E2F-1-MEDIATED TRANSACTIVATION IS INHIBITED BY COMPLEX-FORMATION WITHTHE RETINOBLASTOMA SUSCEPTIBILITY GENE-PRODUCT

Previous studies have shown that the carboxyl-terminal region of E2F-1 (residues 368-437) can support transcriptional activation when linked to the DNA-binding domain of the yeast transcription factor GAL4. Thi s region also contains an 18-residue retinoblastoma (RB)-binding seque nce, raising the possibility that RB binding might inhibit the ability of E2F-1 to form protein-protein contacts required for activation. He re we report a further analysis of the E2F-1 activation domain. In add ition, we show that overexpression of RB, but not the RB mutant, RBd22 , can inhibit GAL4/E2F-1 activity in vivo. Moreover, expression of the simian virus 40 large tumor antigen (T antigen), but not the RB-bindi ng defective T antigen point mutant, K1, can overcome this repression. Three different GAL4/E2F-1 mutants that activate transcription, but f ail to bind to RB, are not significantly affected by overexpression of RB. These findings support a model wherein RB suppresses E2F-1-mediat ed transcriptional activation through direct physical association.