ATP + L-lysine + tRNA(Lys) = AMP + diphosphate + L-lysyl-tRNA(Lys) Binds 3 Mg(2+) ions per subunit. Homodimer. There are two lysyl-tRNA ligases in E.coli: lysS is expressed constitutively, while lysU is heat inducible. Belongs to the class-II aminoacyl-tRNA synthetase family.