GLYCYL-TRANSFER-RNA SYNTHETASE FROM THERMUS-THERMOPHILUS - WIDE STRUCTURE DIVERGENCE WITH OTHER PROKARYOTIC GLYCYL-TRANSFER-RNA SYNTHETASESAND FUNCTIONAL INTERRELATION 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 understan d better this structural diversity and its functional consequences. th e 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 pro tein 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 I in all these class 2 synthetases. Interestingly, the sequence of the prokary otic 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 a bout functional inter-relations between the glycylation systems, Effic ient glycylations involving partners from T. thermophilis and Esherich ia coli showed conservation of the recognition process in prokaryotes despite strong structural variations of the synthetases. However Gly-t RNA synthelase from T. thermophilus acylates eukaryotic tRNA(Gly) whil e the charging ability of tile E. coli enzyme is restricted to prokary otic tRNA(Gly). A similar behaviour is found in eukaryotic systems whe re the restricted species specificity for tRNA glycylation of mammalia n Gly-tRNA synthetase contrasts with the relaxed specificity of the ye ast enzyme. The consensus sequence of the tRNAs charged by the various Gly-tRNA synthetase reveals conservation of only G1-C72 in the accept or arm, C35 and C36 in the anticodon, and the (G10-Y25)-G45 triplet in volved in tRNA including. Conservation of these nucleotides indicates their key role in glycation and suggests that they were part of the an cestral glycine identity set. These features are discussed in the cont ext of the phylogenic connections between prokaryotes, eukaryotes, and archaebacteria, and of the particular place of T. thermophilus in thi s phylogeny.