Cooley L, Appel B, Söll D
Proc Natl Acad Sci U S A. 1982 Nov;79(21):6475-9. doi: 10.1073/pnas.79.21.6475.
All sequenced histidine tRNAs have one additional nucleotide at the 5' end when compared to other tRNA species. Sequence analysis of histidine tRNA genes from Drosophila melanogaster and Schizosaccharomyces pombe showed that the terminal guanylate residue of the mature tRNAs is not encoded by the genes. Analysis of the products from in vitro transcription of these genes in extracts from Drosophila Kc cells demonstrated that the 5'-terminal nucleotide present in the mature tRNA is added post-transcriptionally. The addition reaction requires ATP. A portion of the mature tRNAs are then modified at the 5'-terminal pG. Analysis of the RNA species formed during the in vitro maturation of the Drosophila histidine tRNA primary transcript uncovered the following maturation scheme: (i) the primary transcript is processed by RNase P at the 5' end to form an intermediate precursor; (ii) the 3'-flanking sequence is endonucleolytically removed, and a guanylate moiety is added to the 5' end to form mature-sized histidine tRNA; and (iii) a fraction of the 5'-terminal guanylate residues then undergoes modification. In contrast to the capping of eukaryotic mRNA, the guanylate addition to histidine tRNA results in the formation of a (3'-5')-phosphodiester bond. There are no precedents for the post-transcriptional addition of nucleotides (in phosphodiester linkage) to the 5' end of RNA precursors.
与其他tRNA种类相比，所有已测序的组氨酸tRNA在5'端都有一个额外的核苷酸。对黑腹果蝇和粟酒裂殖酵母的组氨酸tRNA基因进行序列分析表明，成熟tRNA的末端鸟苷酸残基并非由基因编码。对这些基因在果蝇Kc细胞提取物中进行体外转录产物的分析表明，成熟tRNA中存在的5'-末端核苷酸是在转录后添加的。添加反应需要ATP。然后，一部分成熟tRNA在5'-末端的pG处发生修饰。对果蝇组氨酸tRNA初级转录本体外成熟过程中形成的RNA种类进行分析，揭示了以下成熟模式：（i）初级转录本在5'端由RNase P加工形成中间前体；（ii）3'-侧翼序列通过内切核酸酶去除，鸟苷酸部分添加到5'端形成成熟大小的组氨酸tRNA；（iii）一部分5'-末端鸟苷酸残基随后发生修饰。与真核mRNA的加帽不同，组氨酸tRNA的鸟苷酸添加导致形成（3'-5'）-磷酸二酯键。在RNA前体的5'端进行转录后添加核苷酸（以磷酸二酯键连接）没有先例。
