Human adenovirus is a useful model system to study cellular processes such as DNA replication, transcription, splicing and the initiation of malignant transformation by DNA tumour viruses. The HAdV5 early region 1A protein (E1A) is the first protein to be expressed upon viral infection. Here we report, for the first time, the viral early protein E4orf3 as a novel binding partner of the E1A protein. E4orf3 is a 15kDa protein that weaves through the nucleus of infected cells self-polymerizing into cable-like tracks. E4orf3 functions by forming multiple protein binding interfaces to capture and disrupt numerous cellular protein complexes including members of the DNA damage complex composed of Mre11, Nbs1 and Rad50 (MRN complex) and the oligomeric proteins Promyelocytic Leukemia (PML), amongst others. E4orf3 has also been found to block activation of p53 responsive genes via the formation of repressive heterochromatin at p53 target promoters. We found that E4orf3 binds directly to E1A via the N terminus of E1A. The presence of E4orf3 enhances E1A mediated transcriptional activity of viral genes thus driving their upregulation. In addition, we found that while the E4orf3 protein contributes to inactivation of p53, it is unlikely to be sufficient to block p53 mediated transcription. Instead, our results suggest that E1A and E4orf3 may be working together to achieve suppression of the p53 response pathway. The second project in this thesis focuses on the functions of the second exon-encoded region of E1A referred to as the C terminus of E1A. This region, is involved in a variety of processes including the regulation of viral and cellular gene expression, immortalization and transformation. Using various exon 2 deletion mutants we found that the C terminus of E1A makes important contributions to the viral life cycle in terms of kinetics of virus growth, activation of viral and cellular gene expression, efficiency of viral genome replication and deregulation of the cell cycle for the induction of S-phase.