Molecular Characterization of Glutamine Synthetase from the Nitrogen-Fixing Phototrophic Bacterium Rhodopseudomonas palustris1

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Abstract
The phototrophic bacterium R. palustris assimilated ammonium via glutamine synthetase [EC 6.3.1.2] and glutamate synthase. Diazotrophic and ammonium-grown cells had high levels of both enzymes, whereas enzymes of alternative assimilatory pathways were absent or had only low activities. Glutamine synthetase was purified to electrophoretic homogeneity within 3 steps by dye-ligand and ion exchange chromatography. EM revealed a dodecameric molecular entity which was in accordance with parameters derived from electrophoretic techniques. The MW of the enzyme monomer was 55,800 and that of the dodecamer was 670,000. The amino acid composition of R. palustris glutamine synthetase was determined and compared by a statistical method with other known enzyme compositions from prokaryotic and eukaryotic origins.