The role of glutamine in regulation of ammonium transport in Azotobacter vinelandii

Abstract

Under N2-fixing conditions, Azotobacter vinelandii expresses a specific transport system for methylammonium (ammonium) [E. M. Barnes, Jr. and P. Zimniak (1981) J. Bacteriol. 146, 512-516]. This activity is decreased markedly by culture of cells in the presence of 10 mM ammonium or 2 mM methylammonium; in both cases, the Vmax values for methylammonium uptake were 25% of those of N2-fixing cells. Mixing experiments with assay medium indicate that transport activity is controlled by intracellular rather than extracellular metabolites. Glutamine synthetase activity of cells cultured with ammonium was 33% that of N2-fixing cultures, but activity was unaffected by incubation with methylammonium. Thus ammonium transport and ammonium fixation are regulated independently. When ammonium was removed from the medium, cells recovered over 90% of the initial transport activity after 1 h; this recovery was not affected by addition of chloramphenicol. The loss of uptake activity in cells incubated with ammonium or methylammonium correlated with over sixfold increases in intracellular levels of glutamine and gamma-glutamylmethylamide, respectively. Recovery of transport was accompanied by similar reductions in pools of these compounds. Over one-half of methylammonium transport activity could be blocked by direct addition of 10 mM glutamine or gamma-glutamylmethylamide to transport assays; these concentrations were similar to those observed in vivo. The glutamine analog, 6-diazo-5-oxo-L-norleucine, was the most potent inhibitor found (68% inhibition at 10 microM). These results indicate that the regulation of ammonium transport by ammonium and methylammonium is due to inhibition of the transporter by intracellular gamma-glutamyl amides rather than by repression of transporter synthesis.