Functional aspects of modified nucleosides in tRNA

Abstract: Transfer ribonucleic acids (tRNAs) are extensively modified, especially their anticodon loops. Modifications at position 34 (wobble base) and 37 in these loops affect the tRNAs’ decoding ability, while modifications outside the anticodon loops, e.g. m1A58 of tRNAMeti, may be crucial for tRNA structure or stability. A number of gene products are required for the formation of modified nucleosides, e.g. at least 26 proteins (including Elongator complex) are needed for U34 modifications in yeast, and methyl transferase activity of the Trm6/61p complex is needed to form m1A58. The aim of the studies which this thesis is based upon was to investigate the functional aspects of tRNA modifications and regulation of the modifying enzymes’ activity.First, the hypothesis that ncm5U34, mcm5U34, or mcm5s2U34 modifications may be essential for reading frame maintenance was investigated. The results show that mcm5 and s2 group of mcm5s2U play a vital role in reading frame maintenance. Subsequent experiments showed that the +1 frameshifting event at Lys AAA codon occurs via peptidyl-tRNA slippage due to a slow entry of the hypomodified tRNA-Lys.Moreover, the hypothesis that Elp1p N-terminal truncation may regulate Elongator activity was investigated. Cleavage of Elp1p was found to occur between residue 203 (Lys) and 204 (Ala) and to depend on the vacuolar protease Prb1p. However, including trichloroacetic acid (TCA) during protein extraction abolished the appearance of truncated Elp1p, showing that its truncation is a preparation artifact.Finally, in glioma cell line C6, PKCα was found to interact with TRM61. RNA silencing of TRM6/61 causes a growth defect that can be partially suppressed by tRNAMeti overexpression. PKCα overexpression reduces the nuclear level of TRM61, likely resulting in reduced level of TRM6/61 complex in the nucleus. Furthermore, lower expression of PKCα in the highly aggressive GBM (relative to its expression in less aggressive Grade II/III glioblastomas) is accompanied by increased expression of TRM6/61 mRNAs and tRNAMeti, highlighting the clinical relevance of the studies.