A protein domain corresponding to residues 31 to 149 of the E. coli Lysyl-tRNA synthetase species corresponding to the lysS gene was expressed and 15N-labelled. 1H and 15N NMR resonance assignments for this domain were obtained by two-dimensional and three-dimensional homonuclear and heteronuclear spectroscopy. Using distance geometry and simulated annealing, a three-dimensional structure could be calculated using 701 NOE and 86 dihedral angle restraints. It is composed of a five-stranded antiparallel beta-barrel capped by three alpha-helices at its ends. This structure closely resembles that of the N-terminal domain of the other E. coli lysyl-tRNA synthetase species expressed from the lysU gene and is highly homologous to the fold observed for the corresponding region of aspartyl-tRNA synthetase. It is shown that the isolated N-terminal fragment of lysyl-tRNA synthetase can interact with tRNA(Lys) as well as with poly (U), which mimics the anticodon sequence. Amino acid residues involved in these interactions were identified and, in the case of poly-U, a number of specific protein-RNA contacts were characterized. Specific recognition of tRNA(Lys) involves a cluster of four structurally well-defined aromatic residues, anchored on the beta-strands, and basic residues located on the surrounding loops. This organization is reminiscent of other RNA binding proteins, such as the U1A small nuclear ribonucleoprotein.