FIG. 1.

Schematic view of two-component and phosphorelay systems. Activation signals recognized by sensor domains of histidine kinases result in autophosphorylation of a histidine in the histidine phosphotransferase domain (His PTase). The phosphoryl group (P) is transferred directly to the phosphorylated aspartate domain (PA) of a response regulator in a two-component system, causing a conformational change that activates the output domain. In a phosphorelay, the phosphoryl group is transferred to a PA domain that serves as a substrate for a phosphotransferase whose role is to transfer the phosphoryl group to the PA domain of a response regulator. Note that all the steps are reversible in many systems, which may result in dephosphorylation in the absence of a signal.