NtrB

NtrB is a sensor histidine kinase that forms part of the nitrogen regulation two-component system NtrB/NtrC, found in many bacteria, particularly proteobacteria such as Escherichia coli. It functions together with the response regulator NtrC to adjust gene expression in response to nitrogen availability. NtrB and NtrC typically regulate transcription from sigma-54-dependent promoters, linking nitrogen metabolism to changes in cellular transcription.

Mechanism and signaling: In response to nitrogen status, NtrB autophosphorylates on a conserved histidine residue and transfers the phosphate to NtrC, which receives it on an aspartate residue. Phosphorylated NtrC acts as an enhancer-binding protein with an AAA+ ATPase domain that activates transcription by remodeling the RNA polymerase holoenzyme at sigma-54 promoters. This regulatory cascade promotes expression of genes involved in nitrogen assimilation and metabolism, including those for amino acid synthesis and ammonium transport in some species.

Sensing and regulation: NtrB activity is modulated by the PII signal transduction proteins GlnB and GlnK, which sense cellular nitrogen and energy status through the uridylylation state controlled by GlnD. Under nitrogen limitation, these PII proteins convey signals that promote NtrB kinase activity; in nitrogen-replete conditions, they contribute to reduced NtrB activity. The exact interactions can vary among bacteria, but the general principle is that NtrB responds to intracellular nitrogen signals via PII proteins to control NtrC phosphorylation.

Distribution and significance: The NtrB/NtrC system is widespread in bacteria and plays a central role in coordinating nitrogen utilization with transcriptional regulation. Its activity influences a range of nitrogen-related operons and integrates with broader metabolic networks essential for adaptation to fluctuating nutrient environments.