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Abstract. Regulation of glutamine synthetase (EC 6.3.1.2) in Escherichia coli is mediated by adenylylation and deadenylylation of the enzyme. The present studies show that one protein is a common component of both the adenylylation and deadenylylation systems. Thus, the ATP: glutamine synthetase adenylyltransferase, which catalyzes adenylylation of glutamine synthetase, and one of the two proteins required for deadenylylation (the PI protein) are inseparable by a variety of fractionation procedures. The adenylyltransferase and PIdeadenylylating activities behave as a single protein upon filtration through Agarose A 0.5 gel, and during chromatography on DE32 cellulose and hydroxyapatite columns. They migrate as a single protein band during electrophoresis on polyacrylamide gel and have identical susceptibilities to heat inactivation. These data indicate that the adenylyltransferase and the PI-deadenylylation activity are associated with the same protein complex.Glutamine synthetase (>-glutamate: ammonia ligase, EC 6.3.1.2) activity in Escherichia coli is regulated by adenylylation and deadenylylation of the enzyme in response to variations in the concentrations of glutamine and a-ketoglutarate.'-4 The enzyme is adenylylated by a specific ATP: glutamine synthetase adenylyltransferase215 (ATase) that catalyzes attachment in phosphodiester linkage of the adenylyl moiety of ATP to the hydroxyl group of a particular tyrosyl residue on each one of the 12 subunits of glutamine synthetase (GS), reaction 1. Recently, Mantel and Holzer6 demonstrated reversibility of reaction (1) and established the energy rich nature of the adenylyl-O-tyrosyl bond. The physiologically important deadenylylation reaction, however, is catalyzed by a complex enzyme system (DA system) that consists of at least two separable protein components, PI and PI,, and is also stimulated by a-ketoglutarate, ATP, and UTP. Recent studies, unpublished results, with purified preparations of PI 1417

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