Role of the SV40 enhancer in the early to late shift in viral transcription.

Simian virus 40 large tumor antigen is a multifunctional protein, with two of its roles being the promotion of viral DNA replication and replication-independent activation of viral transcription. Replication leads to a shift in transcription from the early-early to the late and late-early cap sites, through mechanisms poorly understood. The viral transcription enhancer contains sequences important for both early and late transcription, and we therefore have carried out experiments to evaluate its role in these events. We find that the ability of replication to lead to a shift diminishes when early-early transcription is made increasingly stronger by multimerizing the enhancer, and suggest that replication might lead to the shift by interfering with the ability of the enhancer to direct initiation to those sites. The natural situation in the virus of having two copies of this element might represent a compromise between maximizing both T antigen expression early in infection and late gene expression after replication begins. We also show that replication-independent transcription activation by T antigen is bidirectional and involves at least in part elements to which the factor TEF-1 binds.