不同於大部分的tRNA合成酵素，酵母菌中的細胞質與粒腺體Valine-tRNA合成酵素（ValRS）是由同一個細胞核基因所解碼出來;這兩個異構型唯一的不同處是在於新合成的粒腺體合成酵素多了一段由46個胺基酸組成的"粒腺體標的訊號"。雖然在活體外這兩個合成酵素有幾乎相同的活性，但是在活體內這兩個合成酵素因為存在不同的胞器內，所以無法互相取代。我們發現此合成酵素的胺基端具有一段富含Lysine的附加區段（胺基酸1~97），但是此附加區段並不直接參與胺醯化作用，也不是細胞內蛋白質合成功能之所需;利用two-hybrid系統我們發現ValRS的附加區段能夠活化細胞核內回報基因的轉錄作用，而這種轉錄活化功能並未發現於其他已知的aaRS附加區段，且其序列亦與已知的轉錄活化蛋白質不同，因此推斷可能是一個新的轉錄活化因子。 In yeast, there are typically two distinct nuclear-encoded tRNA synthetases for each amino acids; one function in the cytoplasm and the other in the mitochondria. Previous studies showed that VAS1 is the only gene coding for valyl-tRNA synthetase （ValRS） in the yeast genome. This gene encodes both the cytoplasmic and mitochondrial forms of valyl-tRNA synthetase. These two isoforms have essentially identical polypeptide sequences except for a 46-amino acid leader at the N-terminus of the mitochondrial precursor, which functions as a mitochondrial targeting signal. In vitro, the two isoforms of ValRS cannot distinguish between tRNA species from the cytoplasm and mitochondria, but the enzymes cannot substitute for each other in vivo due to differential partitioning. Also, like many other eukaryotic tRNA synthetase, the cytoplasmic form of yeast ValRS has a lysine-rich polypeptide extension of 97 amino acids appended to its N-terminus. I found that the appended domain did not participate in aminoacylation and was largely dispensible for function in vivo. Surprisingly, our results showed that this appended domain exhibited a strong transcriptional activation activity that was never found in any other tRNA synthetase tested. This suggests that this appended domain may be a novel transcriptional activator.
