Gam1, an early adenoviral CELO protein, is required for viral replication. Consistent with its ability to inhibit histone deacetylation by HDAC1, Gam1 activates transcription. In this report, we identify the cellular transcription factor p120(E4F) as a Gam1 interaction partner. p120(E4F) is a low-abundance transcription factor that represses the adenovirus E4 promoter. Here we demonstrate that p120(E4F) interacts with HDAC1 in vivo and in vitro, and that E4F-mediated transcriptional repression is alleviated by the HDAC inhibitor trichostatin A or by overexpressing Gam1. A mutant E4 promoter unresponsive to E4F-mediated transcriptional repression is also not stimulated by Gam1. Moreover, our cofractionation experiments demonstrate that p120(E4F), HDAC1 and Gam1 may be concomitantly present in protein complexes. We conclude that Gam1 activates E4-dependent transcription possibly by inactivating HDAC1.