Structure of nondiscriminating glutamyl-tRNA synthetase from Thermotoga maritima

Aminoacyl-tRNA synthetases produce aminoacyl-tRNAs from the substrate tRNA and its cognate amino acid with the aid of ATP. Two types of glutamyl-tRNA synthetase (GluRS) have been discovered: discriminating GluRS (D-GluRS) and non­discriminating GluRS (ND-GluRS). D-GluRS glutamylates tRNA^Glu only, while ND-GluRS glutamylates both tRNA^Glu and tRNA^Gln. ND-GluRS produces the intermediate Glu-tRNA^Gln, which is converted to Gln-tRNA^Gln by Glu-tRNA^Gln amidotransferase. Two GluRS homologues from Thermotoga maritima, TM1875 and TM1351, have been biochemically characterized and it has been clarified that only TM1875 functions as an ND-GluRS. Furthermore, the crystal structure of the T. maritima ND-GluRS, TM1875, was determined in complex with a Glu-AMP analogue at 2.0 Å resolution. The T. maritima ND-GluRS contains a characteristic structure in the connective-peptide domain, which is inserted into the catalytic Rossmann-fold domain. The glutamylation ability of tRNA^Gln by ND-GluRS was measured in the presence of the bacterial Glu-tRNA^Gln amidotransferase GatCAB. Interestingly, the glutamylation efficiency was not affected even in the presence of excess GatCAB. Therefore, GluRS avoids com­petition with GatCAB and glutamylates tRNA^Gln.