alanyl-tRNA synthetase（丙胺酸-tRNA合成酶）是至今唯一仍保留其 原型結構的胺基酸-tRNA合成酶 (aminoacyl-tRNA synthetase)。AlaRS 原型結 構由催化、tRNA辨認、編輯和 C-Ala 結構區域共同組成。在 AlaRS 的四個 結構區域中，C-Ala 的蛋白質序列變異最大，因此在演化過程中，伴隨著功 能上的轉變。大腸桿菌C-Ala 強力結合 tRNA ，且在胺醯化扮演重要功能， 人類C-Ala 強力結合DNA，但是在胺醯化反應中卻是可有可無。為了進一步 了解C-Ala的功能及演化，我們研究及分析二個演化上疏離的低等真核生物 C-Ala的核酸結合能力。我們發現酵母菌Saccharomyces cerevisiae的 C-Ala與 tRNAAla 有很強的結合能力，但是不與 DNA 結合，在胺醯化反應中扮演重要 角色，這個特性與大腸桿菌 C-Ala 相似；然而黏菌Dictyostelium discoideum C-Ala 則可以同時結合tRNAAla與DNA，且在胺醯化反應中扮演重要功能， 這個特性與線蟲 C-Ala 相似。這些結果顯示，C-Ala由原核演化到真核過程 中，它的功能也由tRNA結合逐漸演變成DNA結合，而一些真核細胞的C-Ala 可能可以同時結合tRNAAla及DNA;AlaRS is the only aminoacyl-tRNA synthetase (aaRS) that still retains a conserved prototype structure. AlaRS consists of catalytic, tRNA-recognition, editing, and C-Ala domains. Among these four domains, C-Ala is highly diverged in sequence. E. coli C-Ala robustly binds tRNA and plays an important role in dimerization and aminoacylation, while human C-Ala robustly binds DNA and is dispensable for aminoacylation. Paradoxically, C. elegans (nematode) C-Ala robustly binds both tRNA and DNA and plays an important role in aminoacylation. To gain further insight into the evolution of C-Ala, we explored the nucleic acidbinding properties of C-Ala domains obtained from distantly-related lower eukaryotes. Our data showed that Saccharomyces cerevisiae C-Ala binds tRNAAla but not DNA and plays an important role in aminoacylation, a feature similar to E. coli C-Ala, whereas Dictyostelium discoideum (slime mold) C-Ala binds both tRNAAla and DNA and plays an important role in aminoacylation, a feature similar to C. elegans C-Ala. It thus appears that as prokaryotes evolved to eukaryotes, CAla has been repurposed from mediating tRNAAla binding to DNA binding, with certain eukaryotes binding to both ligands.
