ISSN:
1573-5087
Keywords:
cytokinins
;
cytokinin-binding proteins
;
protein kinase
;
transcription regulation
Source:
Springer Online Journal Archives 1860-2000
Topics:
Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
Notes:
Abstract A role of endogenous cytokinins as the hormonal signal, by which the roots regulate leaf metabolism and prevent leaf senescence in particular, is discussed. Cytokinin signal perception and transduction in leaf cells were studied on fully expanded first leaves of 10–13-day-old barley plants (Hordeum vulgare L. cv. Viner). Their high sensitivity to exogenous cytokinins depends on dramatic decrease in endogenous cytokinin content during leaf growth. Cytokinin-binding protein of 28–30 kD was isolated from barley leaf cytosol by its affinity to synthetic cytokinin benzyladenine (BA). The 30-kD protein was involved in cytokinin-dependent activation of RNA synthesis in vitro in the system containing chromatin-bound RNA polymerase I from barley leaves. Multistage purification of the protein included affinity chromatography on trans-zeatin-Sepharose or zeatin riboside-Sepharose resulted in isolation of barley leaf cytosol 67-kD protein up to electrophoretic homogeneity. The protein was revealed as a single band in Western blot analysis developed with anti-idiotype antibodies from antiserum to zeatin. In concert with trans-zeatin, the 67-kD protein activated transcription elongation directed by RNA polymerase I (in the system containing chromatin-bound RNA polymerase I from barley leaves) and by RNA polymerase II (in nuclei isolated from barley leaves). The protein effect strongly depended on cytokinin concentration. The maximum activation was observed at trans-zeatin concentration of 10−8 M. cis-Zeatin had no effect. These results together with data on reversible [3H]zeatin-binding moiety of the 67-kD protein [8] provide a definitive proof to consider this protein as one of the cytokinin receptors in barley leaf cells which is responsible for cytokinin activation of transcription elongation directed by both RNA polymerase I and RNA polymerase II. Cytokinin-sensitive protein kinase activity was detected in barley leaf chromatin. Stimulation of this protein kinase was specific for physiologically active cytokinin trans-zeatin and was not realized by its non-active isomer cis-zeatin. The protein kinase was co-isolated from chromatin with RNA polymerase I and phosphorylated its subunits. The results of this study are discussed in the context of the latest evidence on regulation of transcription elongation in eucariotic cells. The suggestion was put forward on the involvement of protein kinase and cytokinin-binding protein in cytokinin-dependent transcription regulation in leaves.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00028485
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