An ecf Mutation in Escherichia coZi Pleiotropically Affecting Energy Coupling in Active Transport but Not Generation or Maintenance of Membrane Potential*

A temperature-sensitive ecf mutant of Escherichia coli has been isolated and is described in this paper. The mutant is pleiotropically defective in the active transport of amino acids and /3-methyl-n-thiogalactoside at 37” or above. A membrane potential of about -86 mV (interior negative), however, is established and maintained under conditions in which active transport is defective (pH 7.0 at 39”). In the wild type a membrane potential of -86 mV under these conditions is sufficient for active transport. This result indicates that the electrochemical gradient, although necessary, is not sufficient to drive the active transport of these substrates in the mutant. At the nonpermissive temperature in the presence of glucose the initial rate of uptake for all solutes is normal. This uptake is always followed by efflux of the accumulated solute at a rate essentially identical with the proton conductor carbonylcyanide m-chlorophenylhydrazone-induced efflux of accumulated solute from wild type cells. The inability of the mutant to accumulate solutes could be ascribed to an inhibition of the energy-coupling process by glucose metabolites, as cells pretreated with glucose have been shown to have a significant inhibition of transport ability. Two models for energy coupling of active transport are discussed, one in the context of the chemiosmotic hypothesis and the other in the context of the conformational coupling theory. A simple procedure for the preparation of radioactive triphenylmethylphosphonium bromide is also described.