Ethylene and cytokinin signalling in plants: the role of two-component systems.

Two-component systems involve the use of histidine kinases and response-regulator proteins, and allow for the perception of a variety of signals. ETR1 is an ethylene receptor in plants, and contains domains with sequence similarity to histidine kinases and response regulators. Ethylene signalling involves a phosphorylation cascade, employing both histidine kinases and serine/threonine kinases. CKI1 is involved in the cytokinin signalling pathway in plants, and contains domains with sequence similarity to histidine kinases and response regulators. The identification of two-component systems in eukaryotes indicates that this is a general signalling mechanism, not limited solely to prokaryotes.