Membrane properties of an extreme thermophile. I. Detection of the phase transition and its dependence on growth temperature

Various membrane properties of Thermus thermophilus HB8 were studied in order to elucidate the mechanism and implications of cell adaptation to a high temperature environment. 1. The profile of temperature dependence of the amino acid uptake rates was similar for the membrane obtained from cells grown at both 61 °C and 77°C. 2. The remaining uptake activity after heat treatment paralleled the membrane potential generating ability, as measured by auramine O fluorescence response to the respiration sub-strate, and was not dependent on the growth temperature. 3. The temperature dependence of the efflux of the accumulated amino acid reflects the physical phase of the membrane and is dependent on the growth temperature. Many reports have been published on the cor-relation of membrane physical properties with the physiological functions (7). In the preceding paper, we estimated the phase changes in the membrane of an extreme thermophile (2). A systematic change in the phase transition temper-ature was observed when the growth temperature was varied. In the present study, we compared the following membrane properties, which may possibly be dependent on the physical state of the membrane (so that the temperature profiles are expected to depend on the growth temperature) of an extreme thermophile, Thermus thermophilus, grown at various temperatures. 1. Temperature dependence of amino acid uptake rates. 2. Thermostabilities of the membrane func-tions. 3. Temperature dependence of the efflux of uptaken amino acid. It was shown that the efflux rates were critically dependent on the physical state of the membrane