E2F transcription factors play pivotal roles in controlling the expression of genes involved in cell-cycle progression. Different viruses affect E2F1/retinoblastoma (Rb) interactions by diverse mechanisms releasing E2F1 from its suppressor Rb, enabling viral replication. We show that in T cells infected with human herpesvirus 6A (HHV-6A), the E2F1 protein and its cofactor DP1 increased, whereas the Rb protein underwent massive degradation without hyperphosphorylation at three sites known to control E2F/Rb association. Although E2F1 and DP1 increased without Rb suppression, the E2F1 target genes-including cyclin A, cyclin E, and dihydrofolate reductase-were not up-regulated. To test whether the E2F1/DP1 complexes were used for viral transcription, we scanned the viral genome for genes containing the E2F binding site in their promoters. In the present work, we concentrated on the U27 and U79 genes known to act in viral DNA synthesis. We constructed amplicon-6 vectors containing a GFP reporter gene driven by WT viral promoter or by promoter mutated in the E2F binding site. We found that the expression of the fusion U27 promoter was dependent on the presence of the E2F binding site. Test of the WT U79 promoter yielded >10-fold higher expression of the GFP reporter gene than the mutant U79 promoter with abrogated E2F binding site. Moreover, by using siRNA to E2F1, we found that E2F1 was essential for the activity of the U79 promoter. These findings revealed a unique pathway in HHV-6 replication: The virus causes Rb degradation and uses the increased E2F1 and DP1 factors to transcribe viral genes.H uman herpesvirus 6A (HHV-6A) and HHV-6B infect >90% of children by the age of 2 y (1). Recent studies have found that ∼1% of children are born with chromosomally integrated HHV-6, suggesting that the virus is vertically transmitted in a Mendelian manner (2). HHV-6B is the causative agent of roseola infantum, a brief febrile infection with skin rash (3). In a minority of patients, there are neurological complications up to lethal encephalitis (4). After productive infection, HHV-6B enters into latency from which it can be reactivated-e.g., following bone marrow and hematopoietic stem cell transplantations. Viral reactivation can result in delayed transplant engraftment and severe complications up to lethal encephalitis (5). Furthermore, transplantation of solid organs-including kidney, liver, lung, and heart-results in high rates of HHV-6 reactivation, although only 1% of transplant recipients were found to develop severe complications (6). There is no acute disease known to be caused by HHV-6A, but recent studies have suggested potential involvement in multiple sclerosis (MS) aggravation (7). HHV-6 was found more often in MS plaques than in MS normal-appearing white matter or non-MS brains. HHV-6 reactivation has been reported during MS clinical relapses (7). Recent evidence (8) has suggested the association of HHV-6A with Hashimoto thyroiditis, the most common of all thyroid diseases.E2F1 acts as a transcription factor of ...