| Summary: | 博士 === 國立中央大學 === 生命科學研究所 === 99 === After decades of exploration, knowledge of structure and functions of aminoacyl-tRNA synthetases (aaRSs) has progressed by leaps and bounds. However, their evolution paths are still obscure, especially for the dual-functional genes such as valyl-tRNA synthetase (ValRS). We focus herein on the evolution of yeast ValRS and its appended domain. Previous studies showed a single yeast gene, VAS1, encodes both cytosolic and mitochondrial forms of ValRS. We show that ValRS of Escherichia coli per se cannot substitute for yeast ValRS, but can be converted into a dual-functional yeast enzyme by fusion to the appended domain of yeast ValRS or a nonspecific tRNA-binding domain (TRBD) and a mitochondrial targeting signal (MTS). Interestingly, except for the fission yeast Schizosaccharomyces pombe, all yeast species studied contained a single ValRS gene. There are two homologous ValRS genes in the chromosomes of S. pombe, named SpVAS1 and SpVAS2, which specify cytosolic and mitochondrial functions, respectively. Surprisingly, both of them are of mitochondria origin. This finding suggests that evolution of eukaryotic ValRS invloved processes more than endosymbiosis and mitochondria-to-nucleus gene transfer. In addition, we found that the tRNA-binding protein Asc1p of S. pombe associates with ribosomal proteins. These findings not only suggest an evolutionary path for ValRS from prokaryotes to modern yeasts, but also underscore the possibility that both the appended domain of yeast ValRS and Asc1p play a role in aminoacyl-tRNA channeling.
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