微生物に学ぶアルカリ性環境適応戦略(セミナー「生物のストレス受容のメカニズム-分子レベルから細胞レベルまで-」)

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Alkaliphiles, which are capable of growth in an extremely alkaline environment, serve as a model system to understand the physiological mechanisms allowing microbial growth under extreme environmental conditions. The internal pH of alkaliphiles is kept 2 units lower than the external pH 10 or higher. In alkaliphilic Bacillus, Na^+-dependent pH homeostasis involves the Na^+/H^+ antiport activities as well as Na^+ re-entry routes. A predominant role was evident for the NaChBac (voltage-dependent) Na channel of alkaliphilic B. pseudofirmus OF4 at sub-optimal [Na^+]. NaChBac mutants exhibit a significant loss of pH homeostasis capacity. Alkaliphilic Bacillus halodurans C-125 produces negatively charged acidic polymers that are associated with the peptidoglycan as secondary cell-wall polymers. Mutant strains which introduced mutations in genes encoding acidic polymers biosynthesis result in a growth defect according by elevated cytoplasmic pH. Alkaliphilic Bacillus clausii KSM-K16 was recently identified as the first bacterium in which a single stator-rotor of bacterial flagellar motor uses both protons and sodium ions for ion-coupling, depending upon the environmental pH.
2011-04-15