There is considerable evidence for the rapid diffusion of
ammonia across cytoplasmic membranes, and hence until recently
the concept of active transport of ammonium ions has
received relatively little attention. However, the large ammonia
gradients which can occur across bacterial membranes argue
strongly for an active ammonium transport (Amt) system,
and there is some evidence for such systems in about 50 prokaryotic
species (for reviews, see references 13, 128, and 129).
In most cases, Amt activity is repressed in the presence of high
extracellular ammonium concentrations, and studies with the
ammonium analog [14C]methylammonium in E. coli indicated
that Amt expression is controlled by the Ntr system