The catalytically active from of threonyl/alanyl tRNA synthetase is a dimer. Within the tRNA synthetase class II dimer, the bound tRNA interacts with both monomers making specific interactions with the catalytic domain, the C-terminal domain, and this domain (the second additional domain). The second additional domain is comprised of a pair of perpendicularly orientated antiparallel beta sheets, of four and three strands, respectively, that surround a central alpha helix that forms the core of the domain [1].

1: The structure of threonyl-tRNA synthetase-tRNA(Thr) complex enlightens its repressor activity and reveals an essential zinc ion in the active site. Sankaranarayanan R, Dock-Bregeon AC, Romby P, Caillet J, Springer M, Rees B, Ehresmann C, Ehresmann B, Moras D; Cell 1999;97:371-381. PMID:10319817