第17章 RNA的合成课件.ppt

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 sequence sigma subunit transcription bubble rho (r) protein TATA box enhancer 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 from 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 then 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 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; their similar active sites are the products of convergent evolution. RNA Polymerase Structures. The three-dimensional structures of RNA polymerases from a prokaryote(Thermus aquaticus) and a eukaryote (Saccharoromyces cerevisiae). The two largest subunits for each structureare shown in dark red and dark blue. The similarity of these structures reveals that these enzymes have the sameevolutionary origin and have many mech