Abstract
The systems of tRNA glycylation belong to the most complex aminoacylation systems since neither the oligomeric structure of glycyl-tRNA synthetases (GlyRS) nor the discriminator bases in tRNA super(Gly) are conserved in the phylae. To better understand the structure-function relationship in glycylation systems of various origins and the functional peculiarities related to their structural divergences, the elements in tRNA conferring its glycine identity in Thermus thermophilus were characterized and compared to those of other systems. Thermophilic identity is conferred by the G sub(1)-C sub(72), C sub(2)-G sub(71), G sub(3)-C sub(70), and C sub(50)-C sub(64) pairs together with the G sub(10), U sub(16), C sub(35), and C sub(36) single residues. In contrast to most other aminoacylation systems, the discriminator base is not directly involved in identity. Transplantation of these elements in tRNA super(Asp) and tRNA super(Phe) converts specificity toward glycine albeit conservation of nucleotide 73. Analysis of the functional interrelation of the identity elements shows coupling in synthetase recognition of the elements from anticodon and G sub(10) whereas those from acceptor arm are recognized independently. Despite nondirect implication in identity, the discriminator base contributes cooperatively with C sub(36) in specificity of glycylation. The link between the structural heterogeneity and the functional divergence of the glycylation systems and the phylogenic interrelation of these systems were approached by comparing the ability of GlyRSs of various phylae to glycylate heterologous tRNA super(Gly). Dimeric GlyRSs from mammalian and archaebacteria acylate efficiently only eukaryotic and archaebacterial tRNA super(Gly) with a discriminatory A sub(73), whereas tetrameric Escherichia coli GlyRS acylates only eubacterial tRNA super(Gly) with a discriminatory U sub(73). In contrast, dimeric yeast GlyRS acylates efficiently both eukaryotic and archaebacterial tRNA super(Gly) as well as peculiar prokaryotic isoacceptors. Species specificity is lost with the dimeric GlyRS from Thermus thermophilus that acylates efficiently eubacterial, archaebacterial, and eukaryotic tRNA super(Gly). These features are discussed in the context of the evolution of the glycylation systems and the phylogenic interrelation of the organisms.