Preliminary X-ray crystallographic analysis of an engineered glutamyl-tRNA synthetase from Escherichia coli

The nature of interaction between glutamyl-tRNA synthetase (GluRS) and its tRNA substrate is unique in bacteria in that many bacterial GluRS are capable of recognizing two tRNA substrates: tRNA^Glu and tRNA^Gln. To properly understand this distinctive GluRS–tRNA interaction it is important to pursue detailed structure–function studies; however, because of the fact that tRNA–GluRS interaction in bacteria is also associated with phylum-specific idiosyncrasies, the structure–function correlation studies must also be phylum-specific. GluRS from Thermus thermophilus and Escherichia coli, which belong to evolutionarily distant phyla, are the biochemically best characterized. Of these, only the structure of T. thermophilus GluRS is available. To fully unravel the subtleties of tRNA^Glu–GluRS interaction in E. coli, a model bacterium that can also be pathogenic, determination of the E. coli GluRS structure is essential. However, previous attempts have failed to crystallize E. coli GluRS. By mapping crystal contacts of a homologous GluRS onto the E. coli GluRS sequence, two surface residues were identified that might have been hindering crystallization attempts. Accordingly, these two residues were mutated and crystallization of the double mutant was attempted. Here, the design, expression, purification and crystallization of an engineered E. coli GluRS in which two surface residues were mutated to optimize crystal contacts are reported.