PRESENCE OF A 2ND MECHANISM FOR THE POSTTRANSLATIONAL REGULATION OF NITROGENASE ACTIVITY IN AZOSPIRILLUM-BRASILENSE IN RESPONSE TO AMMONIUM

Although ADP-ribosylation of dinitrogenase reductase plays a significa nt role in the regulation of nitrogenase activity in Azospirillum bras ilense. it is not the only mechanism of that regulation. Tile replacem ent of an arginine residue at position 101 in the dinitrogenase reduct ase eliminated this ADP-ribosylation and revealed another regulatory s ystem. While the constructed mutants had a low nitrogenase activity, N H4+ still partially inhibited their nitrogenase activity, independent of the dinitrogenase reductase ADP-ribosyltransferase/dinitrogenase re ductase activating glycohydrolase (DRAT/DRAG) system, These mutated di nitrogenase reductases also were expressed in a Rhodospirillum rubrum strain that lacked its endogenous dinitrogenase reductase, and they su pported high nitrogenase activity, These strains neither lost nitrogen ase activity nor modified dinitrogenase reductase in response to darkn ess and NH4+, suggesting that the ADP-ribosylation of dinitrogenase re ductase is probably the only mechanism for posttranslational regulatio n of nitrogenase activity in R. rubrum under these conditions.