The structure of threonyl-tRNA synthetase-tRNAThr complex enlightens its repressor activity and reveals an essential zinc ion in the active site

E. coli threonyl-tRNA synthetase (ThrRS) is a class II enzyme that represses the translation of its own mRNA. We report the crystal structure at 2.9 Angstrom resolution of the complex between tRNA(Thr) and ThrRS, whose structural features reveal novel strategies for providing specificity in tRNA selection. These include an aminoterminal domain containing a novel protein fold that makes minor groove contacts with the tRNA acceptor stem. The enzyme induces a large deformation of the anticodon loop, resulting in an interaction between two adjacent anticodon bases, which accounts for their prominent role in tRNA identity and translational regulation. A zinc ion found in the active site is implicated in amino acid recognition/discrimination.