| Summary: | 碩士 === 國立中央大學 === 生命科學系 === 104 === Aminoacyl-tRNA synthetases (aaRSs) are a family of essential translation enzymes, each of which catalyzes the coupling of a specific amino acid to its cognate tRNAs. Histidine tRNA (tRNAHis) is unique among tRNA species as it carries an additional nucleotide at its 5' terminus. This unusual G-1 residue is the major identity element of tRNAHis, and is essential for recognition by histidyl-tRNA synthetase (HisRS). In yeast, G-1 of mitochondrial tRNAHis (denoted as tRNAmHis) is genome-encoded, while G-1 of cytoplasmic tRNAHis (denoted as tRNAnHis) is added post-transcriptionally by tRNAHis guanylyltransferase (Thg1). Thg1 possesses efficient 3'–5' polymerase activity that specifically adds the G-1 residue by recognizing the anticodon of tRNAHis. We reported herein that the Thg1 homologues of Homo sapiens and Drosophila melanogaster possess a mitochondrial targeting signal (MTS) that can deliver the protein into mitochondria for functioning. Moreover, we found that Arabidopsis thaliana contains two THG1 genes, the sequences of which consist of two similar repeats. The specific aims of this project are to elucidate the mechanism by which G-1 can be specifically added to tRNAHis and to decipher how certain HisRSs can recognize tRNAHis without G-1. In addition, we would like to characterize the four Thg1-like proteins (TLPs) of Dictyostelium discoideum. Despite the fact that Caenorhabditis elegans lacks a THG1 gene, its HisRS can recognize both cytoplasmic and mitochondrial tRNAHis isoacceptors. Our results showed that C. elegans HisRS preferred tRNAHis with G-1. It is still unclear how this enzyme can recognize the mitochondrial tRNAHis without G-1.
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