The interaction between electron transfer, proton motive force and solute transport in bacteria.

The properties of proton solute symport have been studied in Streptococcus cremoris, Rhodopseudomonas sphaeroides and Escherichia coli. In the homolactic fermentative organism S. cremoris the efflux of lactate is a membrane protein-mediated process, which can lead to the generation of a proton motive force. These observations support the energy-recycling model that postulates the generation of metabolic energy by end-product efflux. Studies with oxidants and reductants and specific dithiol reagents in E. coli membrane vesicles demonstrated the presence of two redox-sensitive dithiol-disulphide groups in the transport proteins of proline and lactose. The redox state of these groups is controlled by the redox potential of the environment and by the proton motive force. One redox-sensitive group is located at the inner surface, the other at the outer surface of the membrane. In Rps. sphaeroides and E. coli the activity of several transport proteins depends on the activity of the electron transfer systems. On the basis of these results a redox model for proton solute transport coupled in parallel to the electron transfer system is postulated.