Potassium uptake with low affinity and high rate in Enterococcus hirae at alkaline pH
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O R I G I N A L PA P E R
Miyuki Kawano · Ryoko Abuki · Kazuei Igarashi · Yoshimi Kakinuma
Potassium uptake with low affinity and high rate in Enterococcus hirae at alkaline pH
Received: 7 August 2000 / Revised: 12 October 2000 / Accepted: 19 October 2000 / Published online: 21 November 2000 © Springer-Verlag 2000
Abstract Two high-affinity K+ uptake systems, KtrI and KtrII, have been reported in Enterococcus hirae. A mutant, JEMK1, defective in these two systems did not grow at pH 10 in low-K+ medium (less than 1 mM K+), but grew well when supplemented with 10 mM KCl. In this mutant, we found an energy-dependent K+ uptake at pH 10 with a low affinity for K+ (Km of ~20 mM) and an extremely high rate [Vmax of 1.6 µmol min–1 (mg protein)–1]. Rb+ uptake [Km of ~40 mM and Vmax of 0.5 µmol min–1 (mg protein)–1], which was inhibited competitively by K+ and less prominently by Cs+, was also observed. The specificity of this transport is likely to be K+>Rb+>Cs+. This peculiar K+ transport plays a role as a salvage mechanism against defects in high-affinity systems in the K+ homeostasis of this bacterium. Keywords K+ transport · KtrI · KtrII · Enterococcus hirae
Introduction Potassium is a major cytoplasmic cation in growing bacterial cells and plays important roles in cell physiology (Bakker 1993a; Harold 1990). Cellular K+ activates various cytoplasmic enzymes and is required for protein synthesis (Harold 1990; Harold and Maloney 1996). K+ movement across the cell membrane is involved in the maintenance of turgor pressure and in the regulation of cytoplasmic pH (Harold and Maloney 1996; Silver 1996). For these physiological processes, bacteria have evolved diverse potassium transport systems (Bakker 1993a). Two K+-uptake systems, KtrI (Bakker and Harold 1980) and KtrII (Kobayashi 1982; Kakinuma and Harold 1985),
Miyuki Kawano · Ryoko Abuki · Kazuei Igarashi · Yoshimi Kakinuma (✉) Faculty of Pharmaceutical Sciences, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan e-mail: [email protected], Tel.: +81-43-2902898, Fax: +81-43-2902900
have been reported in the gram-positive bacterium Enterococcus hirae. KtrI recognizes K+ as well as Rb+ with an apparent Km of 0.2 mM, and is likely to be constitutive. KtrI K+ uptake requires both generation of an electrochemical gradient of protons (proton potential) and ATP or a related high-energy compound (Bakker and Harold 1980). The activity of this system is optimal around pH 6–6.5 (Kakinuma 1993; Kakinuma and Igarashi 1988) and negligible at high pH (Kobayashi 1982). KtrII selectively recognizes K+ with a Km of 0.5 mM, and has a pH optimum around 9–10. KtrII K+ uptake is independent of the proton potential (Kobayashi 1982; Kakinuma and Harold 1985). Although these K+ transport systems have not been well characterized at the molecular level, we recently found that the ntpJ gene, a cistron at the tail end of the vacuolar-type Na+-ATPase (ntp) operon, encodes a component of the KtrII system (Murata et al. 1996b). KtrII K+ uptake activity was shown to be missing i
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