The structure of a 1:1 molar complex between Escherichia coli elongation factor (EF) Tu-GDP and the cyclic thiazolyl peptide antibiotic, GE2270A, has been determined by X-ray diffraction analysis to a resolution of 2.35 A and refined to a crystallographic refinement factor of 20.6%. The antibiotic binds in the second domain of EF-Tu-GDP, making contact with three segments of amino acids (residues 215-230, 256-264, and 273-277). The majority of the protein-antibiotic contacts are van der Waals interactions. A striking feature of the antibiotic binding site is the presence of a salt bridge, not previously observed in other EF-Tu complexes. The ionic interaction between Arg 223 and Glu 259 forms over the antibiotic and probably accounts for the strong affinity observed between EF-Tu and GE2270A. Arg 223 and Glu 259 are highly conserved, but not invariant throughout the prokaryotic EF-Tu family, suggesting that the antibiotic may bind EF-Tu from some organisms better than others may. Superposition of the antibiotic binding site on the EF-Tu-GTP conformation reveals that one region of the antibiotic would form steric clashes with the guanine nucleotide-binding domain in the GTP, but not the GDP, conformation. Another region of the antibiotic binds to the same site as the aminoacyl group of tRNA. Together with prior biochemical studies, the structural findings confirm that GE2270A inhibits protein synthesis by blocking the GDP to GTP conformational change and by directly competing with aminoacyl-tRNA for the same binding site on EF-Tu. In each of the bacterial strains that are resistant to GE2270A, the effect of a site-specific mutation in EF-Tu could explain resistance. Comparison of the GE2270A site in EF-Tu with sequence homologues, EF-G and EF-1alpha, suggests steric clashes that would prevent the antibiotic from binding to translocation factors or to the eukaryotic equivalent of EF-Tu. Although GE2270A is a potent antibiotic, its clinical efficacy is limited by its low aqueous solubility. The results presented here provide the details necessary to enhance the solubility of GE2270A without disrupting its inhibitory properties.