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 tRNAGlu only, while ND‐GluRS glutamylates both tRNAGlu and tRNAGln. ND‐GluRS produces the intermediate Glu‐tRNAGln, which is converted to Gln‐tRNAGln by Glu‐tRNAGln 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 tRNAGln by ND‐GluRS was measured in the presence of the bacterial Glu‐tRNAGln amidotransferase GatCAB. Interestingly, the glutamylation efficiency was not affected even in the presence of excess GatCAB. Therefore, GluRS avoids competition with GatCAB and glutamylates tRNAGln.