Purification, physical characterization, and NH2-terminal sequence of glutamine synthetase from the cyanobacterium Anabaena 7120.

A procedure for the complete purification of glutamine synthetase (L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2) from the cyanobacterium (blue-green alga) Anabaena 7120 is described. The enzyme has structural characteristics in common with glutamine synthetases from other sources: a subunit molecular weight of approximately 50,000 and a native structure, determined by low dose exposure electron microscopy, consisting of a two-layered regular hexagon made up of 12 subunits. Sequence analysis suggests that the subunits are identical. There was no indication that the enzyme from cyanobacteria is adenylylated, a feature shared with the glutamine synthetase from Bacillus subtilis but not with the enzyme from Escherichia coli. NH2-terminal sequence analysis and the predicted conformation of the NH2-terminal regions showed definite homology among the enzymes from all three sources. The limited analysis suggested a stronger structural homology between the enzyme from Anabaena 7120 and that from E. coli.