Potassium uptake with low affinity and high rate in Enterococcus hirae at alkaline pH

Area of Science:

• Microbiology
• Molecular Biology
• Biochemistry

Background:

• Enterococcus hirae has two known high-affinity potassium (K+) uptake systems: KtrI and KtrII.
• A mutant lacking both KtrI and KtrII (JEMK1) exhibits impaired growth at pH 10 in low-K+ conditions.

Purpose of the Study:

• To investigate the K+ uptake mechanisms in Enterococcus hirae, particularly in mutants lacking high-affinity systems.
• To characterize the properties and function of an alternative K+ transport system under alkaline stress.

Main Methods:

• Growth assays of the JEMK1 mutant in varying K+ concentrations and pH.
• Measurement of energy-dependent K+ and Rubidium (Rb+) uptake in the mutant.
• Determination of kinetic parameters (Km, Vmax) and substrate specificity (K+, Rb+, Cs+).

Main Results:

• The JEMK1 mutant demonstrated an energy-dependent, low-affinity K+ uptake system (Km ≈ 20 mM, Vmax = 1.6 µmol·min⁻¹·(mg protein)⁻¹) at pH 10.
• This system also transported Rb+ (Km ≈ 40 mM, Vmax = 0.5 µmol·min⁻¹·(mg protein)⁻¹), with K+ showing stronger competitive inhibition than Cs+.
• The transport specificity was determined as K+ > Rb+ > Cs+.

Conclusions:

• A novel, low-affinity K+ uptake system functions as a salvage mechanism in Enterococcus hirae.
• This system is essential for maintaining K+ homeostasis when high-affinity uptake pathways are compromised, especially under alkaline stress.