Abstract
The canonical function of glutamyl-tRNA synthetase (GluRS) is to glutamylate tRNAGlu. Yet, not all bacterial GluRSs glutamylate tRNAGlu; many glutamylate both tRNAGlu and tRNAGln, while some glutamylate only tRNAGln and not the cognate substrate tRNAGlu. Understanding the basis of this unique tRNAGlx-specificity is important. Mutational studies have hinted at hotspot residues, both on tRNAGlx and GluRS, that play crucial roles in tRNAGlx-specificity. But the underlying structural basis remains unexplored. Majority of biochemical studies related to tRNAGlx-specificity have been performed on GluRS from Escherichia coli and other proteobacterial species. However, since the early crystal structures of GluRS and tRNAGlu-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 tRNAGlx-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 tRNAGlx-specificity of bacterial GluRSs. Specifically, our analysis suggests a unique role played by a tRNAGlx D-helix contacting loop of GluRS in modulation of tRNAGln-specificity. While earlier studies had identified functional hotspots on tRNAGlx that controlled tRNAGlx-specificity of GluRS, this is the first report of complementary signatures of tRNAGlx-specificity in GluRS.
A short running title: tRNAGlx-specific signatures in GluRS
Key words: GluRS, tRNA-Gln, tRNA-discrimination, E. coli , proteobacteria, protein-RNA interaction