The crystal structure of the EF-Tu . EF-Ts complex from Escherichia coli has been determined to a resolution of 2.5 Angstrom. The complex contains two subunits of each of the elongation factors. The two EF-Ts molecules form a tight dimer, but there is little contact between the two EF-Tu molecules. The interaction of EF-Ts with EF-Tu results principally in the disruption of the Mg2+ ion binding site, thereby reducing the affinity of EF-Tu for guanine nucleotides.