Posttranslational modification of nitrogenase

In the photosynthetic bacteria Rhodospirillum rubrum and Rhodopseudomonas capsulatus post‐translational regulation of nitrogenase is due to ADP‐ribosylation of the Fe‐protein, the dinitrogenase reductase [Burris, R. H. (1991) J. Biol. Chem. 266 , 9339–9342]. This mechanism has been assumed to be responsible for nitrogenase modification in a variety of organisms. In the present study, we examined whether ADP‐ribosylation holds true for the filamentous cyanobacterium Anabaena variabilis . Genes coding for the nitrogenase‐modifying enzymes dinitrogenase reductase‐activating glycohydrolase (DRAG) and dinitrogenase reductase ADP‐ribosyl transferase (DRAT) from R. rubrum have been subcloned and overexpressed in Escherichia coli . After isolation under anaerobic conditions, both proteins were functional as determined by in‐vitro assays using nitrogenase from R. rubrum as substrate. In contrast to the R. rubrum enzyme, nitrogenase from A. variabilis was not affected by DRAG or DRAT. Neither could inactive nitrogenase be restored by DRAG, nor nitrogenase activity suppressed by DRAT. Using specific antibodies against arginine‐bound ADP‐ribose [Meyer, T. & Hilz, H. (1986) Eur. J. Biochem. 155 , 157–165], immunoblotting of the inactive, modified form of the Fe‐protein from R. rubrum but not that from A. variabilis showed a strong cross reaction. Furthermore, differently to R. rubrum no ADP‐ribosylated proteins could be detected at all, indicating the absence of this posttranslational modification in A. variabilis .