Fig. 4.

Model for synergistic regulation of nitrate and bicarbonate uptake. As shown in this model, both the Nrt (cyan) and Cmp (green) complexes are inhibited by nitrate (yellow balls). If energy or nutrient supplies were to become limiting, both nitrogen and carbon fixation would be blocked and the levels of ammonium (mauve balls) would rise. This, in turn, increases the concentration of nitrate and shuts down both bicarbonate and nitrate uptake. It is logical to have nitrate act as the feedback regulator because its reduction to membrane permeable ammonia (dark blue balls) would waste a great deal of cellular reductive potential if not fully used by carbon and nitrogen fixation pathways.