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Translational control by tRNA modifications
Of the more than 170 distinct RNA modifications, more than 90 are found in the 40 known tRNAs. These modifications are located on specific residues including the tRNA anticodon loop, and vary among the three kingdoms of life. Modified nucleotides influence all aspects of tRNA activity and are deposited at different steps during tRNA maturation. We aim to elucidate the biological function of these modifications with a special focus on 5-methylcytidine and queuosine. In particular, we study the molecular mechanisms that couple tRNA modifications to tRNA processing, with a focus on tRNA splicing, and examine how tRNA modifications contribute to the efficiency and accuracy of protein synthesis. We further explore the importance of tRNA modifications for the physiology of multicellular organisms using two organismal models, Caenorhabditis elegans and Mus musculus.
In eukaryotes, methylation of carbon 5 in cytidine (5-methylcytidine) it is catalysed by Nsun2 at positions 48, 49, 50 of the majority of tRNAs, and in the anticodon loop at positions 34 of tRNALeu. A second enzyme, Dnmt2, methylates position 38 of tRNAAsp, tRNAGly and tRNAVal. Queuosine (Q) is a hyper-modified guanosine analogue that comprises a 7-deaza-guanine core structure covalently linked to an amino-methyl side chain and a cyclo-pentanediol moiety. The tRNA guanine transglycosylase, TGT, which comprises the Qtrt1 and Qtrt2 subunits, introduces Q at position 34 of tRNAAsp, tRNAAsn, tRNATyr and tRNAHis. tRNAs are transcribed as precursor molecules that undergo several maturation steps. For certain tRNAs like tRNALeu, tRNAIle and tRNATyr, processing involves excision of an intron located between nucleotides 37 and 38 of the mature tRNA, adjacent to the anticodon loop (positions 34 to 36). This non-conventional splicing mechanism requires separate enzyme-catalysed cleavage (TSEN) and ligation (Ligase) reactions. Site-specific processing and modification of tRNAs are tightly controlled and interdependent processes.
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