AmtB‐mediated NH3 transport in prokaryotes must be active and as a consequence regulation of transport by GlnK is mandatory to limit futile cycling of NH 4 + / NH 3

The nature of the ammonium import into prokaryotes has been controversial. A systems biological approach makes us hypothesize that AmtB‐mediated import must be active for intracellular NH 4 + concentrations to sustain growth. Revisiting experimental evidence, we find the permeability assays reporting passive NH3 import inconclusive. As an inevitable consequence of the proposed NH 4 + transport, outward permeation of NH3 constitutes a futile cycle. We hypothesize that the regulatory protein GlnK is required to fine‐tune the active transport of ammonium in order to limit futile cycling whilst enabling an intracellular ammonium level sufficient for the cell's nitrogen requirements.