Two-component regulation systems

Two-component systems serve as a basic stimulus-response coupling mechanism to allow organisms to sense and respond to changes in many different environmental conditions.^[1] They consist of a membrane-bound histidine kinase that senses a specific environmental stimulus and a corresponding response regulator that mediates the cellular response, mostly through differential expression of target genes.^[2] Two component signalling systems are widely occurring in prokaryotes and a whereas only a few two-component systems have been identified in eukaryotic organisms.^[1] Signal transduction occurs through the transfer of phosphoryl groups from adenosine triphosphate (ATP) to histidine residues in the histidine kinases. Susequently the histidine kinase catalyses the transfer of the phosphate group on the phosphorylated histidine residues to aspartic acid residues on the response regulator. The level of phosphorylation of the response regulator controls its activity.^[3] Many of the two-component systems are involved in signalling systems for sensing the changes of external environment such as temperature, osmolarity, chemo attractants and pH.^[4] In E. coli the EnvZ/OmpR osmoregulation system controls the differential expression of the outer membrane porin proteins OmpF and OmpC.^[5]

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