ENERGY TRANSDUCTION AND AMINO-ACID-TRANSPORT IN THERMOPHILIC AEROBIC AND FERMENTATIVE BACTERIA

Comparative studies on energy-transducing enzymes from thermophilic and mesophilic bacilli showed that functional analogues of enzymes from the thermophilic species are more thermostable and possess higher maximal turnover rates at the respective growth temperatures than those of the mesophilic species, Differences in membrane fatty-acid composition of thermophilic and mesophilic bacilli have been found which explain the similar apparent membrane-microviscosities of these bacteria at their respective growth temperatures. At elevated temperatures the efficiency of energy transduction is substantially declined in the thermophilic Bacillus stearothermophilus due to an extreme increase in H+-conductivity. In membrane vesicles of B. Stearothermophilus the acidic amino acids l-gultamate and l-aspartate are transported by a Na+-H+-solute symport mechanisms in a 1:1:1 stoichiometry. This is concluded from the observations that the membrane potential, pH-gradient and Δμ Na+ are equivalent as driving force for uptake. Uptake of neutral (brached chain) amino acids is also facilitated by Na+-solute mechanisms. This is confirmed by the stimulatory effect of monensin on the steady-state level of accumulation of these amino acids and the absence of uptake in the presence of nonactin in membrane vesicles. Furthermore, the initial rate of l-alanine or l-leucine uptake is specifically enchanced by the addition of sodium ions and to some extent by lithium ions. The affinities of both transport systems for sodium ions was rather low (mM-range). In contrast to this the Na+-H+-spl|-glutamate transport system in B. Stearothermophilus possesses an extremely high affinity fro sodium ionds (Kt<5 μM). l-Glutamate and l-alanine trasnport activities in B. stearothermophilus are allosterically influenced by the pH. Detailes mechanistic studies of amino acid transport have also been performed in a hybridmembrane system composed out of membrane vesicles of Clostridium fervidus and proteoliposomes containing the thermostable and thermoactive primary proton pump cytochrome c oxidase from B. stearothermophilus. The effects of ionophores, uptake driven by artificial gradients and the observed sodium-ion dependence of transport of neutral, acidic and aromatic amino acids indicates that these amino acids are transported in symport with one Na+- or Li+ in Cl. fervidus. © 1990.