Site-directed mutagenesis of the target arginine for ADP-ribosylation of nitrogenase component II in Rhodobacter capsulatus.

The role of the conserved residue arginine-102 in the functioning and the regulation of the nitrogenase component II protein in Rhodobacter capsulatus has been studied by site-directed mutagenesis. The arginine at position 102 was replaced by thirteen different amino acids, and the effect of these substitutions on diazotrophic growth, in vivo and in vitro nitrogenase activity, and ADP-ribosylation of the component II protein was tested. The results show that although arginine is the optimal amino acid at this position, it is not essential for activity. However, the mutant proteins were not modified by ADP-ribosylation, either in the dark or after addition of NH4+, consistent with the specificity of the post-translational regulatory mechanism for the Arg-102. Indirect evidence suggest that this residue may be involved in interaction with the in vivo low-potential electron donor.