Kawano M, Abuki R, Igarashi K, Kakinuma Y
Faculty of Pharmaceutical Sciences, Chiba University, Japan.
Arch Microbiol. 2001 Jan;175(1):41-5. doi: 10.1007/s002030000234.
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 approximately 20 mM) and an extremely high rate [Vmax of 1.6 micromol x min(-1) (mg protein)(-1)]. Rb+ uptake [Km of approximately 40 mM and Vmax of 0.5 micromol x 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.
在平肠球菌中已报道了两种高亲和力钾离子摄取系统，即KtrI和KtrII。一个在这两个系统中存在缺陷的突变体JEMK1，在低钾培养基（钾离子浓度低于1 mM）中pH 10时无法生长，但添加10 mM KCl后生长良好。在这个突变体中，我们发现在pH 10时存在一种能量依赖的钾离子摄取，对钾离子的亲和力较低（Km约为20 mM）且速率极高[Vmax为1.6微摩尔·分钟⁻¹·（毫克蛋白）⁻¹]。还观察到铷离子摄取[Km约为40 mM，Vmax为0.5微摩尔·分钟⁻¹·（毫克蛋白）⁻¹]，其受到钾离子竞争性抑制，铯离子的抑制作用较弱。这种转运的特异性可能是钾离子>铷离子>铯离子。这种特殊的钾离子转运在该细菌钾离子稳态中作为一种针对高亲和力系统缺陷的补救机制发挥作用。
