Uridylyl transferase is an enzyme that uridylates (puts a uridyl group onto) the regulatory protein, PII (Figure 20.9). PII forms a complex with the enzyme, adenylyl transferase (AT), forming an AT-PII complex. When PII is uridylated, the AT-PII complex catalyzes deadenylylation of glutamine synthetase. This, in turn, causes glutamine synthetase to become active. Conversely, when PII is deuridylated (by a different enzyme), the AT-PII complex catalyzes deadenylylation of glutamine synthetase, inactivating it.