Glutamine Synthetase and Nitrogenase Activity in the Blue‐green Alga Anabaena Cylindrica

Summary On adding NH4+ to nitrogen‐fixing cultures of Anabaena cylindrica, the glutamine pool of the intact filament increases within seconds. Nitrogenase activity and then glutamine synthetase activity decline. There are no changes in the activities of glutamate‐aspartate aminotrans‐ferase, or in the pools of alanine, glycine and aspartate during this period. Nitrogenase activity is usually inversely related to the size of the glutamine pool and shows little correlation with glutamine synthetase activity per se. There is no evidence of adenylylation or dead‐enylylation of glutamine synthetase (snake venom phosphodiesterase has no effect on activity, there is no serological homogeneity with the Escherichia coli enzyme which shows a covalent modification; the ratio of biosynthetic: transferase activity does not change markedly on the addition of NH4+ and the kinetics of decrease in glutamine synthetase activity are not consistent with adenylylation of the enzyme). Feed‐back inhibition occurs, with alanine and glycine being particularly effective inhibitors and with some inhibition occurring with AMP, carbamoyl phosphate and aspartate. Histidine, lysine, ornithine, citrulline and arginine were non‐inhibitory at the concentrations used. There is no evidence of inhibition of glutamine synthetase by glutamine. The addition of azaserine inhibits glutamine uptake by intact filaments and reduces the subsequent conversion of glutamine to glutamate within the filament by 77%. Such results are consistent with the operation of the glutamine synthetase‐glutamate synthase pathway in Anabaena cylindrica.