IN-VIVO MODIFICATION OF AZOTOBACTER-CHROOCOCCUM GLUTAMINE-SYNTHETASE

A monospecific anti-(glutamine synthetase) antibody raised against glu tamine synthetase of the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 immunoreacted with glutamine synthetase from the N2-fi xing heterotrophic bacterium Azotobacter chroococcum. In Western-blott ing experiments this antibody recognized a single protein of a molecul ar mass of 59 kDa corresponding to glutamine synthetase subunit. This protein was in vivo-labelled in response to addition of ammonium, both [H-3]adenine and (H3PO4)-P-32 preincubation of the cells being equall y effective. Nevertheless, the amount of glutamine synthetase present in A. chroococcum was independent of the available nitrogen source. Mo dified, inactive glutamine synthetase was re-activated by treatment wi th snake-venom phosphodiesterase but not by alkaline phosphatase. L-Me thionine-DL-sulphoximine, an inhibitor of glutamine synthetase, preven ted the enzyme from being covalently modified. We conclude that, in A. chroococcum, glutamine synthetase is adenylylated in response to ammo nium and that for the modification to take place ammonium must be meta bolized.