核糖核酸酶Z(Ribonuclease Z、RNase Z、3′ tRNase,在不同生物中的名稱包括ElaC、ZiPD、RNase BN、TRZ1等)是一種參與tRNA生合成的核糖核酸酶,為內切酶,屬依賴型金屬水解酶,編碼此蛋白的基因於2002年被發現[1][2]

核糖核酸酶Z
枯草桿菌的核糖核酸酶Z與tRNA結合的結構圖
识别码
EC編號 3.1.26.11
CAS号 98148-84-6
数据库
IntEnz IntEnz浏览
BRENDA英语BRENDA BRENDA入口
ExPASy英语ExPASy NiceZyme浏览
KEGG KEGG入口
MetaCyc英语MetaCyc 代谢路径
PRIAM英语PRIAM_enzyme-specific_profiles 概述
PDB RCSB PDB PDBj PDBe PDBsum

tRNA基因轉錄產生tRNA前驅物(pre-tRNA)後,其5′端會被核糖核酸酶P切割,3′端則被核糖核酸酶Z切割,隨後再由CCA tRNA核苷酸轉移酶英语CCA tRNA nucleotidyltransferase在其3′端加上CCA三個鹼基,以生成成熟的tRNA[1][3][4]。核糖核酸酶Z切割位點下游位點的CC會抑制其切割活性,因此已被加上CCA的成熟tRNA不會再被其切割[2][註 1],此外tRNA前驅物5′端序列的長度也可能影響核糖核酸酶Z切割的活性[6][7]

釀酒酵母[8]大腸桿菌[9]枯草桿菌[10]海栖热袍菌[11]等生物的核糖核酸酶Z結構均已被解出[6]

演化與功能

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三域生物皆有核糖核酸酶Z,已被定序的所有真核生物古菌以及許多細菌皆有之,但變形菌門的細菌多不具此酵素。自然界中存在兩型的核糖核酸酶Z,較短的RNase ZS長280至360個氨基酸,見於三域生物[2];較長的RNase ZL長度約為前者兩倍,在演化上應是由前者經基因重複產生,只見於真核生物[2]。RNase ZS會以二聚體的形式切割tRNA,RNase ZL則是以單體的形式作用,且有研究顯示後者的切割活性比前者的高許多[6]

脊椎動物植物以外的真核生物(包括釀酒酵母、粟酒裂殖酵母黑腹果蠅秀麗隱桿線蟲模式生物[2])經常只有RNase ZL[12];而同時具有RNase ZL和RNase ZS的生物中兩者在細胞中的位置可能不同[2],例如模式植物阿拉伯芥分別有兩個RNase ZS與RNase ZL,前者一個位於細胞質,一個位於葉綠體中,後者一個位於細胞核與粒線體,一個僅見於粒線體[13][14];釀酒酵母僅有RNase ZL,位於細胞核與粒線體中[8]

人類的RNase ZL(ELAC2)基因有兩個起始密碼子,可轉錄產生兩種不同的mRNA,其中較長者包含一粒線體導向序列,會被送入粒線體中,負責切割粒線體基因組編碼的tRNA前驅物;較短者則會被送入細胞核中,切割細胞核編碼的tRNA前驅物,除產生成熟tRNA外,也參與tRNA片段(tRNA fragment)的生成,進而影響細胞內各種小RNA量的平衡[2][15][16]已知有ELAC2基因的突變前列腺癌心肌病變相關[6][8][17][18]。人類的RNase ZS(ELAC1)則位於細胞質中,其功能仍不甚清楚,有研究指其可能參與解決轉譯核糖體停滯的反應途徑,停滯的核糖體上P位點的tRNA 3′端會被內切酶ANKZF1英语ANKZF1切割,將與其連結的多肽鏈和末端的CCA鹼基一起移除,造成tRNA最末端的核苷酸形成2′,3′-環磷酸(2′,3′-cyclic phosphate),ELAC1可能可切割此結構,使tRNA重新產生有活性的3′端,得以再被CCA tRNA核苷酸轉移酶作用接上CCA而重新利用[19][20]

切割其他RNA

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除tRNA前驅物外,核糖核酸酶Z可能還可切割其他與tRNA前驅物結構相似的RNA。核糖核酸酶P與核糖核酸酶Z可切割MALAT1(一個長鏈非編碼RNA)的3′端,產生MALAT1相關胞漿小RNA(mascRNA)[21];另有一3′端和MALAT1高度相似的長鏈非編碼RNAMEN β RNA可能也可被核糖核酸酶P與核糖核酸酶Z切割,產生類似mascRNA的小RNA[22]。阿拉伯芥編碼tRNAGly的基因下游緊接著編碼snoR43家族的snoRNA基因,兩者會共同轉錄成一RNA前驅物,並被核糖核酸酶Z切割,以產生成熟的tRNA與snoRNA[23]

參見

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  • 核糖核酸酶E英语Ribonuclease E:某些細菌具有的一種核糖核酸酶,亦為內切酶,tRNA前驅物的3′端可被其切割後,再經由其他外切酶切割產生成熟tRNA,為tRNA 3′成熟的另一機制,不依賴核糖核酸酶Z。部分真核生物可能也有類似機制[2]

註腳

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  1. ^ 此機制有許多例外情形,相較於真核生物tRNA末端的CCA皆是轉錄後才加上,許多細菌的tRNA基因末端即編碼CCA,意即tRNA前驅物中具有CCA,但仍可被核糖核酸酶Z以較低的活性切割。海栖热袍菌的核糖核酸酶Z可能不受CCA抑制,可正常切割含有CCA的tRNA前驅物[2][5]

參考文獻

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