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转录前起始复合体

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真核生物mRNA转录的转录前起始复合体示意图

转录前起始复合体(Transcription preinitiation complex,简称PIC)是真核生物古菌细胞基因转录前由约100个蛋白质组成的复合体,真核生物转录mRNA的前起始复合体包括负责转录的RNA聚合酶Ⅱ[1][2]、6种通用转录因子TFIIA英语TFIIATFIIB英语TFIIBTFIID英语TFIIDTFIIE英语TFIIETFIIF英语TFIIFTFIIH英语TFIIH)与其他辅助蛋白,组成过程中TFIID的TATA结合蛋白次单元首先在TFIIA的帮助下与DNA启动子TATA盒(或起始子元件英语initiator element等其他元件)结合[3],接着TFIIB也会与启动子上的BRE元件(TFIIB-recognition element)结合[4],D、A、B三者与启动子结合组成的复合体可再吸引TFIIF和RNA聚合酶Ⅱ前来[3],随后TFIIE、TFIIH再先后与复合体结合[5],后者结合DNA中的模板股,具解旋酶活性,可将启动子区域的DNA双股螺旋解开以形成转录泡,此外还具激酶活性,可将RNA聚合酶Ⅱ的C端结构域磷酸化以活化聚合酶开始转录[5]。除了以上六种通用转录因子外,转录前起始复合体还包括中介体染色质重塑酶英语Chromatin remodeling、其他共同活化物英语Coactivator (genetics)等调控蛋白[6],其中中介子与转录前起始复合体结合后,可与距离较远的调控序列强化子结合以促进转录进行[7]。古菌的转录前起始复合体与真核生物的类似,但较为简化,仅需TATA结合蛋白、TFIIB英语Archaeal transcription factor B与RNA聚合酶,且转录因子与DNA结合的方向和真核生物的可能为相反[8]

真核生物的转录起始后,聚合酶合成约10个碱基的RNA时可能发生流产性起始英语abortive initiation,即聚合酶停止转录、将此10nt左右的RNA释出,但仍与启动子结合,随后聚合酶成功脱离启动子和转录前起始复合体(启动子解离)才可继续转录产生完整的mRNA(此过程中RNA聚合酶Ⅱ的C端结构域会被磷酸化[9][10],在此之前可能会发生数次的流产性起始[11]

RNA聚合酶I英语RNA polymerase I(转录rRNA)与RNA聚合酶III(转录tRNA等小RNA)转录起始的机制则与RNA聚合酶II的不同。RNA聚合酶I转录前,UBTF先与起始位点上游100至200nt处的上游控制元件(UCE)结合,再与选择因子I英语Selective factor 1复合体(或称TIF-IB)结合,促使上游控制元件与启动子核心区域接触,随后RNA聚合酶I再与此复合体结合并开始转录[12][13];RNA聚合酶III的转录起始则仰赖位于起始位点下游的调控序列(internal control sequences),具体机制因转录小RNA的种类而异[14]

参考文献

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