The uncE114 mutation (Gln42----Glu) in subunit c of the Escherichia coli H+ ATP synthetase causes uncoupling of proton translocation from ATP hydrolysis (Mosher, M. E., White, L. K., Hermolin, J., and Fillingame, R. H. (1985) J. Biol. Chem. 260, 4807-4814). In the background of strain ER, the mutation led to dissociation of F1 from the membrane. Ten revertants to the uncE114 mutation were isolated, and the uncE gene was cloned and sequenced. Six of the revertants were intragenic and had substitutions of glycine, alanine, or valine for the mutant glutamate residue at position 42. The intragenic, revertant uncE genes were incorporated into an otherwise wild type chromosome of strain ER. Membrane vesicles prepared from each of the revertants showed a restoration of F1 binding to F0. The Val42 revertant differed from the other two revertants in that the ATPase activity of F1 was inhibited when membrane bound. This was shown by the stimulation of ATPase activity when F1 was released from the membrane. The Gly42 and Ala42 revertants demonstrated membrane ATPase activity that was resistant to dicyclohexylcarbodiimide treatment. Resistance was shown to be due to the increased dissociation of F1 from the membrane under ATPase assay conditions. The Ala42 revertant showed a significant reduction in ATP-dependent quenching of quinacrine fluorescence that was attributed to less efficient coupling of ATP hydrolysis to H+ translocation, whereas the other revertants showed responses very near to that of wild type. Minor changes in the F1-F0 interaction in all three revertants were indicated by an increase in H+ leakiness, as judged by reduced NADH-dependent quenching of quinacrine fluorescence. The minor defects in the revertants support the idea that residue 42 is involved in the binding and coupling of F1 to F0 but also show that the conserved glutamine (or asparagine) is not absolutely necessary in this function.