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Publisher Summary This chapter focuses on the adenylylation of bacterial glutamine synthetase. The enzyme glutamine synthetase catalyzes the synthesis of glutamine from ammonia and glutamate in an ATP-dependent reaction. In bacteria and higher plants, it also participates in net synthesis of glutamate from ammonia and 2-oxoglutarate by functioning in a cycle with glutamate synthase. Physiological studies of Holzer and colleagues indicated that the GS of Escherichia coli was very rapidly adenylylated when the organism was shifted from N-limited medium to a medium containing excess ammonium. Strains of S. typhimurium (glnE) that lack the ability to adenlylate GS show a large growth defect upon shift from N-limited media, media in which they have accumulated high levels of GS, to media containing a high concentration of NH 4 +. This growth defect appears to be because of excessive GS activity after the shift because it is eliminated in glnE strains that also synthesize abnormally low amounts of GS in both N-limited and NH4+, it is prolonged in glnE strains that also synthesize abnormally high amounts of GS in NH4+ excess media and the latter strains continue to show a growth defect even after adaptation to NH 4 +, and glnE strains have normal amounts of both of the glutamatebiosynthetic enzymes under N-limited conditions and after NH 4 + upshift. At present, there appears to be no simple explanation for the distribution of this covalent modification. Although adenylylation of GS occurs widely among gram-negative bacteria, a more complete understanding of the physiological requirements for adenylylation of GS will contribute to understanding the basis for distribution of this covalent modification among the prokaryotes. |
