DIVERGENCE OF GLUTAMATE AND GLUTAMINE AMINOACYLATION PATHWAYS - PROVIDING THE EVOLUTIONARY RATIONALE FOR MISCHARGING

Aminoacyl-tRNA for protein synthesis is produced through the action of a family of enzymes called aminoacyl-tRNA synthetases. A general rule is that there is one aminoacyl-tRNA synthetase for each of the standa rd 20 amino acids found in all cells. This is not universal, however, as a majority of prokaryotic organisms and eukaryotic organelles lack the enzyme glutaminyl-tRNA synthetase, which is responsible for formin g Gln-tRNA(Gln) in eukaryotes and in Gram-negative eubacteria. Instead , in organisms lacking glutaminyl-tRNA synthetase, Gln-tRNA(Gln) is pr ovided by misacylation of tRNA(Gln) with glutamate by glutamyl-tRNA sy nthetase, followed by the conversion of tRNA-bound glutamate to glutam ine by the enzyme Glu-tRNA(Gln) amidotransferase. The fact that two di fferent pathways exist for charging glutamine tRNA indicates that ance stral prokaryotic and eukaryotic organisms evolved different cellular mechanisms for incorporating glutamine into proteins. Here, we explore the basis for diverging pathways for aminoacylation of glutamine tRNA . We propose that stable retention of glutaminyl-tRNA synthetase in pr okaryotic organisms following a horizontal gene transfer event from eu karyotic organisms (Lamour et al. 1994) was dependent on the evolving pool of glutamate and glutamine tRNAs in the organisms that acquired g lutaminyl-tRNA synthetase by this mechanism, This model also addresses several unusual aspects of aminoacylation by glutamyl- and glutaminyl -tRNA synthetases that have been observed.