人類細胞核基因體擁有兩個不同的基因，分別解碼細胞質及粒線體histidyl-tRNA synthetase (HisRS)，細胞質中的HisRS ( HsHisRSc)主要辨識G-1和A73核?酸；相反的，粒線體內的HisRS ( HsHisRSm)不僅辨識G-1:C73，也辨識粒線體tRNAHis的反密碼。在此篇論文中，我們探討人類野生型與突變型HisRS對於tRNAHis G-1和G34 (反密碼子上最有影響力的核?酸)的辨識，胺基酸R169中負責辨識G-1， 胺基酸EQE (469 to 471)負責辨識G34，隨後我們將此進行突變，分析其互補活性 (體內)與胺醯化活性(體外)，有趣的是，突變EQE時，削弱了部分HsHisRSc的活性，而 R169 的突變則完全阻斷了其體外和體內活性。相比之下，HsHisRSm 的 EQE 或 R170（相當於HsHisRSc 中的 R169）突變完全阻斷了體內活性。此外，HsHisRSc 會結合自身的 mRNA 的一個小片段，這個小片段會摺疊成反密碼子類似結構（一種具有 GUG 的反密碼子的stem-loop結構），這個結果顯示HsHisRSc可能可以抑制它自己的蛋白質表達。本研究強調二個演化相近的HisRS異構?具有截然不同的tRNA喜好。;Humans possess two distinct nuclear genes encoding closely related mitochondrial and cytoplasmic isoforms of histidyl-tRNA synthetase (HisRS), respectively. The human cytoplasmic form of HisRS (HsHisRSc) recognizes a G nucleotide with 5’-monophosphate at position -1 (G-1) and A73 as the major identity elements. In contrast, its mitochondrial counterpart (HsHisRSm) recognizes not only G-1:C73 but also the anticodon of the mitochondrial tRNAHis. In this thesis, we focused on recognition of G-1 and G34 (the most influential nucleotide of the anticodon) of human tRNAHis isoacceptors by the WT and mutant HisRS isoforms. The amino acid R169 in the motif 2, which is responsible for recognition of G-1, and EQE (469 to 471) in the anticodon-binding domain, which is responsible for recognition of G34, were mutated, and the complementation (in vivo) and aminoacylation activities (in vitro) of the resultant mutants were analyzed. Interestingly, mutation at EQE partially impaired the activity of HsHisRSc, while mutation at R169 completely blocked its activity in vitro and in vivo. In contrast, mutation at EQE or R170 of HsHisRSm (equivalent to R169 of HsHisRSc) completely blocked its activity in vivo. In addition, HsHisRSc robustly bound an anticodon mimic (an anticodon-like stem-loop structure with GUG in the loop) retrieved from its own mRNA, suggesting that this enzyme might regulate its own expression via autogenous translation repression. This study underscores the specialized tRNA preferences of two closely related HisRS isoforms.
