Catalyzes the attachment of threonine to tRNA(Thr) in a two-step reaction: L-threonine is first activated by ATP to form Thr-AMP and then transferred to the acceptor end of tRNA(Thr). Also edits incorrectly charged L-seryl-tRNA(Thr). ATP + L-threonine + tRNA(Thr) = AMP + diphosphate + H(+) + L-threonyl-tRNA(Thr) Binds 1 zinc ion per subunit. Homodimer. The protein structure shows 2 N-terminal domains, the central catalytic and C-terminal anticodon recognition domain. Comparison with the E.coli structure shows the N-terminal domains (editing region) are quite flexible with respect to the catalytic domain. The C-terminal domain recognizes the anticodon region of the tRNA while the acceptor arm is sandwiched between the N-terminal domains and the catalytic domain (PubMed:12875846). Belongs to the class-II aminoacyl-tRNA synthetase family.