Glutamate is required to maintain the steady-state potassium pool in Salmonella typhimurium

In many bacteria, accumulation of K+ at high external osmolalities is accompanied by accumulation of glutamate, To determine whether there is an obligatory relationship between glutamate and K+ pools, we studied mutant strains of Salmonella typhimurium with defects in glutamate synthesis, Enteric bacteria synthesize glutamate by the combined action of glutamine synthetase and glutamate synthase (GS/GOGAT cycle) or the action of biosynthetic glutamate dehydrogenase (GDH), Activity of the GS/GOGAT cycle is required under nitrogen-limiting conditions and is decreased at high external ammonium/ammonia (NH4+) concentrations by lowered synthesis of GS and a decrease in its catalytic activity due to covalent modification (adenylylation by GS adenylyltransferase). By contrast, GDH functions efficiently only at high external NH4+ concentrations, because it has a low affinity for NH4+. When grown at low concentrations of NH4+ (less than or equal to 2 mM), mutant strains of S. typhimurium that lack GOGAT and therefore are dependent on GDH have a low. glutamate pool and grow slowly; we now demonstrate that they have a low K+ pool, When subjected to a sudden NH4+ upshift, strains lacking GS adenylyltransferase drain their glutamate pool into glutamine and grow very slowly; we now find that they also drain their K+ pool, Restoration of the glutamate pool in these strains at late times after shift was accompanied by restoration of the K+ pool and a normal growth rate, Taken together, the results indicate that glutamate is required to maintain the steady state K+ pool-apparently no other anion can substitute as a counter-ton for free K+-and that K+ glutamate is required for optimal growth.