A mutant of Methanobacterium thermoautotrophicum with a lesion in membrane Na + -translocating ATPase (synthase) was isolated. The total ATPase activity in permeabilized cells of this mutant was elevated three-fold as compared with the wild-type strain. In contrast to wild-type cells, mutant ATPase was neither inhibited by DCCD nor stimulated by Na + ions. The methane formation orate of the mutant cells at pH 7.5 under non-growing conditions was nearly twice that of the wildtype strain and was stimulated by sodium ions. On the other hand, the ATP synthesis driven by methanogenesis under the same conditions was lower that of the wild-type under the same conditions, and contrary to the wild-type was not stimulated by Na + ions. ATP synthesis driven by a potassium diffusion potential in the presence of sodium ions was markedly diminished in the mutant cells. The membrane potential values of the wildtype and the mutant cells in the presence of 10 mM NaCl at pH 7.0 were comparable at energized conditions (3223 mV and 3230 mV respectively). The Mg 2+ -dependent ATPase activity of the 10 S Ug supernatant of broken cells from the mutant cells was 30% higher than in the wild-type. On the other hand, two bands with Mg 2+ -dependent ATPase activity were identified by native PAGE in this fraction in both wild-type as well as in mutant. These data suggest that the binding of Na + -translocating ATPase (synthase) to the membrane spanning part is changed in the mutant strain.z 1997 Federation of European Biochemical Societies.
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