Regulation of Glutamine Synthetase from Bacillus subtilis by Divalent Cations, Feedback Inhibitors, and l-Glutamine

Glutamine synthetase purified to apparent homogeneity from Bacillus subtilis is subject to feedback inhibition by multiple end products of glutamine metabolism, as well as by the reaction product l-glutamine. AMP, glutamine, and histidine are potent inhibitors when any of the substrates glutamate, MnATP, or ammonia are present in limiting concentration during assay, but the strong inhibition by glycine and alanine is influenced far less by substrate concentration. In the Mg2+-dependent biosynthetic assay, AMP and glutamine are potent inhibitors of glutamine synthetase, whereas alanine and glycine do not inhibit this activity at all. Tryptophan, which is not an inhibitor in the Mn2+ biosynthetic assay, exhibits some inhibition in the Mg2+ assay. Synergistic inhibition of the Mn2+-dependent glutamine synthetase activity is present when AMP and glutamine are studied together; glutamine alone is not inhibitory when present at 5 mm concentration in the saturating assay, but produces 85% inhibition when studied in the presence of 2.5 mm AMP. Histidine and AMP are also synergistic, but glutamine acts independently of histidine. Alanine and glycine reciprocally reduce the effectiveness of one another as inhibitors. Glutamine appears to play a major role in the over-all regulation of its own synthesis. In Escherichia coli, glutamine modulates the glutamine synthetase adenylylation-deadenylylation system, thereby controlling over-all enzyme activity as well as responsiveness to feedback inhibitors. B. subtilis appears to lack the mechanisms to adenylylate and deadenylylate preformed glutamine synthetase. The organism does, however, appear to achieve similar regulatory effects by the direct inhibition of glutamine synthetase by glutamine, and by the striking potentiation of AMP inhibition that occurs with low concentrations of glutamine. Therefore, in B. subtilis as well as E. coli, intracellular glutamine levels play an important role in the fine control of the over-all glutamine synthetase activity, and in the control of cellular anabolic nitrogen metabolism.