第17章RNA的合成.ppt
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1、这是位于甘肃省酒泉市阿克塞哈萨克族自治县县城西约50公里红柳沟的柱廊状宫殿式丹霞地貌局部(4月20日摄)。 甘肃省酒泉市阿克塞哈萨克族自治县红柳沟的丹霞地貌呈南北走向、全长约6公里,处在阿尔金山主峰脚下,以柱廊、宫殿等形状为主,壮观美丽,堪称鬼斧神工。 这段丹霞地貌过去曾长期不被外界了解,2007年经有关丹霞地貌专家考察,确定其为柱廊状宫殿式丹霞地貌。,壮美的南极,第十七章RNA的生物合成,Key Terms,transcription RNA polymerase promoter sites transcription factor footprinting consensus seque
2、nce sigma subunit transcription bubble rho (r) protein TATA box enhancer,pre-mRNA 5 cap poly(A) tail RNA editing RNA splicing spliceosome small nuclear RNA (snRNA) alternative splicing catalytic RNA self-splicing III. Synthesizing,RNA的合成过程,包括起始,延伸和终止三个部分 (RNA)n +NTP (RNA)n +1 + PPi RNA合成需要; 双链DNA 活化
3、前提(NTP) 镁离子或锰离子 酶类,识别阶段;酶与启动子的结合(全酶)DNA链的 局部打开. 转录的起始阶段; DNA摸板和RNA碱基配对 RNA链的延长; 核心酶向前移动,RNA链延长. 转录的终止;酶到达基因转录终点. 新合成的RNA 和 RNA 聚合酶从DNA上脱落.,Transcription Is Catalyzed by RNA Polymerase,We begin our consideration of transcription by examining the process in bacteria such as E. coli. RNA polymerase fro
4、m E. coli is a very large (400 kd) and complex enzyme consisting of four kinds of subunits .The subunit composition of the entire enzyme, called the holoenzyme, is a 2 b b s. The s subunit helps find a promoter site where transcription begins, participates in the initiation of RNA synthesis, and the
5、n dissociates from the rest of the enzyme. RNA polymerase without this subunit (a 2 b b ) is called the core enzyme. The core enzyme contains the catalytic site. This catalytic site resembles that of DNA polymerase in that it includes two metal ions in its active form. One metal ion remains bound to
6、 the enzyme, whereas the other appears to come in with the nucleoside triphosphate and leave with the pyrophosphate. Three conserved aspartate residues of the enzyme participate in binding these metal ions. Note that the overall structures of DNA polymerase and RNA polymerase are quite different; th
7、eir similar active sites are the products of convergent evolution.,一 RNA聚合酶,1960-1961年发现,全酶分子量465000,有五种亚基 组成,含有两个锌原子 全酶中无希格码亚基的酶称为核心酶,核心酶只能使已开始的RNA链延长,不具有起始合成RNA的能力 希格码亚基称为起始亚基 RNA聚合酶的作用是合成一条与被转录的DNA互补的反平行的RNA链。RNA以53方向合成,而RNA聚合酶沿着模板链35方向移动。,RNA Polymerase Structures. The three-dimensional structur
8、es of RNA polymerases from a prokaryote (Thermus aquaticus) and a eukaryote (Saccharoromyces cerevisiae). The two largest subunits for each structure are shown in dark red and dark blue. The similarity of these structures reveals that these enzymes have the same evolutionary origin and have many mec
9、hanistic features in common.,RNA Chains Are Formed de Novo and Grow in the 5 -to-3 Direction,RNA聚合酶的组成,二 真核生物RNA聚合酶,RNA聚合酶 I: 存在于核仁, 转录 28S,18S, 5.8S rRNA. RNA聚合酶II: 存在于核质,转录mRNA RNA聚合酶III: 存在于核质,转录tRNA和 5SrRNA 结构含有8-14个亚基,转录泡,三 摸板链 负链 无意义链 非摸板链 正链 有意义链,DNA Unwinding. RNA polymerase unwinds about 17
10、 base pairs of template DNA.,Transcription Bubble. A schematic representation of a transcription bubble in the elongation of an RNA transcript. Duplex DNA is unwound at the forward end of RNA polymerase and rewound at its rear end. The RNA-DNA hybrid rotates during elongation.,四 RNA聚合酶与启动子的识别,启动子(Pr
11、omoter),Prokaryotic Promoter Sequences. A comparison of five sequences from prokaryotic promoters reveals a recurring sequence of TATAAT centered on position -10. The -10 consensus sequence (in red) was deduced from a large number of promoter sequences. The sequences are from the (A) lac, (B) gal, a
12、nd (C) trp operons of E. coli; (D) from l phage; and (E) from F C174 phage. III. Synthesizing,The first nucleotide (the start site) of a transcribed DNA sequence is denoted as +1 and the second one as +2; the nucleotide preceding the start site is denoted as -1. These designations refer to the codin
13、g strand of DNA. Recall that the sequence of the template strand of DNA is the complement of that of the RNA transcript (see Figure 5.26). In contrast, the coding strand of DNA has the same sequence as that of the RNA transcript except for thymine (T) in place of uracil (U). The coding strand is als
14、o known as the sense (+) strand, and the template strand as the antisense (-) strand.,启动子,在RNA合成中用作模板的链称模板链或负(-)链。与模板链互补的DNA链称为非模板链或正(+)链。非模板链与转录的RNA在碱基序列上是一致的,只是DNA中用T代替了U。非模板链有时又称为编码链,它在转录和蛋白质合成中没有直接功能。RNA合成中转录起始点标上+1,这个位点前面的碱基对标以负数,它们是不被转录的,而起始之后的碱基序列标以正数。这里不设0数。5端方向称上游(up stream)3端方向称下游(downstre
15、am)根据习惯是指非模板链中的序列。,实验证明,启动子序列中某些核苷酸是保守的,即在大多数情况下它们都会出现,称为共有序列。原核与真核生物启动子通常含有两个共有序列。原核生物共有序列出现在-10和-35所以称-10区和-35区。真核生物共有序列出现在-25和-75,分别称为-25区(TATA box或Hoghess box)和-75区(CAAT box)。紧邻转录起始点的共有序列是一个A/T富含区,它使DNA双链容易局部解链。,五 起 始,1亚基与核心酶结合形成RNA聚合酶全酶。 2全酶以静电作用和启动子上游非特异性DNA 结合。 3RNA聚合酶全酶沿DNA上下游动,包括共有序列在内与启动子特
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