Abstract
Nitrogen fixation in phototrophic bacteria has been studied in greatest detail at the molecular level in Rhodospirillum rubrum and Rhodobacter capsulatus. The general characteristics of nitrogenase are very similar to those of the enzymes from e.g. Klebsiella pneumoniae and Azotobacter vinelandii (reviewed in 1). There is however one very interesting difference: nitrogen fixation in R. rubrum, Rb. capsulatus and a number of other phototrophic bacteria, is regulated at the metabolic level in addition to the transcriptional control demonstrated in all diazotrophs studied. This metabolic regulation is manifested as an inhibition of nitrogenase activity when cells are subjected to “switch-off” effectors, such as ammonium ions, glutamine or darkness. At the molecular level this inhibition is due to the conversion of dinitrogenase reductase (the Fe-protein) from an active to an inactive form by ADP-ribosylation of an arginine residue in one of the subunits. The modified form can not interact with dinitrogenase (the MoFe-protein). ADP-ribosylation is catalyzed by ADP-ribosyl transferase (DRAT) with NAD+ as ADP-ribose donor and dinitrogenase reductase activating glycohydrolase (DRAG) catalyzes the reverse reaction.
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