B-MYB, A REPRESSED TRANSACTIVATING PROTEIN

B-Myb belongs to a family of related transcription factors which share a unique DNA binding domain. B-Myb plays an important role in regulat ion of the cell cycle. Its expression is upregulated by the human papi lloma virus HPV16 E7 oncoprotein. Overexpression of B-Myb can bypass p 53-mediated cell cycle arrest. The founding member of the myb gene fam ily, c-Myb, and A-Myb are involved in hematopoiesis and neurogenesis, respectively, and are both activators of gene transcription. Whether B -Myb is a transactivator or a repressor, however, has remained a matte r of discussion. We reviewed the transactivation potential of B-Myb in yeast, taking advantage of the fact that inducible gene activation is an evolutionarily conserved process. By mutational analysis we locali zed a conserved activation domain in B-Myb. In vertebrate cells the tr ansactivation potential of B-Myb is concealed by the C-terminal part o f the protein. We show that the cell cycle regulators cyclin A and cyc lin E activate B-Myb by eradicating the inhibition mediated by its car boxy-terminus. Our data suggest that in vertebrates the trans-activati ng function of B-Myb is regulated during the cell cycle and link Myb f unctions to cell cycle progression.