DIRECT TRANSCRIPTIONAL REPRESSION BY PRB AND ITS REVERSAL BY SPECIFICCYCLINS

It was recently shown that the E2F-pRB complex is a negative transcrip tional regulator. However, it was not determined whether the whole com plex or pRB alone is required for repression. Here we show that pRB an d the related protein p107 are capable of direct transcriptional repre ssion independent of E2F. When fused to the DNA binding domain of GAL4 , pRB or p107 represses transcription of promoters with GAL4 binding s ites. Thus, E2F acts as a tether for pRB or p107 but is not actively i nvolved in repression of other enhancers. This function of pRB maps to the pocket and is abrogated by mutation of this domain. This result s uggests an intriguing model in which the pocket has a dual function, f irst to bind E2F and second to repress transcription directly, possibl y through interaction with other proteins. We also show that direct tr anscriptional repression by pRB is regulated by phosphorylation. Mutat ions which render pRB constitutively hypophosphorylated potentiate rep ression, while phosphorylation induced by cyclin A or E reduces repres sion ninefold.