Covalent regulation of glutamine synthetase. Adenylyl transferase (AT), in association with the regulatory protein PII, adenylylates and inactivates the synthetase. When associated with PII bound to UMP, AT deadenylylates the synthetase, thereby activating the enzyme. Uridylyl transferase (UT), the enzyme that modifies PII, is allosterically regulated by α-ketoglutarate, ATP, and glutamine.
[Information from D. L. Nelson and M. M. Cox, Lehninger Principles of Biochemistry 6th ed. (W. H. Freeman and Company, 2013), Fig. 22.9.]