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 activates L-serine and transfers it to tRNA(Thr) but cannot deacylate incorrectly charged amino acid; unlike most archaea the editing function is found in a freestanding protein. ATP + L-threonine + tRNA(Thr) = AMP + diphosphate + H(+) + L-threonyl-tRNA(Thr) Binds 1 zinc ion per subunit. Homodimer (By similarity). Probably interacts with its editing subunit (By similarity). Belongs to the class-II aminoacyl-tRNA synthetase family.