We described previously the isolation, by genetic selection, of a mutant of Escherichia coli glutaminyl-tRNA synthetase that misaminoacylates supFtRNATyr with glutamine. The single amino acid change responsible for the mischarging phenotype was identified at amino acid 235 in the translated glnS gene. The mutant, called glnS7, has an Asp----Asn change, and studies with the purified glnS7 gene product show it may mischarge a number of presumably different tRNAs. We have carried out extensive homology searches that show E. coli glutaminyl-tRNA synthetase shares regions of homology with other aminoacyl-tRNA synthetases, although little apparent similarity at the site of the glnS7 mutation. In addition to the conserved 'HIGH' motif implicated in aminoacyl adenylate formation, there are regions of homology of glutaminyl-tRNA synthetase with other synthetases which may be involved in tRNA binding. These include short stretches of homology in sequences acting as a tRNA 'anchor' as well as homology of some aminoacyl-tRNA synthetases to a recently identified motif in ribonucleoproteins. Therefore, our results show that E. coli glutaminyl-tRNA synthetase may share with other aminoacyl-tRNA synthetases regions responsible for tRNA binding, while other regions of the protein, of which the glnS7 mutation may be a component, are responsible for tRNA discrimination.