| Abstract
| - The aminoacyl-tRNA synthetases covalently link transfer RNAs to their cognate amino acids.Some of the tRNA synthetases have employed an editing mechanism to ensure fidelity in this first stepof protein synthesis. The amino acid editing active site for Escherichia coli leucyl-tRNA synthetase resideswithin the CP1 domain that folds discretely from the main body of the enzyme. A portion of the editingactive site is lined with conserved threonines. Previously, we identified one of these threonine residues(Thr252) as a critical amino acid specificity factor. On the basis of X-ray crystal structure information,two other nearby threonine residues (Thr247 and Thr248) were hypothesized to interact with the editingsubstrate near its cleavage site. Single mutations of either of these conserved threonine residues had minimaleffects on amino acid editing. However, double mutations that deleted the hydroxyl group from theneighboring threonine residues abolished amino acid editing activity. We propose that these threonineresidues, which are also conserved in the homologous isoleucyl-tRNA synthetase and valyl-tRNA synthetaseediting active sites, play a central role in amino acid editing. It is possible that they collaborate in stabilizingthe transition state.
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