Glycyl-tRNA synthetase from Thermus thermophilus--wide structural divergence with other prokaryotic glycyl-tRNA synthetases and functional inter-relation with prokaryotic and eukaryotic glycylation systems

The tRNA glycylation system is amongst the most complex aminoacylation systems since neither the oligomeric structure of the enzymes nor the discriminator base in tRNAs are conserved in the phylae. To understand better this structural diversity and its functional consequences, the prokaryotic glycylation system from Thermus thermophilus, an extreme thermophile, was investigated and its structural and functional inter-relations with those of other origins analyzed. Alignments of the protein sequence of the dimeric thermophilic glycyl-tRNA synthetase (Gly-tRNA synthetase) derived from its gene with sequences of other dimeric Gly-tRNA synthetases revealed an atypical character of motif 1 in all these class 2 synthetases. Interestingly, the sequence of the prokaryotic thermophilic enzyme resembles eukaryotic and archaebacterial Gly-tRNA synthetases, which are all dimeric, and diverges drastically from the tetrameric enzymes from other prokaryotes. Cross aminoacylations with tRNAs and synthetases of different origins provided information about functional interrelations between the glycylation systems. Efficient glycylations involving partners from T. thermophilus and Escherichia coli showed conservation of the recognition process in prokaryotes despite strong structural variations of the synthetases. However, Gly-tRNA synthetase from T. thermophilus acylates eukaryotic tRNA(Gly) while the charging ability of the E. coli enzyme is restricted to prokaryotic tRNA(Gly). A similar behaviour is found in eukaryotic systems where the restricted species specificity for tRNA glycylation of mammalian Gly-tRNA synthetase contrasts with the relaxed specificity of the yeast enzyme. The consensus sequence of the tRNAs charged by the various Gly-tRNA synthetases reveals conservation of only G1-C72 in the acceptor arm, C35 and C36 in the anticodon, and the (G10-Y25)-G45 triplet involved in tRNA folding. Conservation of these nucleotides indicates their key role in glycylation and suggests that they were part of the ancestral glycine identity set. These features are discussed in the context of the phylogenic connections between prokaryotes, eukaryotes, and archaebacteria, and of the particular place of T. thermophilus in this phylogeny.