Two-component signal transduction pathways typically composed of a sensor histidine kinase that receives the input stimuli and then phosphorylates a response regulator that effects an appropriate change in cellular physiology. Histidine kinases and response regulators have an intrinsic modularity that separates signal input, phosphotransfer, and output response; this modularity has allowed bacteria to dramatically expand and diversify their signaling capabilities. Plants likely gained two-component pathways through the integration of chloroplast genes into the nuclear genome. Whereas two-component genes are found in yeasts, filamentous fungi, slime molds, and plants, they are conspicuously absent from higher eukaryotes and metazoans. Type-B response regulators comprise a receiver domain along with an extended C-terminal output domain, and their expression is not regulated by any stimulus tested so far. The output domains of B-type ARRs usually contain a GARP DNA-binding motif, a C-terminal transactivation domain and at least one NLS. Several molecular and cell biological studies have provided strong evidence that B-type response regulators function as nuclear transcription factors binding to target promoters that contain the 5??-W/GAT/W-motif, where W is either A or T.
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