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.
