Summary
Most photosynthetic bacteria are capable of nitrogen fixation and the biochemistry and genetics of nitrogen fixation have been studied extensively in a few of these. Both the traditional, molybdenum-dependent, nitrogenase and the third alternative form of nitrogenase have been found in anoxygenic phototrophs and the properties of these are similar to those in heterotrophic nitrogen fixers. The nif genes for the molybdenum nitrogenase system and the anf genes for the alternative nitrogenase system have been isolated. Nitrogenase activity in some photosynthetic bacteria is regulated by reversible ADP-ribosylation at arg 101 of the dinitrogenase reductase protein, which prevents it from transferring electrons to the dinitrogenase protein. ADP-ribosylation of dinitrogenase is catalyzed by dinitrogenase reductase ADP-ribosyl transferase (DRAT) and the removal of ADP-ribose is performed by dinitrogenase reductase activating glycohydrolase (DRAG). These are encoded by the draT and draG genes, respectively.