Functional and physiological evidence for a Rhesus-type ammonia transporter in Nitrosomonas europaea

Ammonium transporters form a conserved family of transport proteins and are widely distributed among all domains of life. The genome of Nitrosomonas europaea codes for a single gene (rh1) that belongs to the family of the AMT/Rh ammonium transporters. For the first time, this study provides functional and physiological evidence for a Rhesus-type ammonia transporter in bacteria (N. europaea). The methylammonium (MA) transport activity of N. europaea correlated with the Rh1 expression. The K. value for the MA uptake of N. europaea was 1.8 +/- 0.2 mM (pH 7.25), and the uptake was competitively inhibited by ammonium [Ki(NH4+) 0.3 +/- 0.1 mM at pH 7.25]. The MA uptake rate was pH dependent, indicating that the uncharged form of MA is transported by Rh I. An effect of the glutamine synthetase on the MA uptake was not observed. When expressed in Saccharomyces cerevisiae, the function of Rh1 from N. europaea as an ammonia/MA transporter was confirmed. The results suggest that Rh1, equilibrates the uncharged substrate species. A low pH value in the periplasmic space during ammonia oxidation seems to be responsible for the ammonium accumulation functioning as an acid NH4+ trap.