Nuclear and chloroplast cytokinin-binding proteins from barley leaves participating in transcription regulation

The 67 cytokinin-binding protein (CBP) previously isolated from the cytosol of mature leaves of 10-day-old barley seedlings has now been purified from nuclei isolated from the same leaves. The procedure of CBP isolation included protein purification by Sephadex G-50, hydrophobic chromatography on phenyl-Sepharose and affinity chromatography on zeatin-Sepharose. Interaction of trans-zeatin with the nuclear protein was demonstrated by the ELISA technique based on cytokinin competition with anti-idiotype antibodies (raised against antibodies to trans-zeatin) for complex formation with the protein immobilized on polystyrene microtiter plates. Nuclear 67 protein in concert with trans-zeatin activated transcription elongation in vitro in systems containing chromatin associated with RNA polymerase I or nuclei isolated from barley leaves. Nuclear 67 protein had no effect on the chloroplast transcription system. A 64 CBP was isolated from chloroplasts of barley leaves by the same procedure as that used for CBP isolation from nuclei. The cytokinin-binding properties of the chloroplast protein were demonstrated by competition of trans-zeatin with Ab(a)-i in complex formation with the protein in ELISA. Chloroplast CBP activated RNA synthesis markedly in chloroplast lysate without any effect on transcription in the chromatin-containing system. Therefore both nuclear and chloroplast transcription machineries are regulated by cytokinins by means of special nuclear and chloroplast cytokinin-binding proteins which can be considered cytokinin receptors with the properties of transfactors regulating the elongation phase of transcription.