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转运-信使RNA

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维基百科,自由的百科全书
转运-信使RNA
tmRNA (PDB 3IYR)
识别符
代号 tmRNA
Rfam RF00023
其他数据
RNA类型 基因
tmRNA同时具有tRNA与mRNA的功能与结构

转运-信使RNA(Transfer-messenger RNA,简称tmRNA,旧称10Sa RNASsr RNA)是细菌中一种同时具有mRNAtRNA功能与结构的RNA,此RNA本身如mRNA般具有编码区,3'端可如tRNA般由丙胺酸-tRNA合成酶英语Alanine—tRNA ligase加上一个丙胺酸[1],长度多介于320至400nt之间(少数结构退化者可能更短)[2],当细胞中的核糖体转译mRNA发生停滞时(包括mRNA突变而失去终止密码子等原因),tmRNA可与SmpB蛋白与EF-Tu等组合成一核糖核蛋白,并结合至核糖体空出的A位英语A-site以启动反式转译(trans-translation),即将核糖体合成中的多肽转移至其3'端的丙胺酸上,以tmRNA的编码区为模板继续转译数个胺基酸后再遇到终止密码子而结束,转译出的蛋白末端带有丙胺酸和tmRNA编码的小段多肽序列(在大肠杆菌中为AANDENYALAA),此段序列通常会被细胞中的蛋白酶识别而将整个蛋白质降解[3],造成转译停滞的mRNA也会被水解移除[4]

几乎所有细菌基因组中皆具有编码tmRNA的基因[5],此RNA对某些细菌的生长相当重要,也有细菌在较为不利的生长条件下才需tmRNA[6]。tmRNA的合成途径和tRNA相似,转录后其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英语Ribonuclease T核糖核酸酶PH英语RNase PH切除[7][8],可连接丙胺酸的3'-CCA序列在某些物种是由基因编码,因此转录完成时即具备,某些物种则是在tmRNA转录与切割之后才由tRNA核苷酸转移酶英语tRNA nucleotidyltransferase加上[9]。另外α-变形菌、某些蓝绿菌与少数β-变形菌的tmRNA是由两段RNA组成的,其中一段RNA的3'端结合丙胺酸,另一段RNA则具编码序列,两者仅以碱基配对的氢键相互结合[10]

tmRNA最早于1979年研究人员以电泳分析大肠杆菌RNA时与核糖核酸酶P上的M1RNA(旧称10Sb RNA)一起被发现,当时被命名为10Sa RNA[11] ,后续研究发现10Sa RNA具有常见于tRNA的假尿嘧啶核苷,且其3'端与tRNA的结构相似[12]。1995年始发现tmRNA还有编码蛋白序列的功能[13]。除细菌外,少数真核生物(雅各比虫英语jakobid与属卵菌大豆疫霉菌英语Phytophthora sojae粒线体中的mtDNA也有编码tmRNA,但皆已失去与mRNA相似的编码区,仅存与tRNA相似的区域[9],还有数种藻类的质粒体中具有tmRNA,结构简化但仍具编码区和tRNA相似区[14]

反式转译过程示意图

参见

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参考文献

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