinprokaryotes Palindromicsequence G Crichregionthatformshairpinsofvaryinglengths 7 20bp inRNA G CrichhairpinarefollowedbyarunofUresiduesinRNA Structure Rho independentterminator Intrinsicterminator Function G Crich transcriptiondelay RNAstem loop polyA U RNApolleave inhibitedbyKfactor RNApol DNAseparated 因子 促进RNApol NTP RNAelongation 完成NMP之间的磷酸脂键的连接 Editing 与Rho 因子竞争RNA3 end 构成HoloEnzyme后 因子含有两个位点 Isite RifS Esite RifR 要求高浓度的ATPorGTP 专一性地结合ATPorGTP 对NTP非专一性结合 Rho factor lHexamer55kdneutral lContestRNA3 endwith factor lATPase like GTPase likeactivityneedsspacerof 50Nt l转录终止效率取决于紧随回文的下游序列 保证RNA DNA具有一定松弛的结合力 过低的结合力 成为Rho independentterminator 过高的结合力 引起RNApolread through 各依赖Rho因子的终止子间的效率差异取决于终止子区DNA RNA间结合力的差异 Modelofrho dependenttermination b Whenthehairpinformsinthetranscript thepolymerasepauses givingRhoachancetocatchup c RhohelicaseunwindstheRNA DNAhybridandreleasesthetranscript a Rhobindstothetranscriptandpursuesthepolymerase Theactionofrhofactormaycreatealinkbetweentranscriptionandtranslationwhenarho dependentterminatorliessoonafteranonsensemutation Antiterminationdependsonspecificsites 4 4 2Anti terminationinRho dependentterminatorofProkaryotes Antiterminationproteins allowRNApolymerasetotranscribethroughcertainterminatorsites AntiterminationcanbeusedtocontroltranscriptionbydeterminingwhetherRNApolymeraseterminatesorreadsthroughaparticularterminatorintothefollowingregion Switchesintranscriptionalspecificitycanbecontrolledatinitiationortermination AncillaryfactorsbindtoRNApolymeraseasitpassescertainsites Thenutsiteconsistsoftwosequences NusB S10joincoreenzymeasitpassesboxA ThenNusAandpNproteinbindaspolymerasepassesboxB ThepresenceofpNallowstheenzymetoreadthroughtheterminator producingajointmRNAthatcontainsimmediateearlysequencesjoinedtodelayedearlysequences RNApolymerasemayalternatebetweeninitiation competentandtermination competentformsassigmaandNusfactorsalternativelyreplaceoneanotheronthecoreenzyme nusA与核心酶结合 但并不与全酶发生作用 当Sigma因子加入到酶复合体中 它代替了nusA蛋白质 重新组成了全酶 这也就是说RNA酶以准备起始和准备终止的交替形式存在 经历了如图所示的循环 当全酶与启动子结合后 释放了Sigma因子 产生了核心酶来合成RNA 当一个nusA蛋白质辩识了核心酶并与之结合 就产生了酶复合物 当聚合酶与DNA结合时 nus成分不能被替换 当发生终止时 酶被释放 nus因子或者被释放 或者被sigma因子所取代 phageregulatoryregioncontainsaclusteroftrans cis factor Np Np Cis F PR OR NuTR TR PRE PRM PLOL NuTL TL1 TL2 PintTrans F CI Cro CII N Q CIII int Xis 4 4 3Principleofanti terminationin phage N p结合在NuTR1 NuTL1 位点 等待RNApol Np使RNApol变构并不能与Rho因子结合 RNApol经过terminator并发生通读现象 read through HostRNApolymerasetranscribeslambdagenesandterminatesattsites pNallowsittoreadthroughterminatorsintheLandR1units pQallowsittoreadthroughtheR terminator ThesitesatwhichpNacts nut andatwhichpQacts qut arelocatedatdifferentrelativepositionsinthetranscriptionunits pQ通过在qut位点起作用 qut序列位于后期转录单位的起点 它的前部分位于起动子之中 后部分在被转录部分的开始 这暗示了pQ起作用与DNA的识别是联系着的 它作用的机理 至少涉及最初结合复合体 一定与pN不同 pQ的基本作用是干扰暂停 一旦pQ对RNA聚合酶起作用 酶在包括rho 依赖性和真实终止子在内的所有位点处的暂停将缩短 这样 pQ没有直接作用于终止子本身 而是让酶迅速通过终止子 使核心酶和 或 辅助因子没有机会引起终止 抗终止机制是否多发生于原核生物内 4 5Pre RNAprocessinginEukaryotes 4 5 1概念 pre RNA cappingtailingsplicingmethylationediting 生物学意义 matureRNA preventpre RNAfromdigestedbyRNase astemplate proteintranslation linterruptedgene interruptedRNA removeintrons stopcodon RNAsplicingistheprocessofexcisingthesequencesinRNAthatcorrespondtointrons sothatthesequencescorrespondingtoexonsareconnectedintoacontinuousmRNA pre mRNAheterogeneousnuclearRNA hnRNA 核内不均一RNA hnRNP ribonucleoproteosome Types structureofCAPin