The Na+-Responsive ntp Operon Is Indispensable for Homeostatis of K+ and Na+ in Enterococcus hirae at Limited Proton Potential

Enterococcus hirae ATCC 9790 grew well in Na⁺-deficient, low-K⁺ medium, but growth was inhibited by carbonylcyanide m-chlorophenylhydrazone (CCCP). Growth inhibition and decrease of cellular K⁺ levels in the presence of CCCP were relieved by the addition of Na⁺ and a high concentration of K⁺. In contrast, in the mutant defective in Na⁺-ATPase or the NtpJ component of the KtrII K⁺ uptake system, CCCP-induced growth inhibition was rescued by a high concentration of K⁺ but not of Na⁺. These transporters are thus indispensable for homeostatis of K⁺ and Na⁺ at low proton potential.