Simian virus 40 small t antigen trans activates the adenovirus E2A promoter by using mechanisms distinct from those used by adenovirus E1A.

As reported previously for simian virus 40 small t antigen, polyomavirus small t antigen stimulates transcription directed by the adenovirus E2A and VA-I promoters during transient transfection assays. To determine whether papovaviral small t antigens might employ biochemical mechanisms during transcription activation that are either similar to or distinct from other viral trans activators, I compared the abilities of simian virus 40 small t antigen and adenovirus E1A to regulate the E2A promoter during transient transfection assays. I determined that, whereas activation of the E2A promoter by E1A involves the transcription factors ATF and EIIF, activation by small t antigen involves only EIIF. The effects of cotransfecting maximal concentrations of plasmids encoding small t antigen with E1A suggested that they activate the E2A promoter by different mechanisms. To determine whether small t antigen employs a mechanism different from that encoded in E1A domain II, domain III, or both, I compared the effects of transfecting plasmids expressing small t antigen, the 12S product of E1A, or the 13S product with a mutation in domain II on trans activation of the E2A promoter in two cellular backgrounds. On the basis of these comparisons, it appears that small t antigen does not activate transcription by a mechanism similar to either of the activities encoded in E1A. This suggests that papovavirus small t antigens belong to a distinct class of trans-acting proteins.