A NOVEL DNA-BINDING PROTEIN WITH HOMOLOGY TO MYB ONCOPROTEINS CONTAINING ONLY ONE REPEAT CAN FUNCTION AS A TRANSCRIPTIONAL ACTIVATOR

A cDNA clone encoding a novel Myb-related protein, designated MybSt1, was isolated from a potato cDNA expression library by South Western screening using the CaMV 35S promoter domain A as a probe. Sequence comparison shows a small region with some homology to the highly conserved DNA binding domain of the c-myb proto-oncogene consisting of three imperfect repeats. The Myb motif of the MybSt1 protein is distinct from the plant Myb DNA binding domain described so far. In contrast to the known plant Myb proteins, with two repeats required for the DNA binding activity, the clone mybSt1 contains only one such repeat. Nevertheless, the Myb-related protein MybSt1 is able to bind to DNA in a sequence-specific manner. In addition to the Myb-like region, the protein MybSt1 contains an acidic segment in its central region as well as a proline-rich region near the C-terminus. Applying the random binding site selection technique, high-affinity DNA binding sites for MybSt1 were identified, sharing the core motif GGATA. In transient expression assays using plant protoplasts, clear evidence was obtained for this myb clone functioning as a transcriptional activator.