Activation of simian virus 40 transcription in vitro by T antigen

Simian virus 40 is repressed when the viral early gene product large tumor antigen (TAg) binds to specific sites within the viral origin and DNA replication ensues. Late transcription is activated by TAg, even in the absence of viral DNA replication. We show here that TAg produced in human 293 cells can selectively activate Simian virus 40 transcription in a cell-free system. In the absence of DNA binding by TAg, early and late transcription are both activated, as they are in vivo, suggesting that the effect might be mediated by a cellular component(s) utilized by both the early and late promoters. When TAg binds to the viral origin of replication, early transcription is repressed but the late promoter activation is unaffected. Various preparations of TAg differed in their activities, with some able both to bind DNA and to activate transcription and others able to do only one or the other. Since these variations might be explained by variable amounts of associated factors that copurified with TAg, we asked whether a bacterially derived protein could regulate transcription. An NH2-terminal 272-amino-acid fragment of TAg, produced in Escherichia coli as a glutathione S-transferase fusion protein, retains the ability to activate transcription in vitro, similar to that of the full-length protein. Structural features of this region that might be important are discussed.