Methylammonium uptake by Rhizobium sp. strain 32H1.

We present evidence that methylammonium is transported into cowpea Rhizobium sp. strain 32H1 cells by a membrane carrier whose natural substrate is ammonium. After growth in low (0.2%) oxygen, which is necessary for nitrogen fixation by these cells, respiring rhizobial cells took up [14C]methylammonium to high intracellular levels. Cells grown in atmospheric (21%) oxygen did not take up methylammonium. Uptake (transport plus metabolism) was maximal in cells harvested in the early stationary phase of batch culture and had a distinct pH optimum of 6.5 to 7.0. Uptake was inhibited by metabolic poisons that dissipate the proton motive force or inhibit ATP synthesis. Inhibition of uptake by ammonium and the counterflow phenomenon indicated that ammonium and methylammonium share a transport carrier. Of the methylammonium taken up, about 15% was accumulated to intracellular levels 20 times higher than those in the medium; most of the methylammonium was metabolized to gamma-N-methylglutamine.