Freestanding tRNA editing subunit of threonine--tRNA ligase, the catalytic subunit is AC Q97VW8. Deacylates (edits) mischarged L-seryl-tRNA(Thr) in trans, removing L-serine, has no aminoacylation activity. In vitro when both subunits are present, or if the 2 subunits are fused, L-seryl-tRNA(Thr) is no longer produced. Has no activity on correctly acylated L-seryl-tRNA(Ser) or L-threonyl-tRNA(Thr) (PubMed:15240874). Editing is probably catalyzed by the 2'-OH of A76 of tRNA(Thr) (By similarity). Probably interacts with its catalytic subunit (AC Q97VW8); a subunit fusion (in the order edit-catalytic) is fully functional. Not essential, however growth is severely inhibited in the presence of 0.5 mM L-serine, but not in the presence of 0.5 mM L-ser plus 0.5 mM L-threonine. Belongs to the class-II aminoacyl-tRNA synthetase family. Archaea-specific ThrRS editing domain subfamily.