GlnE

GlnE is a bacterial enzyme that regulates the activity of glutamine synthetase (GS) by reversible adenylylation. It is encoded by the glnE gene. In many bacteria, GS catalyzes the ATP-dependent synthesis of glutamine from ammonium and glutamate, and its activity is controlled by GlnE through the addition or removal of an adenylyl group on a specific tyrosine residue of GS.

Mechanism and function: GlnE is a bifunctional adenylyltransferase that can both adenylylate (inactivate) GS and deadenylylate (activate) GS. The direction of modification depends on the cell’s nitrogen status and regulatory signals from the PII family proteins GlnB and GlnK, which themselves respond to nitrogen availability via the uridylylation status controlled by GlnD. In nitrogen-replete conditions, GlnE promotes adenylylation of GS, reducing GS activity; in nitrogen limitation, GlnE promotes deadenylylation, restoring GS activity to support ammonium assimilation.

Distribution and structure: The glnE gene is widespread among bacteria that possess nitrogen regulation systems. GlnE is typically a cytosolic protein and often functions as a multimer with distinct catalytic regions responsible for the addition and removal of adenylyl groups from GS, enabling rapid switching of GS activity in response to environmental cues.

Biological significance: By controlling GS activity, GlnE links nitrogen sensing to nitrogen assimilation, enabling bacteria to balance energy expenditure with nutrient uptake. The GlnE–GlnB/GlnD regulatory axis is a central component of bacterial nitrogen regulation and has been the focus of extensive biochemical and genetic study.