酵母菌valyl-tRNA synthetase附加區段的 生物功能之探討;Elucidating the biological functions of the appended domain of yeast ValRS

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题名:	酵母菌valyl-tRNA synthetase附加區段的生物功能之探討;Elucidating the biological functions of the appended domain of yeast ValRS
作者:	謝佳容;Chia-Jung Hsieh
贡献者:	生命科學研究所
关键词:	tRNA合成酶;胺醯化作用;valine;valine tRNA synthetase;Aminoacylation
日期:	2002-06-21
上传时间:	2009-09-22 10:17:00 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	在酵母菌的系統中，相對應於每種胺基酸分別有兩套解碼自細胞核基因的tRNA 合成酵素。其中一套用於細胞質中的蛋白質合成作用，另一套則是在粒線體中執行反應。然而，在之前的研究中顯示 VAS1 是酵母菌染色體中唯一能解碼valine tRNA 合成酵素的基因。此基因利用選擇性轉錄及轉譯作用同時合成細胞質及粒線體的valine tRNA 合成酵素。這兩個同功酵素具有幾乎完全相同的胺基酸序列，但是在粒線體valine tRNA 合成酵素的胺基端多了一組由46 個胺基酸組成的”粒線體標的訊號”，由於這兩個tRNA 合成酵素存在於不同的胞器內，使得它們在功能上並不能相互的取代。有趣的是，相較於原核生物的valine tRNA 合成酵素，酵母菌的細胞質valine tRNA 合成酵素在其胺基端多了一段由98 個胺基酸組成的附加區段。序列分析的結果顯示這個區段含有許多帶正電荷的胺基酸，不同於大部分合成酵素的附加區段，純化的valine tRNA 合成酵素的附加區段並不會鍵結tRNA ，在功能上也是可有可無，將大部分的附加區段由合成酵素中刪除並不影響它的活體內及活體外功能。值得注意的是，我們發現這個附加區段本身具有活化轉錄的功能，或許valine tRNA 合成酵素本身就是一個轉錄活化因子。 In yeast, there typically are two distinct nuclear-encoded tRNA synthetases for each amino acid; one functions in the cytoplasm and the other in the mitochondria. However, evidence shows that VAS1 is the only gene coding for valyl-tRNA synthetase (ValRS) in the complete yeast genome. This gene encodes not only the cytoplasmic form of ValRS but also its mitochondrial isoform. These two ValRS enzymes have essentially identical polypeptide sequences, except for a 46-amino acid leader peptide at the N-terminus of the mitochondrial precursor, which functions as a mitochondrial targeting signal. Biochemical studies show that these two ValRS isoforms cannot distinguish tRNA species from the cytoplasm and mitochondria in vitro. Despite these similarities, these two ValRS species cannot substitute for each other in vivo, presumably due to differential partitioning. We show here that the appended domain of yeast ValRS, which is absent in its prokaryotic counterparts, is largely dispensable for the enzyme’s aminoacylation as well as complementing activities. Moreover, unlike the appended domains of other yeast tRNA synthetases, the appended domain of yeast ValRS does not appear to bind tRNA. Interestingly, this dispensable domain exhibits a strong transcriptional activation activity in a two-hybrid test, suggesting that ValRS might be involved in regulation of gene expression.
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