Multiple, distinct trans-activation functions are encoded by the simian virus 40 large T and small t antigens, only some of which require the 82-residue amino-terminal common domain.

Simian virus 40 (SV40) small t and large T antigens can each trans activate the adenovirus (Ad) E2A and the Ad VA-I promoters. The first 82 amino acids of large T and small t are identical. However, this large T-small t common domain between residues 1 and 82 does not trans activate, suggesting that large T and small t each encode separate trans-activation functions. To determine whether the large T or small t unique domains, which are required for trans activation of the E2A promoter, are sufficient for this activity, we have employed expression plasmids separately encoding the common and unique domains of large T and small t. Cotransfection of a large T unique domain expression plasmid efficiently trans activated the E2A promoter. Optimal trans activation by large T required the motif that binds cellular proteins such as the retinoblastoma gene product, which is located in the large T unique domain, and additional large T structures outside this motif. In contrast, the small t unique domain did not trans activate the E2A promoter. Experiments utilizing E2A promoter mutants containing only the ATF- or EIIF-binding sites demonstrated that trans activation by small t involves only the EIIF transcription factor and that this function requires both the common (residues 1 to 82) and the small t unique domains expressed as a colinear protein. trans activation by large T, in contrast, involves at least three mechanisms. There appear to be at least two mechanisms that involve the EIIF transcription factor, at least one of which does not require the common domain (residues 1 to 82) and one mechanism that involves the ATF factor and does require both the common and the large T unique domains.