Retracing the evolution of amino acid specificity in glutaminyl-tRNA synthetase

Molecular phylogenetic studies of glutaminyl-tRNA synthetase suggest that it has relatively recently evolved from the closely related enzyme glutamyl-tRNA synthetase. We have now attempted to retrace one of the key steps in this process by selecting glutaminyl-tRNA synthetase mutants displaying enhanced glutamic acid recognition. Mutagenesis of two residues proximal to the active site, Phe-90 and Tyr-240, was found to improve glutamic acid recognition 3-5-fold in vitro and resulted in the misacylation of tRNA(Gln) with glutamic acid. In vivo expression of the genes encoding these misacylating variants of glutaminyl-tRNA synthetase reduced cellular growth rates by 40%, probably as a result of an increase in translational error rates. These results provide the first biochemical evidence that glutaminyl-tRNA synthetase originated through duplication and consequent diversification of an ancestral glutamyl-tRNA synthetase-encoding gene.