Receptor of Trans-Zeatin Involved in Transcription Activation by Cytokinin

Zeatin‐binding protein (67 ± 2 kDa) was isolated from the cytosol of the first leaf of 10‐day‐old barley plants. The protein fits to all requirements for a zeatin receptor: (i) it binds [3H]trans‐zeatin reversibly and specifically, (ii) it is recognized by anti‐idiotype antibodies from antiserum raised against trans‐zeatin, (iii) in concert with 10−8 M trans‐zeatin it activates rRNA synthesis in vitro in a transcription elongation system containing chromatin from barley leaves associated with RNA‐polymerase I. In the presence of trans‐zeatin, the protein activates also RNA synthesis directed by RNA‐polymerase I and RNA‐polymerase II in isolated nuclei from barley leaves.