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 domains of life. Although their products are required for transport of the ammonium analogue methylammonium in washed cells, only in Saccharomyces cerevisiae have they been shown to be necessary for growth at low NH4+ concentrations. We now demonstrate that an amtB strain of Escherichia coli also grows slowly at low NH4+ concentrations in batch culture, 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 observed at pH 6.1 is relieved at pH 7.1. These results provide direct evidence 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 indicate that AmtB/MEP proteins function in acquisition of the uncharged form. We confirmed that accumulation of [14C]methylammonium depends on its conversion to γ-N-methylglutamine, an energy-requiring reaction catalyzed by glutamine synthetase, and found that at pH 7, constitutive expression of AmtB did not relieve the growth defects of a mutant strain of Salmonella typhimurium that appears to require a high internal concentration of NH4+/NH3. Hence, contrary to previous views, we propose that AmtB/MEP proteins increase the rate of equilibration of the uncharged species, NH3, across the cytoplasmic membrane rather than actively transporting—that is, concentrating—the charged species, NH4+.