ADAPTATION OF MICROORGANISMS AND THEIR TRANSPORT-SYSTEMS TO HIGH-TEMPERATURES

Growth of Bacteria and Archaea has been observed at temperatures up to 95 and 110 degrees C, respectively. These thermophiles are adapted to environments of high temperature by changes in the membrane lipid com position, higher thermostabilities of the (membrane) proteins, higher turnover rates of the energy transducing enzymes, and/or the (exclusiv e) use of sodium-ions rather than protons as coupling ion in energy tr ansduction. The proton permeability of the cytoplasmic membrane of bac teria and archaea was observed to increase with the temperature. This increased proton permeability limits the maximum temperature of growth of bacteria. Higher growth temperatures can be reached by an increase d proton pumping activity by using the less permeable sodium ions as c oupling ions or by changing the lipid composition of the cytoplasmic m embrane. The Na+/H+/glutamate transport proteins of the thermophiles B acillus stearothermophilus (GltT(Bc)) and Bacillus caldotenax (GltT(Bc )) were studied extensively. These transportproteins have unique featu res. Transport of L-glutamate occurs in symport with 1 Na+ and 1 H+ wh en the transport proteins are expressed in their natural environment. The sodium ion dependency of the GltT transporters of these Bacillus s trains was found to increase with temperature. However, when the GltT proteins are expressed in the mesophile Escherichia coli, electrogenic symport of L-glutamate occurs with greater than or equal to 2 H+. The se observations suggest that the conformation of the transport protein s in the E. coli and the Bacillus membranes differs, and that the conf ormation influences the coupling ion selectivity. The Na+/H+/glutamate transport proteins of B. stearothermophilus (GltT(Bc)) and B. caldote nax (GltT(Bc)) are homologous to transport systems of glutamate and st ructurally related compounds from mesophilic organisms. Both sodium, a s well as proton coupled transporters, belong to this family of carbox ylate transporters (FCT). (C) 1997 Elsevier Science Inc.