Ammonium/methylammonium transport (Amt) proteins facilitate diffusion of NH3 bidirectionally

The ammonium/methylammonium transport (Amt) proteins of enteric bacteria and their homologues, the methylammonium/ammonium permeases of Saccharomyces cerevisiae, are required for fast growth at very low concentrations of the uncharged species NH3. For example, they are essential at low ammonium (NH4+ + NH3) concentrations under acidic conditions. Based on growth studies in batch culture, the Amt protein of Salmonella typhimurium (AmtB) cannot concentrate either NH3 or NH4+ and this organism appears to have no means of doing so. We now show that S. typhimurium releases ammonium into the medium when grown on the alternative nitrogen source arginine and that outward diffusion of ammonium is enhanced by the activity of AmtB. The latter result indicates that AmtB acts bidirectionally. We also confirm a prediction that the AmtB protein would be required at pH 7.0 in ammonium-limited continuous culture, i.e., when the concentration of NH3 is less than or equal to50 nM. Together with our previous studies, current results are in accord with the view that Amt and methylammonium/ammonium permease proteins increase the rate of diffusion of the uncharged species NH3 across the cytoplasmic membrane. These proteins are examples of protein facilitators for a gas.