fluid-crystalline state, but at 13℃ the membrane is four fold more viscous than at 50℃. These observations show that homeo-viscous adaptation determines to a limiting extent the membrane fluidity. The more important control mechanism clearly is homeo-proton permeability adaptation which precludes futile cycling of protons at higher growth temperatures and allows cells to maintain the proton motive force to drive essential energy transducing processes. The second suggestion was confirmed by detailed energy-transduction studies in cells and membrane vesicles from the thermophilic bacteria: Clostridium fervidus (3) and Thermoalkalibacter borgoria (4). In both organisms sodium-ions were the coupling ions used in all energy-transducing processes studied. C. fervidus was found to rely only on sodium-ions.
1. Van de Vossenberg, J.L.C.M., Ubbink-Kok, T., Elferink, M.G.L., Driessen, A.J.M., and Konings, W.N. (1995) "Ion permeability of the cytoplasmic membrane limits the maximum growth temperature of bacteria and archaea," Mol. Microbiol., 18, 925-932.
2. Van de Vossenberg, L.C.M., Driessen, A.J.M., da Costa, M.S., and Konings, W.N. (1997) "Homeostasis of the Membrane proton permeability in Bacillus subtilis grown at different temperatures," submitted.
3. Speelman, G., Poolman, B., Abee, T., and Konings, W.N. (1993) "Energy transduction in the thermophilic anaerobe Clostridium fervidus is exclusively coupled to sodium ions," Proc. Natl. Acad. Sci. USA, 90, 7975-7979.
4. Prowe, S.G., van de Vossenberg, J.L.C.M., Driessen, A.J.M., Antranikian, G., and Konings, W.N. (1996) "Sodium-coupled energy transduction in the newly isolated thermoalkaliphilic strain LBS3," J. Bacteriol., 178, 4099-4104.
