The structure of threonyl-tRNA synthetase-tRNA(Thr) complex enlightens itsrepressor activity and reveals an essential zinc ion in the active site

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