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