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) (PubMed:15240874). Also activates L-serine and transfers it to tRNA(Thr); unlike most archaea the editing function is found in a freestanding protein (ACQ980D1) (PubMed:15240874). In vitro when both subunits are present, or if the 2 subunits are fused, L-seryl-tRNA(Thr) is no longer produced, the 2 subunits edit incorrectly charged L-seryl-tRNA(Thr) (PubMed:15240874). Has no activity on correctly acylated L-seryl-tRNA(Ser) or L-threonyl-tRNA(Thr). ATP + L-threonine + tRNA(Thr) = AMP + diphosphate + H(+) + L-threonyl-tRNA(Thr) Binds 1 zinc ion per subunit. Inhibited by 1 uM borrelidin (BN). kcat is 1.3 sec(-1) for L-serine, 1.9 for L-threonine at 60 degrees Celsius (PubMed:15240874). Homodimer (By similarity). Probably interacts with its editing subunit (AC Q980D1); a subunit fusion (in the order edit-catalytic) is fully functional. Belongs to the class-II aminoacyl-tRNA synthetase family.