NapA Na+/H+ antiporter as a sodium extrusion system supplementary to the vacuolar Na+-ATPase in Enterococcus hirae

Enterococcus hirae has two sodium extrusion systems: the NapA Na+/H+ antipo rter and the vacuolar Na+-ATPase. We found that a NapA mutant, WD4, which i s deficient in Na+/H+ antiporter activity, grew well in the pH range of 6 t o 10 up to 200 mM sodium. This was due to active, potential-independent sod ium extrusion by the Na+-ATPase, which was induced under these conditions. The NapA Na+/H+ antiporter is thus not a prerequisite for growth of E. hira e in the presence of sodium, but plays a supplementary role in sodium extru sion at acidic pH.