Expression of the genes for the membrane-bound F0 sector of the Escherichia coli F1F0 proton-translocating ATPase can respond to changes in metabolic conditions, and these changes are reflected in alterations in the subunit stoichiometry of the oligomeric F0 proton channel. Transcriptional and translational lacZ fusions to the promoter and to two F0 genes show that, during growth on the nonfermentable carbon source succinate, transcription of the operon and translation of uncB, encoding the a subunit of F0, are higher than during growth on glucose. In contrast, translation of the uncE gene, encoding the c subunit of F0, is higher during growth on glucose than during growth on succinate. Translation rates of both uncB and uncE change as culture density increases, but transcription rates do not. Quantitation of the c stoichiometry shows that more c subunits are assembled into the F1F0 ATPase in cells grown on glucose than in cells grown on succinate. E. coli therefore appears to have a mechanism for regulating the composition and, presumably, the function of the ATPase in response to metabolic circumstances.