ATP-driven exchange of Na+ and K+ ions by Streptococcus faecalis

We describe the characterization of KtrII, a novel potassium transport system of Streptococcus faecalis, first discovered by H. Kobayashi [1982) J. Bacteriol. 150, 506-511). KtrII requires sodium ions and mediates the stoichiometric exchange of internal Na+ for external K+. Potassium accumulation is not energized by the electrochemical potentials of either H+ or Na+; the energy source is probably ATP. Two lines of evidence indicate that KtrII is a manifestation of the sodium-stimulated ATPase reported earlier (Heefner, D. L., and Harold, F. M. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 2798-2802). (i) Mutants that lack the ATPase also lack KtrII, and revertants recover both in parallel. (ii) KtrII and the Na+-ATPase are induced in parallel when cells are grown on media rich in sodium, particularly under conditions that limit the generation of a proton potential. KtrII is not induced in response to K+ deprivation. We propose that the Na+-ATPase exchanges Na+ for K+ ions.