Figure 1.

Information flow through a two-component signaling pathway. Shown are the standard prokaryotic and eukaryotic signaling components including (a) the sensor module, typically a transmembrane receptor with two putative membrane-spanning helices; (b) a transmitter histidine kinase that is regulated by the receptor and catalyzes autophosphorylation on histidine; and (c) a receiver or response regulator whose active site catalyzes phosphotransfer from the transmitter, thereby yielding autophosphorylation on aspartate. The response regulator can catalyze its own dephosphorylation, but some pathways require a separate phosphatase to generate more rapid dephosphorylation, or to provide additional pathway regulation. Different pathways display highly specialized assemblages of the modular elements; e.g. the sensor, transmitter, and response regulator modules can be separate proteins or can be fused together in various combinations (see text for references).