E2F is required to prevent inappropriate S-phase entry of mammalian cells

E2F is a family of transcription factors that regulates the cell cycle. It is widely accepted that E2F-mediated transactivation of a set of genes is t he critical activity that governs cellular progression through G(1) into S phase. In contrast to this hypothesis, we demonstrate that E2F actually sup presses the onset of S phase in two cell types when the cells are arrested by gamma irradiation. Our findings indicate that in these cells, the critic al event triggering progression from G(0)/G(1) arrest into S phase is the r elease of E2F-mediated transrepression of cell cycle genes, not transactiva tion by E2F. Furthermore, our data suggest that E2F-mediated transactivatio n is not necessary for the G(1)/S-phase transition in these cells.