Leucyl-tRNA synthetase (LeuRS) catalyzes the leucylation of tRNA(Leu). To maintain the fidelity of protein biosynthesis, LeuRS also catalyzes the editing reaction. In the present work, highly conserved T252 in the T-rich region within CP1 domain of Escherichia coli LeuRS was mutated to G, D, or E. Steady-state kinetic of aminoacylation, and combined editing assays indicated that not only the size of the amino acid but also the absence of hydrogen bonds between T252 and adjacent molecules may affect the editing. It is further confirmed by in vivo experiments using the temperature-sensitive strain KL231 (DeltaleuS), which revealed the arrested growth of bacterial cells bearing mutants with highly impaired editing activity in the presence of leucine analog. (C) 2004 Elsevier Inc. All rights reserved.