EVOLUTION OF THE GLX-TRANSFER-RNA SYNTHETASE FAMILY - THE GLUTAMINYL ENZYME AS A CASE OF HORIZONTAL GENE-TRANSFER

An important step ensuring the fidelity in protein biosynthesis is the aminoacylation of tRNAs by aminoacyl-tRNA synthetases. The accuracy o f this process rests on a family of 20 enzymes, one for each amino aci d. One exception is the formation of Gln-tRNA(Gln) that can be accompl ished by two different pathways: aminoacylation of tRNA(Gln) with Gln by glutaminyl-tRNA synthetase (GlnRS; EC 6.1.1.18) or transamidation o f Glu from Glu-tRNA(Gln) mischarged by glutamyl-tRNA synthetase (GluRS ; EC 6.1.1.17). The latter pathway is widespread among bacteria and or ganelles that, accordingly, lack GlnRS. However, some bacterial specie s, such as Escherichia coli, do possess a GlnRS activity, which is res ponsible for Gln-tRNA(Gln) formation. In the cytoplasm of eukaryotic c ells, both GluRS and GlnRS activities can be detected. To gain more in sight into the evolutionary relationship between GluRS and GlnRS enzym e species, we have now isolated and characterized a human cDNA encodin g GlnRS. The deduced amino acid sequence shows a strong similarity wit h other known GlnRSs and with eukaryotic GluRSs. A molecular phylogene tic analysis was conducted on the 14 GlxRS (GluRS or GlnRS) sequences available to date. Our data suggest that bacterial GlnRS has a eukaryo tic origin and was acquired by a mechanism of horizontal gene transfer .