HOW DOES Gln-tRNA SYNTHETASE AMINOACYLATE THE CORRECT tRNA WITH THE CORRECT AMINO ACID? T. A. Steitz, L. F. Silvian, V. L. Rath, Department of Molecular Biophysics and Biochemistry, Howard Hughes Medical Institute, Yale University, New Haven, CT USA

To provide a structural basis for understanding the high specificity of Gln-tRNA synthetase (GlnRS) for its cognate tRNA and amino acid, structures of the enzyme cocrystallized with tRNA^Glns containing mutated anticodon and acceptor stem bases have been correlated with the kinetic consequences of these mutations. Further, these complexes contain a bound Gln-AMP analogue. The mutation sharing the most profound kinetic and structural change is the mutation of U35 in the anticodon to C35. The large change in k_cat may be a consequence of significant alterations in the structure of the anticodon and D-stem and loops and in the N-terminus of the protein.