The structure of the translational initiation factor IF1 from Escherichia coli has been determined with multidimensional NMR spectroscopy. Using 1041 distance and 78 dihedral constraints, 40 distance geometry structures were calculated, which were refined by restrained molecular dynamics. From this set, 19 structures were selected, having low constraint energy and few constraint violations. The ensemble of 19 structures displays a root-mean-square deviation versus the average of 0.49 A for the backbone atoms and 1.12 A for all atoms for residues 6-36 and 46-67. The structure of IF1 is characterized by a five-stranded beta-barrel. The loop connecting strands three and four contains a short 3(10) helix but this region shows considerably higher flexibility than the beta-barrel. The fold of IF1 is very similar to that found in the bacterial cold shock proteins CspA and CspB, the N-terminal domain of aspartyl-tRNA synthetase and the staphylococcal nuclease, and can be identified as the oligomer-binding motif. Several proteins of this family are nucleic acid-binding proteins. This suggests that IF1 plays its role in the initiation of protein synthesis by nucleic acid interactions. Specific changes of NMR signals of IF1 upon titration with 30S ribosomal subunit identifies several residues that are involved in the interaction with ribosomes.