Ammonium (methylammonium) transport by Azotobacter chroococcum

Azotobacter chroococcum synthesizes an ammonium transport system under N2 fixing, but not ammonium-surfeited or ammonifying conditions, as evidenced from a concentrative uptake of the analogue methylammonium (Km= 140 μM, Vmax= 0.5 nmol/p min per mg protein), which was competitively inhibited by ammonium (Ki= 10 μM). Transport was energy-dependent and lead to formation of about 30-fold methylammonium or 100-fold ammonium concentration gradients.