Signatures of tRNA Glx -specificity in bacterial glutamyl-tRNA synthetases

The canonical function of glutamyl-tRNA synthetase (GluRS) is to glutamylate tRNA . Yet, not all bacterial GluRSs glutamylate tRNA ; many glutamylate both tRNA and tRNA , while some glutamylate only tRNA and not the cognate substrate tRNA . Understanding the basis of this unique tRNA -specificity is important. Mutational studies have hinted at hotspot residues, both on tRNA and GluRS, that play crucial roles in tRNA -specificity. But the underlying structural basis remains unexplored. Majority of biochemical studies related to tRNA -specificity have been performed on GluRS from Escherichia coli and other proteobacterial species. However, since the early crystal structures of GluRS and tRNA -bound GluRS were from non-proteobacterial species ( Thermus thermophilus ), the proteobacterial biochemical data have often been interpreted in the context of non-proteobacterial GluRS structures. Marked differences between proteo- and non-proteobacterial GluRSs have been demonstrated and therefore it is important that tRNA -specificity be understood vis-a-vis proteobacterial GluRS structures. Towards this goal we have solved the crystal structure of GluRS from E. coli . Using the solved structure and several other currently available proteo- and non-proteobacterial GluRS crystal structures, we have probed the structural basis of tRNA -specificity of bacterial GluRSs. Specifically, our analysis suggests a unique role played by a tRNA D-helix contacting loop of GluRS in modulation of tRNA -specificity. While earlier studies had identified functional hotspots on tRNA that controlled tRNA -specificity of GluRS, this is the first report of complementary signatures of tRNA -specificity in GluRS.