Summary: Glutamine synthetase has been purified to homogeneity from two N-fixing cyanobacteria, and a species of (the phycobiont of ). The activities of the enzyme in the biosynthetic and transferase assays were, respectively, 9.4 and 32 μmol product formed min (mg protein); the corresponding values for the sp. enzyme were 6.5 and 20. Stabilization of the enzyme required Mg, glutamate, EDTA and a thiol reagent to be present during purification. The molecular weight of the enzyme was 591000 as estimated by sedimentation analysis, 660000 by gel filtration and 565000 by polyacrylamide gel electrophoresis; the sp. enzyme gave values of 630000 by gel filtration and 575000 by electrophoresis. The molecular weights of the sub-units of each enzyme were approximately 49000 to 50 000. Electron microscopy revealed that each molecule was composed of 12 sub-units arranged in two superimposed hexagonal rings. The maximum diameter of the rings was 13.6 nm and the distance between the centres of adjacent sub-units was 4.9 nm. When dialysed in the absence of stabilizing ligands the enzyme lost activity and the protein band characteristic of the native enzyme was replaced by three bands with approximate molecular weights of 510000, 310000 and 130000. These sub-species re-associated and activity was restored by adding 2-mercaptoethanol and substrates. A similar reversible deactivation has been observed with glutamine synthetase from photosynthetic eukaryotes and yeast but no similar data have been reported for a N-fixing prokaryote.


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