Metabolic Regulation of Nitrogenase Activity inRhodospirillum rubrum: The Role of PII Proteins and Membrane Sequestration

Expression of nitrogenase is transcriptionally regulated in all diazotrophs. In some phototrophs and some species of Azospirillum nitrogenase activity is regulated in response to sudden changes in the availability of combined nitrogen or in the energy supply. An increase in combined nitrogen in forms that can be used by the diazotroph or a decrease in energy supply will lead to dramatic decrease in nitrogenase activity. This is due to covalent modification of the Fe-protein, which will hinder the interaction between this nitrogenase component and the other, the MoFe-protein. The addition of the modification, an ADP-ribose moiety, is catalyzed by the regulatory enzyme DraT and is removed in a reaction catalyzed by DraG. The regulation of these two enzymes has for a long time been unknown but recent studies have shown that PII proteins are involved in conveying the signal from the external change in conditions to DraT and DraG. The mechanism also involves the ammonia channel AmtB and membranes sequestration PII proteins and of DraG.