Crystal Structure of a Non-discriminating Glutamyl-tRNA Synthetase

Error-free protein biosynthesis is dependent on the reliable charging of each tRNA with its cognate amino acid. Many bacteria, however, lack a glutaminyl-tRNA synthetase. In these organisms, tRNAGln is initially mischarged with glutamate by a non-discriminating glutamyl-tRNA synthetase (ND-GluRS). This enzyme thus charges both tRNAGlu and tRNAGln with glutamate. Discriminating GluRS (D-GluRS), found in some bacteria and all eukaryotes, exclusively generates Glu-tRNAGlu. Here we present the first crystal structure of a non-discriminating GluRS from Thermosynechococcus elongatus (ND-GluRSTel) in complex with glutamate at a resolution of 2.45 Å. Structurally, the enzyme shares the overall architecture of the discriminating GluRS from Thermus thermophilus (D-GluRSTth). We confirm experimentally that GluRSTel is non-discriminating and present kinetic parameters for synthesis of Glu-tRNAGlu and of Glu-tRNAGln. Anticodons of tRNAGlu (34C/UUC36) and tRNAGln (34C/UUG36) differ only in base 36. The pyrimidine base of C36 is specifically recognized in D-GluRSTth by the residue Arg358. In ND-GluRSTel this arginine residue is replaced by glycine (Gly366) presumably allowing both cytosine and the bulkier purine base G36 of tRNAGln to be tolerated. Most other ND-GluRS share this structural feature, leading to relaxed substrate specificity.