轉運-信使RNA

轉運-信使RNA(Transfer-messenger RNA,簡稱tmRNA,舊稱10Sa RNASsr RNA)是細菌中一種同時具有mRNAtRNA功能與結構的RNA,此RNA本身如mRNA般具有編碼區,3'端可如tRNA般由丙胺酸-tRNA合成酶英語Alanine—tRNA ligase加上一個丙胺酸[1],長度多介於320至400nt之間(少數結構退化者可能更短)[2],當細胞中的核糖體轉譯mRNA發生停滯時(包括mRNA突變而失去終止密碼子等原因),tmRNA可與SmpB蛋白與EF-Tu等組合成一核糖核蛋白,並結合至核糖體空出的A位英語A-site以啟動反式轉譯(trans-translation),即將核糖體合成中的多肽轉移至其3'端的丙胺酸上,以tmRNA的編碼區為模板繼續轉譯數個胺基酸後再遇到終止密碼子而結束,轉譯出的蛋白末端帶有丙胺酸和tmRNA編碼的小段多肽序列(在大腸桿菌中為AANDENYALAA),此段序列通常會被細胞中的蛋白酶識別而將整個蛋白質降解[3],造成轉譯停滯的mRNA也會被水解移除[4]

轉運-信使RNA
tmRNA (PDB 3IYR)
識別符
代號 tmRNA
Rfam RF00023
其他資料
RNA類型 基因
tmRNA同時具有tRNA與mRNA的功能與結構

幾乎所有細菌基因組中皆具有編碼tmRNA的基因[5],此RNA對某些細菌的生長相當重要,也有細菌在較為不利的生長條件下才需tmRNA[6]。tmRNA的合成途徑和tRNA相似,轉錄後其5'端有一段序列需由核糖核酸酶P切割[1],3'端也有些序列需由核糖核酸酶T英語Ribonuclease T核糖核酸酶PH英語RNase PH切除[7][8],可連接丙胺酸的3'-CCA序列在某些物種是由基因編碼,因此轉錄完成時即具備,某些物種則是在tmRNA轉錄與切割之後才由tRNA核苷酸轉移酶英語tRNA nucleotidyltransferase加上[9]。另外α-變形菌、某些藍綠菌與少數β-變形菌的tmRNA是由兩段RNA組成的,其中一段RNA的3'端結合丙胺酸,另一段RNA則具編碼序列,兩者僅以鹼基配對的氫鍵相互結合[10]

tmRNA最早於1979年研究人員以電泳分析大腸桿菌RNA時與核糖核酸酶P上的M1RNA(舊稱10Sb RNA)一起被發現,當時被命名為10Sa RNA[11] ,後續研究發現10Sa RNA具有常見於tRNA的假尿嘧啶核苷,且其3'端與tRNA的結構相似[12]。1995年始發現tmRNA還有編碼蛋白序列的功能[13]。除細菌外,少數真核生物(雅各比蟲英語jakobid與屬卵菌大豆疫黴菌英語Phytophthora sojae粒線體中的mtDNA也有編碼tmRNA,但皆已失去與mRNA相似的編碼區,僅存與tRNA相似的區域[9],還有數種藻類的質體體中具有tmRNA,結構簡化但仍具編碼區和tRNA相似區[14]

反式轉譯過程示意圖

參見

參考文獻

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