AMMONIA ACQUISITION IN ENTERIC BACTERIA - PHYSIOLOGICAL-ROLE OF THE AMMONIUM METHYLAMMONIUM TRANSPORT-B (AMTB) PROTEIN/

Homologues of the amtB gene of enteric bacteria exist in all three dom ains of life. Although their products are required for transport of th e ammonium analogue methylammonium in washed cells, only in Saccharomy ces cerevisiae have they been shown to be necessary for growth at low NH4+ concentrations. We now demonstrate that an amtB strain of Escheri chia coli also grows slowly at low NH4+ concentrations in batch cultur e, but only at pH values below 7. In addition, we find that the growth defect of an S. cerevisiae triple-mutant strain lacking the function of three homologues of the ammonium/methylammonium transport B (AmtB) protein [called methylammonium/ammonium permeases (MEP)] that was obse rved at pH 6.1 is relieved at pH 7.1. These results provide direct evi dence that AmtB participates in acquisition of NH4+/NH3 in bacteria as well as eucarya. Because NH3 is the species limiting at low pH for a given total concentration of NH4+ + NH3, results with both organisms i ndicate that AmtB/MEP proteins function in acquisition of the uncharge d form. We confirmed that accumulation of [C-14]methylammonium depends on its conversion to gamma-N-methylglutamine, an energy-requiring rea ction catalyzed by glutamine synthetase, and found that at pH 7, const itutive expression of AmtB did not relieve the growth defects of a mut ant strain of Salmonella typhimurium that appears to require a high In ternal concentration of NH4+/NH3. Hence, contrary to previous views, w e propose that AmtB/MEP proteins increase the rate of equilibration of the uncharged species, NH3, across the cytoplasmic membrane rather th an actively transporting-that is, concentrating-the charged species, N H4+.