The involvement of the chloroplast zeatin-binding protein in hormone-dependent transcriptional control of the chloroplast genome

Using a multistep system for protein purification, including affinity chromatography on zeatin riboside-Sepharose, we isolated a zeatin-binding protein from the chloroplasts of the first leaf of ten-day-old barley seedlings (ZBP(Chl)). In combination with traps-zeatin, this protein enhanced the RNA synthesis in the chloroplast lysate from the same leaves. A hormonal specificity of ZBP(Chl)-induced transcriptional stimulation was demonstrated. The protein activated the chloroplast but not nuclear genome. Age-dependent changes in the ZBP(Chl) functional activity and in the response of the chloroplast transcription system to this protein and transzeatin were observed. In the chloroplast lysate from aging first leaves of 14-day-old barley plants, ZBP(Chl) from the leaves of 3-, 10-, and 14-day-old barley plants activated transcription in the absence of the hormone, and traps-zeatin suppressed this activation. Thus, traps-zeatin played the role of the ZBP(Chl) activator in chloroplast lysates from ten-day-old seedling and its inhibitor in lysates from 14-day-old seedlings. The combination of ZBP(Chl) and traps-zeatin activated the RNA synthesis in lysates from lupine leaves, which indicates the absence of species-specificity in the ZBP(Chl) action. It was shown that, in the heterologous transcription system, the principal factor of regulation was not a plant species, which was a source of the ZBP(Chl) or chloroplast lysate, but the physiological state of plants used.