Regulation of glutamine synthetase in the blue-green alga Anabaena L-31

Abstract

In N₂-grown cultures of Anabaena L-31, in which protein synthesis was prevented by chloramphenicol, presence of NH₄⁺ caused a drastic decrease of glutamine synthetase (l-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2) activity indicating NH₄⁺-mediated inactivation or degradation of the enzyme. The half-life of glutamine synthetase was more than 24 h, whereas that of nitrogenase (reduced ferredoxin:dinitrogen oxidoreductase (ATP-hydrolysing), EC 1.18.2.1) was less than 4 h, suggesting that glutamine synthetase may not act as positive regulator of nitrogenase synthesis in Anabaena. Glutamine synthetase purified to homogeneity was subject to cumulative inhibition by alanine, serine and glycine. The amino acids, however, exhibited partial antagonism in this behaviour. Glyoxylate, an intermediate in photorespiration, virtually prevented the amino acid inhibition. Kinetic studies revealed inhibition of the enzyme activity by high Mg²⁺ concentration under limiting glutamate level and by high glutamate in limiting Mg²⁺. Maximum enzyme activity occurred when the ratio of glutamate to free Mg²⁺ was 0.5 to 1.0. The results demonstrate that the enzyme is subject to multiple regulation by various metabolites involved in nitrogen assimilation.