Biochemical characterization of a putative cytokinin-responsive his-kinase, CKI1, from Arabidopsis thaliana

His-Asp phosphorelays are evolutionary-conserved powerful biological tactic s for intracellular signal transduction. Such a phosphorelay is generally m ade up of "sensor histidine (His)-kinases", "response regulators", and "his tidine-containing (HPt) phosphotransmitters''. Results from recent intensiv e studies suggested that, in the higher plant Arabidopsis thaliana, His-Asp phosphorelays may be widely used for propagating environmental stimuli, su ch as phytohormones (e.g., ethylene and cytokinin). In this study, we chara cterized, in vitro, the putative cytokinin-responsive CKI1 His-kinase, in t erms of His-Asp phosphorelays. It was demonstrated for the first time that the receiver domain in this sensor exhibits a strong phosphohistidine phosp hatase activity toward some Arabidopsis HPt phosphotransmitters (AHP1 and A HP2), suggesting the functional importance of the receiver domain for a pre sumed interaction of the sensor His-kinase with other His-Asp phosphorelay components.