The Na + -dependent F-ATP synthases ofIlyobacter tartaricusandPropioni-genium modestum contain membrane-embedded ring-shaped c subunit assemblies with a stoichiometry of 11. Subunit c from either organism was overexpressed in Escherichia coliusing a plasmid containing the corres-ponding gene, extracted from the membrane using detergent and then puri-fied.