The conserved, polar loop region of subunit c of the Escherichia coli F1F0 ATP synthase is postulated to function in the coupling of proton translocation through F0 to ATP synthesis in F1. We have used a random mutagenesis procedure to define the essential residues in the region. Oligonucleotide-directed mutagenesis was carried out with a random mixture of mutant oligonucleotides, the oligonucleotide mixture being generated by chemical synthesis by using phosphoramidite nucleotide stocks that were contaminated with the other three nucleotides. Thirty mutant genes coding single-amino-acid substitutions in the region between Glu-37 and Leu-45 of subunit c were tested for function by analyzing the capacity of plasmids carrying the mutant genes to complement a Leu-4----amber subunit c mutant. All substitutions at the conserved Arg-41 residue resulted in loss of oxidative phosphorylation, i.e., transformants could not grow on a succinate carbon source. The other conserved residues were more tolerant to substitution, although most substitutions did result in impaired growth on succinate. We conclude that Arg-41 is essential in the function of the polar loop and that the ensemble of other conserved residues collectively maintain an optimal environment required for that function.