Ammonium and methylammonium transport in Rhodobacter sphaeroides

Rhodobacter sphaeroides maintained intracellular ammonium pools of 1.1 to 2.6 mM during growth in several fixed nitrogen sources as well as during diazotrophic growth. Addition of 0.15 mM NH4+ to washed, nitrogen-free cell suspensions was followed by linear uptake of NH4+ from the medium and transient formation of intracellular pools of 0.9 to 1.5 mM NH4+. Transport of NH4+ was shown to be independent of assimilation by glutamine synthetase because intracellular pools of over 1 mM represented NH4+ concentration gradients of at least 100-fold across the cytoplasmic membrane. Ammonium pools of over 1 mM were also found in non-growing cell suspensions in nitrogen-free medium after glutamine synthetase was inhibited with methionine sulfoximine. In NH4+-free cell suspensions, methylammonium (14CH3NH3+) was taken up rapidly, and intracellular concentrations of 0.4 to 0.5 mM were maintained. The 14CH3NH3+ pool was not affected by methionine sulfoximine. Unlike NH4+ uptake, 14CH3NH3+ uptake in nitrogen-free cell suspensions was repressed by growth in NH4+. These results suggest that R. sphaeroides may produce an NH4+-specific transport system in addition to the NH4+/14CH3NH3+ transporter. This second transporter is able to produce normal-size NH4+ pools but has very little affinity for 14CH3NH3+ and is not repressed by growth in high concentrations of NH4+.