Transactivation of murine cyclin A by polyomavirus large and small T antigens

Polyomavirus large and small T antigens cooperate in the induction of S pha se in serum deprived Swiss 3T3 cells. While the large T antigen is able to induce S phase-specific enzymes, we have recently shown that both T antigen s contribute to the production of the cyclins E and A and that the small T antigen is essential for the induction of cyclin A-dependent cdk2 activity (S. Schuchner and E, Wintersberger, J, Virol, 73:9266-9273, 1999). Here we present our attempts to elucidate the mechanisms by which the large and the small T antigens transactivate the murine cyclin A gene. Using Swiss 3T3 c ells carrying the T antigens and various mutants thereof under the hormone- inducible mouse mammary tumor virus promoter, as well as transient-cotransf ection experiments with the T antigens and cyclin A promoter-luciferase rep orter constructs, we found the following. The large T antigen activates the cyclin A promoter via two transcription factor binding sites, a cyclic AMP responsive element (CRE), and the major negative regulatory site called CD E-CHR. While an intact binding site for pocket proteins is required for the function of this T antigen at the CDF-CHR, its activity at the CRE is larg ely independent thereof. In contrast, an intact J domain and an intact zinc finger are required at both sites. The small T antigen also appears to hav e an influence on the cyclin A promoter through the CRE as well as the CDE- CHR. For this an interaction with protein phosphatase 2A is essential; muta tion of the J domain does not totally eliminate but greatly reduces the tra nsactivating ability.