Fig. 7

Overview of the main ions/osmolytes transporters present in the locus kimA- kup in Enterococcus faecalis responding to diverse stress conditions. (a) Expression of the kimA and kup K⁺ transporters in E. faecalis is induced under stress conditions. In strain JH2-2, the IS6770 insertion sequence rompe la activation at low pH. KimA mediates K⁺ uptake at elevated K⁺ concentrations and exhibits, whereas Kup functions at lower K⁺ and exhibits higher-affinity c-di-AMP binding. (b) Under acidic stress, K⁺ uptake is coupled to proton extrusion via the F₀F₁-ATPase (encoded by the atp operon) and to Na⁺ extrusion by the NapA antiporter. This coupling helps maintain intracellular pH and ionic homeostasis during acid stress. (c) Under alkaline stress, K⁺ uptake is balanced by Na⁺ extrusion via a V-type ATPase (also encoded by the ntp operon), and the potentially assisted by a Ca²⁺-ATPase (caaT). (d) In alkaline media with elevated NaCl, the OpuC osmolyte uptake system also contributes to ionic balance and osmoprotection.