Differences of tRNA identity between Escherichia coli and Saccharomyces cerevisiae (yeast).

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Correct recognition of tRNAs by their cognate aminoacy-tRNA synthetases is essential to the maintenance of accurate translation. To discriminate the cognate tRNA from a pool of various tRNA species sharing a similar L-shaped tertiary structure, the aminoacyl-tRNA synthetase was found to recognize a relatively small number of nucleotides of the tRNA,which offen include the anticodon nucleotides and the discriminator base at position 73. Most of the available data are biased to the Escherichia coli system, although a few studies in other organisms have recently been made. We examined the identity elements of several tRNA species from Saccharomyces cerevisiae (yeast) and Thermus thermophilus using in vitro transcripts. For E.coli, T.thermophilus and yeast tRNA^<Thr>, the first base pair in the acceptor stem and the second and third positions of the anticodon are indeed retained as major identity elements. However the second base pair, C2-G71, in the acceptor stem is required for aminoacylation with threonine in E.coli, but not in T.thermophilus or yeast. In addition, the discriminator base, A73, in E.coli is not involved in the specific aminoacylation, whereas U73 in T.thermophilus and A73 in yeast contribute to the tRNA identity, although with differences in the quantitative effects on mutations. In case of glycine tRNAs, discriminator base, the second base pair in the acceptor stem, and the anticodon nuclotides, C35 and C36, contribute to the specific glycylation of all three glycly-tRNA synthetases, the discriminator base differing between prokaryotes (U73) and eukaryote (A73) . The first base pair, G1-C72, is important for glycylation in E.coli and T.thermophilus, whereas the third base pair is important for glycylation in yeast. These above results indicate that while major identity elements have been conserved throughout evolution, the mechanism by which aminoacyl-tRNA synthetases recognize their substrates seems to have diverged somewhat among different species.