POSTTRANSLATIONAL REGULATION OF NITROGENASE ACTIVITY BY FIXED NITROGEN IN AZOTOBACTER-CHROOCOCCUM

Using anti-(Fe protein) antibody raised against the Fe protein of the photosynthetic bacterium Rhodospirillum rubrum, it was found that the Fe protein component of nitrogenase (EC 1.18.2.1) from Azobacter chroo coccum, cells subjected to an ammonium shock, and hence with an inacti ve nitrogenase. appeared as a doublet in Western blot analysis of cell extracts. The Fe protein incorporated [P-32]phosphate and [H-3]adenin e in response to ammonium treatment, and L-methionine-Dr-sulfoximine, an inhibitor of glutamine synthetase (L-glutamate: ammonia ligase (ADP forming), EC 6.3.1.3), prevented Fe protein from inhibition and radio isotope labelling. These results support that A. chroococcum Fe protei n is most likely ADP-ribosylated in response to ammonium. After ammoni um treatment, when in vivo activity was completely inhibited, Fe-prote in modification was still increasing. This suggests the existence of a nother mechanism of nitrogenase inhibition faster than Fe-protein modi fication. When ammonium was intracellularly generated instead of being externally added, as occurs with the short-term nitrate inhibition of nitrogenase activity observed in A. chroococcum cells simultaneously fixing molecular nitrogen and assimilating nitrate, a covalent modific ation of the Fe protein was likewise demonstrated.