Two distinct complexes between seryl-tRNA synthetase and tRNA Ser from Thermus themophilus have been crystallized using ammonium sulphate as a precipitant. Form III crystals grow from solutions containing a 1:2.5 stoichiometry of synthetase dimer to tRNA. They are of monoclinic space group C2 with unit cell dimensions a = 211.6 Å, b = 126.8 Å, c = 197.1 Å, β = 132.4° and diffract to about 3.5 Å. Preliminary crystallographic results show that the crystallographic asymmetric unit contains two synthetase dimers. Form IV crystals grow from solutions containing a 1:1.5 stoichiometry of synthetase dimer to tRNA. They are of orthorhombic space group P212121 with unit cell dimensions a = 124.5 Å, b = 128.9 Å, c = 121.2 Å and diffract to 2.8 Å resolution. Preliminary crystallographic results show that these crystals contain only one tRNA molecule bound to a synthetase dimer.
