RETRACING THE EVOLUTION OF AMINO-ACID SPECIFICITY IN GLUTAMINYL-TRANSFER-RNA SYNTHETASE

Molecular phylogenetic studies of glutaminyl-tRNA synthetase suggest t hat it has relatively recently evolved from the closely related enzyme glutamyl-tRNA synthetase, We have now attempted to retrace one of the hey steps in this process by selecting glutaminyl-tRNA synthetase mut ants displaying enhanced glutamic acid recognition. Mutagenesis of two residues proximal to the active site, Phe-90 and Tyr-240, was found t o improve glutamic acid recognition 3-5-fold in vitro and resulted in the misacylation of tRNA(Gln) with glutamic acid. In vivo expression o f the genes encoding these misacylating variants of glutaminyl-tRNA sy nthetase reduced cellular growth rates by 40%, probably as a result of an increase in translational error rates. These results provide the f irst biochemical evidence that glutaminyl-tRNA synthetase originated t hrough duplication and consequent diversification of an ancestral glut amyl-tRNA synthetase-encoding gene. (C) 1998 Federation of European Bi ochemical Societies.