Two-component systems

Two component systems are signal transduction systems that consist of a sensor kinase and a response regulator (usually a transcription factor). The kinase autophosphorylates on a His residue (more preciely, one subunit of the dimeric kinase phosphorylates the His residue of the other subunit), and the phosphate group can then be transferred to an aspartate residue in the cognate response regulator. Some response regulators can also autophosphorylate using acety-phosphate as the phosphate donor. More complex signal transduction systems of this class are called phosphorelay. The paradigm for a phosphorelay is the pathway leading to the phosphorylation of Spo0A.