胺基酸-tRNA合成酶（aaRSs）是必需的轉譯酵素，其中每一種都催化特定胺基酸與其同源tRNA的結合。另外5′ G-1的存在是tRNAHis在所有三個生命領域中的獨特特徵。這種不尋常的G-1是tRNAHis的主要辨識決定基，對於胺基酸-tRNA合成酶（HisRS）的辨識至關重要。G-1可由兩種完全不同的機制產生：在細菌（和許多古細菌）中，G-1是由tRNAHis基因轉錄而來；在真核生物中，G-1是在tRNAHis轉錄後，由 Thg1酵素’’添加上去的，Thg1主要辨認tRNAHis的反密碼。一般而言，具有A73的tRNAHis是用轉錄後修飾的方法獲得G-1，具有C73的tRNAHis則是由基因轉錄直接而來。我們的結果顯示：人（Homo sapiens）、果蠅（Drosophila melanogaster）、和家蠶（Bombyx mori）的Thg1酵素都具有線粒體標的訊號（MTS），因此這些Thg1可以同時在細胞質及線粒體中作用。此外，我們發現這些Thg1可以同時將G-1添加在細胞質tRNAnHis (含有A73)和線粒體tRNAmHis (含有C73)。有趣的是，人類和果蠅Thg1酵素利用ATP依賴性機制將G-1加到tRNAnHis，卻是利用GTP依賴性機制將G-1加到tRNAmHis。本研究為真核細胞Thg1酵素如何作用在細胞質及線粒體提供了新的見解。;Aminoacyl-tRNA synthetases (aaRSs) are essential translation enzymes, each catalyzes the coupling of a specific amino acid to its cognate tRNAs. The presence of an additional 5’ guanosine residue (G-1) is a unique feature of tRNAHis in all three domains of life. This unusual G-1 residue is the major identity element of tRNAHis and is essential for recognition by histidyl-tRNA synthetase (HisRS). The functional significance of G-1 is evidenced by the existence of two entirely distinct mechanisms: G-1 is encoded in the tRNAHis gene of bacteria (and many archaea) and G-1 is added post-transcriptionally by tRNAHis guanylyltransferase (Thg1) in eukaryotes. Thg1 possesses an efficient 3’–5’polymerase activity that specifically adds the G-1 residue by recognizing the anticodon of tRNAHis. Typically, G-1 addition to tRNAHis with A73 is mediated via an ATP-dependent mechanism in eukaryotes, while G-1 addition to tRNAHis with C73 is mediated via a GTP-dependent mechanism in prokaryotes. We reported herein that like human Thg1, Thg1 homologs of Drosophila melanogaster and Bombyx mori also possess a mitochondrial targeting signal (MTS). As a result, these Thg1 enzymes can be targeted to both cytoplasm and mitochondria. Moreover, we found that these Thg1 enzymes can attach G-1 to both cytoplasmic tRNAsHis with A73 and mitochondrial tRNAsHis with C73. Interestingly, Drosophila Thg1 enzyme attaches G-1 to tRNAnHis via an ATP-dependent mechanism, while it attaches G-1 to tRNAmHis via a GTP-dependent mechanism. This study provides new insight into the mechanisms of tRNAHis maturation in the cytoplasm and mitochondria.
