E2F3 ACTIVITY IS REGULATED DURING THE CELL-CYCLE AND IS REQUIRED FOR THE INDUCTION OF S-PHASE

Previous work has demonstrated the important role of E2F transcription activity ire the induction of S phase during the transition from quie scence to proliferation. In addition to the E2F-dependent activation o f a number of genes encoding DNA replication activities such as DNA Po l a, we now show that the majority of genes encoding initiation protei ns, including Cdc6 and the Mcm proteins, are activated following the s timulation of cell growth and are regulated by E2F. The transcription of a subset of these genes, which includes Cdc6 cyclin E, and cdk2, is also regulated during the cell cycle. Moreover, whereas overall E2F D NA-binding activity accumulates during the initial G(1) following a gr owth stimulus, only E2F3-binding activity reaccumulates at subsequent G(1)/S transitions, coincident with the expression of the cell-cycle-r egulated subset of E2F-target genes. Finally, we show that immunodeple tion of E2F3 activity inhibits the induction of S phase in proliferati ng cells. We propose that E2F3 activity plays an important role during the cell cycle of proliferating cells, controlling the expression of genes whose products are rate limiting for initiation of DMA replicati on, thereby imparting a more dramatic control of entry into S phase th an would otherwise be achieved by post-transcriptional control alone.