Reactivation in Epstein-Barr virus (EBV) is closely associated with a G0/G1 cell cycle arrest which can be induced either by lytic cycle-inducing agents or by the viral immediate-early transactivator, Zta. Accumulating evidence shows that in epithelial cells, downregulation of the proto-oncogenes, c-Myc and E2F1, plays a central role in lytic cycle-associated cell growth arrest. Here, we provide evidence that c-Myc and E2F1 provide a gatekeeper function to ensure that certain cell cycle inhibitory events have been capitulated prior to full progression into the lytic cycle. Specifically, we show that reconstituted expression of c-Myc in EBV-infected epithelial cells blocks the induction of the EBV lytic cycle. Using luciferase reporter assays, we demonstrate that both c-Myc and E2F1 are sufficient to inhibit Zta mediated transactivation. A genetic dissection of Zta identified a negative regulatory region within Zta transactivation domain, which shows high homology to the proline-rich domain of tumor suppressor, p53. A tryptophan residue within this Zta negative regulatory region plays a crucial role in mediating the inhibitory signals of c-Myc and E2F1. This region of Zta has been shown by others to be important for recruiting the coactivator, CBP. We have now shown that c-Myc and E2F1 can directly inhibit the transactivation function of both CBP and its cellular homolog, p300 suggesting that transcriptional coactivators CBP/p300 are targeted by c-Myc and E2F1 to inhibit Zta transactivation. Overall, our studies demonstrate a new mechanism involved in the regulation of the transition from EBV latency to the lytic replication
