The 2 angstrom crystal structure of leucyl-tRNA synthetase and its complexwith a leucyl-adenylate analogue

Leucyl-, isoleucyl- and valyl-tRNA synthetases are closely related large mo nomeric class I synthetases. Each contains a homologous insertion domain of similar to 200 residues, which is thought to permit them to hydrolyse ('ed it') cognate tRNA that has been mischarged with a chemically similar but no n-cognate amino acid. We describe the first crystal structure of a leucyl-t RNA synthetase, from the hyperthermophile Thermus thermophilus, at 2.0 Angs trom resolution. The overall architecture is similar to that of isoleucyl-t RNA synthetase, except that the putative editing domain is inserted at a di fferent position in the primary structure. This feature is unique to prokar yote-like leucyl-tRNA synthetases, as is the presence of a novel additional flexibly inserted domain. Comparison of native enzyme and complexes with l eucine and a leucyladenylate analogue shows that binding of the adenosine m oiety of leucyl-adenylate causes significant conformational changes in the active site required for amino acid activation and tight binding of the ade nylate. These changes are propagated to more distant regions of the enzyme, leading to a significantly more ordered structure ready for the subsequent aminoacylation and/or editing steps.