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 composition, higher thermostabilities of the (membrane) proteins, higher turnover rates of the energy transducing enzymes, and/or the (exclusive) use of sodium-ions rather than protons as coupling ion in energy transduction. The proton permeability of the cytoplasmic membrane of bacteria 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 increased proton pumping activity by using the less permeable sodium ions as coupling ions or by changing the lipid composition of the cytoplasmic membrane. The Na+/H+/glutamate transport proteins of the thermophiles Bacillus stearothermophilus (GltTBs) and Bacillus caldotenax (GltTBc) were studied extensively. These transportproteins have unique features. Transport of L-glutamate occurs in symport with 1 Na+ and 1 H+ when the transport proteins are expressed in their natural environment. The sodium ion dependency of the GltT transporters of these Bacillus strains 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 > or = 2 H+. These observations suggest that the conformation of the transport proteins in the E. coli and the Bacillus membranes differs, and that the conformation influences the coupling ion selectivity. The Na+/H+/glutamate transport proteins of B. stearothermophilus (GltTBs) and B. caldotenax (GltTBc) are homologous to transport systems of glutamate and structurally related compounds from mesophilic organisms. Both sodium, as well as proton coupled transporters, belong to this family of carboxylate transporters (FCT).