Metabolic Regulation of Nitrogen Fixation in Rhodospirillum rubrum and Evidence for an Energy-Dependent Ammonium Transport System
In photosynthetic R. rubrum, nitrogenase is tightly controlled at both the transcriptional and metabolic level. Nitrogenase activity is regulated by reversible ADP-ribosylation in response to external “switch-off” effectors. In response to a sudden fixed-N increase or energy depletion, the Fe protein is covalently modified by DRAT to inhibit nitrogenase activity. When the fixed-N is metabolized or the energy status of the cell is restored, DRAG removes the ADP-ribose moiety, activating nitrogenase (Nordlund and Ludden, 2004). Two different signal-transduction pathways appear to lead to nitrogen and energy “switch-off” with the pathways merging at or before affecting DRAT/DRAG activities.