Molecular and regulatory properties of glutamine synthetase from the phototrophic bacterium Rhodopseudomonas capsulata E1F1

Abstract

The glutamine synthetase of the phototrophic bacterium Rhodopseudomonas capsulata E1F1 was purified to homogeneity by a procedure which used a single affinity chromatography step. Like enzymes from other photosynthetic procaryotes, native glutamine synthetase from R. capsulata E1F1 was found to be a dodecameric protein of approximately 660 kilodaltons with identical subunits of about 55 kilodaltons each. The Stokes radius and S20,w of the native enzyme were 8.35 nm and 19.20, respectively. The enzyme exhibited different aggregation states with detectable oligomers of 1, 2, 3, 4, 6, 8, 10, and 12 subunits. Disaggregation of the glutamine synthetase occurred after the native protein was subjected to electrophoresis in polyacrylamide gels, as well as occurring spontaneously at low ionic strength. Glutamine synthetase from R. capsulata E1F1 was regulated by an adenylylation-deadenylylation mechanism, and the adenylylation state of the protein depended on the nitrogen source, growth phase, and light intensity. Ammonia repressed glutamine synthetase, whereas glycine, serine, alanine, valine, and aspartate were noncompetitive inhibitors of the glutamine synthetase biosynthetic activity.