Evolution of tRNA Identity.

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Correct recognition of tRNAs by their cognate aminoacyl-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.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 fo … More r 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.In the E. coli system, the discriminator base of threonine tRNA (A73) is not involved in the threonylation activity by threonyl-tRNA synthetase (ThrRS). To investigate the contribution of this discriminator base of archaea threonine tRNA in threonylation by ThrRS, cross-species aminoacylation reactions among E. coli, Haloferax volcanii (halophilic archae), and Aeropyrum pernix (extreme thermophilic archaea) have been studied. It was found that E. coli ThrRS threonylated the archaea threonine tRNAs (U73), but E. coli threonine tRNA was not aminoacylated by archaea ThrRS. Results of the threonylation experiment using in vitro mutants of E. coli threonine tRNA showed that only the mutant threonine tRNA having U73 was threonylated. These findings indicate that the discriminator base U73 of threonine tRNA is a strong determinant for the recognition by archaea ThrRS. Less