FIG. 8.

Model for activation of vaccinia virus intermediate transcription and the switch from intermediate and late transcription. Following entry into the cell, the DNA is amplified by the viral replication machinery. The amplified DNA draws transcription factors such as TBP from the nucleus into the cytoplasm to activate intermediate gene transcription. TBP is capable of targeting both intermediate (I) and late promoters (L) but activates only the intermediate promoters initially because intermediate transcription factors assemble a dysfunctional preinitiation complex on the short late promoter. As late transcription factors accumulate, they displace intermediate factors to activate late promoters and inactivate intermediate transcription by formation of a dysfunctional complex at intermediate promoters. VV, vaccinia virus.