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 i s an obligatory relationship between glutamate and K+ pools, we studie d mutant strains of Salmonella typhimurium with defects in glutamate s ynthesis, Enteric bacteria synthesize glutamate by the combined action of glutamine synthetase and glutamate synthase (GS/GOGAT cycle) or th e action of biosynthetic glutamate dehydrogenase (GDH), Activity of th e 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 t o covalent modification (adenylylation by GS adenylyltransferase). By contrast, GDH functions efficiently only at high external NH4+ concent rations, because it has a low affinity for NH4+. When grown at low con centrations 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 hav e a low K+ pool, When subjected to a sudden NH4+ upshift, strains lack ing GS adenylyltransferase drain their glutamate pool into glutamine a nd grow very slowly; we now find that they also drain their K+ pool, R estoration of the glutamate pool in these strains at late times after shift was accompanied by restoration of the K+ pool and a normal growt h rate, Taken together, the results indicate that glutamate is require d to maintain the steady state K+ pool-apparently no other anion can s ubstitute as a counter-ton for free K+-and that K+ glutamate is requir ed for optimal growth.