【正文】 fferent hydroxyl group ? There is no hydrolysis and no energy loss Alternative splicing Transsplicing Alternative splicing Splicing is precise BUT... ? Individual premRNA may be spliced to more than one kind of mRNA by removing different binations of introns/exons ? Alternative forms can be regulated Sex determination in Drosophila Transsplicing ? Definition 539。 P269圖 大多數(shù) mRNA的 3?未端有 polyA以 ATP為底物，在 RNA末端腺苷酸轉(zhuǎn)移酶的作用下，在 3?端逐個(gè)添加多聚腺苷酸尾巴。 轉(zhuǎn)錄產(chǎn)物的 “加工 ” 返回 mRNA前體的轉(zhuǎn)錄后加工，包括了對(duì) 5‘ 端和3‘ 端（首、尾部）的修飾以及對(duì)中間的部分進(jìn)行剪接（ splicing）。 轉(zhuǎn)錄產(chǎn)物的 “加工 ” 返回 內(nèi)含子（ intron）： 真核細(xì)胞 mRNA前體的加工 基因的插入順序?yàn)椴荒芫幋a時(shí) DNA順序。 三、不同 RNA剪接方式導(dǎo)致一個(gè)基因多種產(chǎn)物 四、核糖體 RNA和 tRNA前體的轉(zhuǎn)錄后加工 轉(zhuǎn)錄出來(lái)的 RNA稱(chēng)為 RNA前體 ，需要進(jìn)行加 工原始轉(zhuǎn)錄產(chǎn)物 → 成熟 RNA分子 此過(guò)程稱(chēng) “轉(zhuǎn)錄后加工 ” 。 Relative positions of GTFs and RNAP II on DNA RNAP II crab claws clamp over the DNA near the initiation site. Melting of the DNA is assisted by the TFIIH helicase (not shown) stimulated by TFIIE Formation of preinitiation plex TBP (TFIID) binds to the promoter DNA (TATA + Inr) recruits TFIIB recruits RNAP II + TFIIF recruit TFIIE recruits and stimulates TFIIH ?Helicase activity unwinds DNA near start site ?Kinase activity phosphorylates CTD An elongation factor called TFIIS stimulates elongation for RNAP II. TFIIF also has a role in elongation. After termination of transcription the CTD is dephosphorylated and the RNAP II can reenter a preinitiation plex (PIC) Elongation by RNAP II POL II IIF CTD MULTISTEP MODEL PREINITIATION COMPLEX INR TATA UPE 20 +1 IID IIA IIB H J E MULTISTEP MODEL INITIATION COMPLEX PHOSPHORYLATION OF CTD INR TATA UPE 20 +1 IID IIA IIB H J E POL II IIF CTD ATP MULTISTEP MODEL PROMOTER CLEARANCE INR TATA UPE 20 +1 IID IIA IIB H J E POL II CTD RIBONUCLEOTIDES premRNA ELONGATION FACTORS INITIATION PROMOTER CLEARANCE ELONGATION 第五節(jié) 新生 RNA的剪接和修飾 一、除去內(nèi)含子的剪接過(guò)程 內(nèi)含子有四種類(lèi)型 二、真核生物 mRNA進(jìn)行裝飾性加工 真核生物轉(zhuǎn)錄特點(diǎn)： ① 轉(zhuǎn)錄和翻譯在時(shí)間及空間上是分割的，保證了精細(xì)的調(diào)節(jié)。 7. 真核細(xì)胞中 DNA與組蛋白結(jié)合在一起，形成染色質(zhì)， 后者進(jìn)一步盤(pán)曲、折疊形成染色體，其中只有一小 部分能轉(zhuǎn)錄。而真核 細(xì)胞，每一種蛋白質(zhì)的基因都有自己獨(dú)立的啟動(dòng)子，所以 真核細(xì)胞轉(zhuǎn)錄產(chǎn)物是單順?lè)醋?mRNA。 順式作用元件并不能直接發(fā)揮作用，要與 反式作用因子（ transacting factors）相互作用來(lái)調(diào)控轉(zhuǎn)錄，反式作用因子是一些特殊的蛋白質(zhì)因子。 沉默子（ silencer）：降低轉(zhuǎn)錄的速度，沉默子也稱(chēng)抑 制子。 ?轉(zhuǎn)錄因子的功能：調(diào)節(jié) RNA聚合酶的活性，將 RNA 聚合酶引到啟動(dòng)子位置。 2. 真核啟動(dòng)子比原核啟動(dòng)子更復(fù)雜和更多樣性，不同的 RNA聚合酶有不同的啟動(dòng)子。 原核生物的轉(zhuǎn)錄過(guò)程 Chain Elongation Core polymerase no sigma factor ? Polymerase is pretty accurate only about 1 error in 10,000 bases (not as accurate as DNAP III) ? Even this error rate is OK, since many transcripts are made from each gene ? Elongation rate is 2050 bases per second slower in G/Crich regions and faster elsewhere ? Topoisomerases precede and follow polymerase to relieve super coiling Science, vol. 281, p 424 (1998) Spatial Organization of Transcription Elongation Complex in E. coli Interactions between nucleic acids and the core enzyme keep RNAP processive 7. ?因子識(shí)別終止信號(hào)（不衣賴(lài) ?因子） 終 止 階 段 8. 核心酶不停止轉(zhuǎn)錄 9. RNA鏈釋放出來(lái) Two mechanisms ? Rho(ρ) the termination factor protein ? rho is an ATPdependent helicase ? it moves along RNA transcript, finds the bubble, unwinds it and releases RNA chain ? Specific sequences termination sites in DNA ? inverted repeat, rich in G:C, which forms a stemloop in RNA transcript ? 68 As in DNA coding for Us in transcript Chain Termination Rhoindependent transcription termination (depends on DNA sequence NOT a protein factor) Stemloop structure Rhoindependent transcription termination ? RNAP pauses when it reaches a termination site. ? The pause may give the hairpin structure time to fold ? The fold disrupts important interactions between the RNAP and its RNA product ? The Uric