Family ARR-B

Two-component systems have originally been found and functionally characterized in prokaryotes. Additionally, they emerged as important sensing/response mechanisms in higher plants. Generally, hybrid histidine kinases function as receptors for the incoming signals and the so-called 'response regulators' transmit this information via canonical His-to-Asp phosphorelay. Histidine containing phosphotransfer domain proteins exist in plants as a kind of shuttle mechanism, which can receive the phosphate residue from the upstream histidine kinases and transfer it to the response regulators. So far, there are 10 A-type ARRs (Arabidopsis Response Regulators) and 11 B-type ARR proteins known from Arabidopsis thaliana. While the A-type ARR ions likely via protein-protein interaction, B-type ARRs constitute a subfamily of the GARP-domain transcription factor family. The B-motif (GARP-like motif) comprises about 60 - 80 AS and has been shown to be responsible for binding to its DNA-binding consensus GAT. Besides these classical ARRs the APRR (Arabidopsis Pseudo response Regulator) family consists of several members of pseudo response regulators, where the phospho-accepting Asp (D) residue has been replaced by a Glu (E) in the receiver domain of these proteins. Thus, these atypical response regulator proteins might not necessarily be involved in a canonical His-to-Asp phosphorelay dependent pathway. Functionally the two-component systems in plants play a major role in cytokinin perception and signalling and contribute to ethylene signal transduction and osmosensing. Furthermore, developmental processes like megagametogenesis in Arabidopsis thaliana and flowering promotion in rice (Oryza sativa) involve elements of two-component systems. Two-component-like elements also function as components of the Arabidopsis circadian clock. Because of the molecular mode of signalling, plant two-component systems also appear to serve as intensive cross talk and signal integration machinery.

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