GlnB

GlnB, commonly referred to as the PII signal transduction protein, is a small, highly conserved regulatory protein that serves as a central sensor of cellular nitrogen status in many bacteria. It forms a homotrimer of approximately 12–15 kDa subunits and functions as a molecular switch that responds to the cellular levels of glutamine, 2-oxoglutarate, and energy charge by binding ATP or ADP and small effector molecules. These interactions influence GlnB’s conformation and its affinity for partner proteins.

Regulation of GlnB is controlled by the uridylyltransferase GlnD, which adds or removes a uridylyl group on a tyrosine residue of GlnB in response to nitrogen availability. The uridylylated and deuridylylated forms of GlnB interact differently with target enzymes and regulators. Key targets include the NtrB/NtrC two-component system, the adenylyltransferase GlnE that controls glutamine synthetase (GS) activity by adenylylation/deadenylylation, and the ammonium transporter AmtB. Through these interactions, GlnB coordinates ammonium uptake, assimilation, and the expression of nitrogen-regulated genes.

GlnB thus plays a central role in balancing carbon and nitrogen metabolism under fluctuating nitrogen conditions and is widely distributed among bacteria; many species also harbor a GlnK paralog with overlapping regulatory roles. While the core mechanism is conserved, the exact regulatory network can vary between species, reflecting different ecological niches and metabolic strategies.