The overall goal of these studies is the elucidation of transcriptional control mechanisms by the immediate early protein of adenovirus, E1A, and by two E1A-like viral regulators, SV40 T antigen and the E7 protein of human papilloma virus (HPV). We have pursued an in-depth analysis of the transactivation functions of these proteins, which though related, originate from different DNA tumor viruses. E1A_12S, one of three major alternatively spliced protein products of the adenoviral E1A gene, has been considered an oncoprotein on the basis of its ability to transform cells in conjunction with an activated ras gene or adenovirus E1B. We find that E1A_12S, in addition to manifesting transforming ability, also has trans-activating activity. The ability of E1A_12S to activate transcription of one early gene of adenovirus, the E2 gene, has served as a useful paradigm for understanding the molecular basis of E1A trans-activating ability. We have studied the details of E2 trans-activation as well as hsp70 activation in which a mechanistically similar model of activation may be applied.

E2 gene trans-activation is now known to occur by E1A mediated disruption of cellular complexes which contain the transcription factor E2F critical for E2 promoter activation. Many of the proteins previously identified as E1A-associated proteins, including the retinoblastoma gene product (Rb) and cyclin A, are initially complexed to E2F and then transferred to E1A as a result of the dissociation event. The regions of E1A required for interactions with these cellular proteins (conserved regions 1 and 2:​ CR1 and CR2), are fundamental to the transforming activity of E1A, and are the same which liberate E2F from complexes with these proteins. Liberated E2F is presumably capable of productive promoter interactions and to thus serve as a trans-activator. Similar sequences to E1A CR1 and CR2 occur in the related viral oncoproteins, T antigen, and E7 where they are also known to be important for the process of malignant transformation. This sequence similarity forms the basis for a shared capacity of the three oncoproteins to disrupt protein complexes containing E2F and with the shared ability to induce transcription of the adenoviral E2 promoter.

The E1A amino-terminus (N-terminus) shares a more limited additional region ofsequence similarity with T antigen. This N-terminal region, distinct from CR1 and CR2, has also been shown essential for E1A oncogenic activity. We have found that the N-terminal region of E1A_12S is responsible for trans-activating another gene, the cellular heat shock 70 (hsp70) gene. The target element in the hsp70 gene appears to be the TATA element. TATA mediated trans-activation by the EIA N-terminus, correlates with the ability of E1A to disrupt complexes of another sort, namely complexes containing a general transcription factor, the TATA-binding protein (TBP) and a transcriptional inhibitory factor, Dr-1. We find that E1A can form stable complexes with Dr-1 dependent on the same E1A N-terminal sequences responsible for hsp70 activation.

Thus, two paradigms of transactivation by E1A_12S, activation of the E2 promoter, and activation of the hsp70 promoter, rely upon the ability of distinct regions of E1A to disrupt protein complexes containing cellular transcription factors, liberating these factors for productive promoter interactions. The ability of E1A to disrupt E2F-containing complexes and trans-activate the E2 promoter extends to both T antigen and E7;​ while the ability of E1A to disrupt TBP/Dr-1 complexes and transactivate hsp70 appears only to be shared with T antigen. The fact that homologous regions in all three oncoproteins are trans-activating targets in common and mediate malignant transformation, suggests an inter-relationship between the ability to transactivate and the ability to act as an oncoprotein. We suggest a model wherein oncoprotein mediated disruption of complexes containing transcription factors, would result in increased transcription factor availability. The consequence of increased availability of the general transcription factor TBP, may be a global increase in transcription since RNA polymerase II, and a subset of RNA polymerase l and III transcripts, utilize the same TBP. In the case of E2F, the expression of a cohort of genes involved in DNA replication (dihydrofolate reductase, thymidine kinase, and DNA polymerase a), which contain E2F sites in their promoters, would be augmented. In either case, that of TBP or E2F, increased transcription factor availability could promote acclerated entry into S phase. Stimulation of cells to enter S phase would result in a favorable environment for growth of these viruses which require host cell components for viral DNA replication. In the absence of a lytic infection, the release of TBP or E2F from cellular complexes by these oncoproteins could lead to continous proliferation and oncogenic transformation.