In molecular biology, a two-component regulatory system serves as a basic stimulus-response coupling mechanism to allow organisms to sense and respond to changes in many different environmental conditions.
[3] Upon detecting a particular change in the extracellular environment, the HK performs an autophosphorylation reaction, transferring a phosphoryl group from adenosine triphosphate (ATP) to a specific histidine residue.
[5][6] This typically triggers a conformational change that activates the RR's effector domain, which in turn produces the cellular response to the signal, usually by stimulating (or repressing) expression of target genes.
[3] Two-component signal transduction systems enable bacteria to sense, respond, and adapt to a wide range of environments, stressors, and growth conditions.
[11] These pathways have been adapted to respond to a wide variety of stimuli, including nutrients, cellular redox state, changes in osmolarity, quorum signals, antibiotics, temperature, chemoattractants, pH and more.
[18] In Escherichia coli, the osmoregulatory EnvZ/OmpR two-component system controls the differential expression of the outer membrane porin proteins OmpF and OmpC.
[19] The KdpD sensor kinase proteins regulate the kdpFABC operon responsible for potassium transport in bacteria including E. coli and Clostridium acetobutylicum.
[3] Two-component systems in eukaryotes likely originate from lateral gene transfer, often from endosymbiotic organelles, and are typically of the hybrid kinase phosphorelay type.
[34] Because of their sequence similarity and operon structure, many two-component systems – particularly histidine kinases – are relatively easy to identify through bioinformatics analysis.
)[3] A database of prokaryotic two-component systems called P2CS has been compiled to document and classify known examples, and in some cases to make predictions about the cognates of "orphan" histidine kinase or response regulator proteins that are genetically unlinked to a partner.