On. Kulaeva et al., Nuclear and chloroplast cytokinin-binding proteins from barley leaves participating in transcription regulation, PLANT GR R, 32(2-3), 2000, pp. 329-335
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 inclu
ded protein purification by Sephadex G-50, hydrophobic chromatography on ph
enyl-Sepharose and affinity chromatography on zeatin-Sepharose. Interaction
of trans-zeatin with the nuclear protein was demonstrated by the ELISA tec
hnique 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 concer
t with trans-zeatin activated transcription elongation in vitro in systems
containing chromatin associated with RNA polymerase I or nuclei isolated fr
om barley leaves. Nuclear 67 protein had no effect on the chloroplast trans
cription system. A 64 CBP was isolated from chloroplasts of barley leaves b
y the same procedure as that used for CBP isolation from nuclei. The cytoki
nin-binding properties of the chloroplast protein were demonstrated by comp
etition of trans-zeatin with Ab(a)-i in complex formation with the protein
in ELISA. Chloroplast CBP activated RNA synthesis markedly in chloroplast l
ysate without any effect on transcription in the chromatin-containing syste
m. Therefore both nuclear and chloroplast transcription machineries are reg
ulated by cytokinins by means of special nuclear and chloroplast cytokinin-
binding proteins which can be considered cytokinin receptors with the prope
rties of transfactors regulating the elongation phase of transcription.