The uncE410 allele differs from the normal uncE gene in that C leads to T base changes occur at nucleotides 190 and 191, resulting in proline at position 64 in the c-subunit of the F1F0-ATPase being replaced by leucine. Two partial-revertant strains were isolated in which alanine-20 of the c-subunit was replaced by proline, owing to a G leads to C base change at nucleotide 58. These c-subunits, coded for by the uncE501 and uncE502 alleles, therefore contained two amino acid changes, namely proline-64 leads to leucine, and alanine-20 leads to proline. Membranes prepared from the partial-revertant strains lacked ATP-dependent atebrin-fluorescence-quenching activity but were able to carry out oxidative phosphorylation. The ATPase activity of the F1-ATPase was inhibited when bound to membranes from strains carrying the uncE410, uncE501 and uncE502 alleles. It is concluded that a bend in the helix axis in one of the arms of the c-subunit hairpin structure is required for integration of the c-subunit into a functional F1F0-ATPase.