Sep-tRNA:Cys-tRNA synthase. This family belongs to the pyridoxal phosphate (PLP)-dependent aspartate aminotransferase superfamily (fold I). Cys-tRNA(Cys) is produced by O-phosphoseryl-tRNA synthetase which ligates O-phosphoserine (Sep) to tRNA(Cys), and Sep-tRNA:Cys-tRNA synthase (SepCysS) converts Sep-tRNA(Cys) to Cys-tRNA(Cys), in methanogenic archaea. SepCysS forms a dimer, each monomer is composed of a large and small domain; the larger, a typical pyridoxal 5'-phosphate (PLP)-dependent-like enzyme fold. In the active site of each monomer, PLP is covalently bound to a conserved Lys residue near the dimer interface.