NarXNarL

NarXNarL is a bacterial two-component regulatory system consisting of NarX, a membrane-bound sensor histidine kinase, and NarL, a cytosolic response regulator. The system senses nitrate in the environment and coordinates gene expression to enable anaerobic nitrate respiration when oxygen is limited. In many bacteria, NarX detects extracellular nitrate and autophosphorylates on a histidine residue. The phosphate is transferred to NarL, and NarL-P (the phosphorylated form) binds promoter regions to modulate transcription of genes involved in nitrate and nitrite reduction, as well as related transport and regulatory components.

The NarXNarL regulon typically includes operons encoding the nitrate reductase enzymes, such as narGHJI for the membrane-bound nitrate reductase, and, in some species, the periplasmic nitrate reductase system. It can also regulate nitrate/nitrite transporters and other genes that support anaerobic respiration. Regulation by NarXNarL is often integrated with other global regulators, and the exact targets can vary among species. In many bacteria, the NarXNarL pathway operates in parallel with the NarQNarP system, allowing different responses to nitrate and nitrite under varying environmental conditions.

Structural and functional features include NarX’s sensor domains and histidine kinase activity, and NarL’s receiver and DNA-binding domains. The system is particularly studied in Escherichia coli and related Enterobacteriaceae, where it plays a key role in adjusting metabolic pathways for growth on nitrate as a terminal electron acceptor. NarXNarL remains a focal point in research on bacterial energy metabolism and regulatory networks.