Short-term Regulation of Glutamine Synthetase Activity in the Unicellular Cyanobacterium Anacystis Nidulans by Ammonium

A short-term regulation of glutamine synthetase (GS) activity by ammonium was studied in the unicellular cyanobacterium Anacystis nidulans. The addition of ammonium (250 juM) to nitrate-grown cells resulted in a short-term inactivation of GS to a level of about 10% of the initial level in 30 min. The enzyme activity showed recovery from ammonium inactivation when the ammonium in the medium was depleted and attained its normal level within 90 min. The intracellular glutamine level was slightly affecteed by the nitrogen source available in the medium, suggesting that glutamine per se is not responsible for GS inactivation. Nitrogen-deprivation of the cells also attenuated the ammoniumpromoted GS inactivation, indicating that regulation of GS is modulated through the internal balance between carbon and nitrogen (oxoglutarate and glutamine) compounds.