A vaccinia virus late transcription factor copurifies with a factor that binds to a viral late promoter and is complemented by extracts from uninfected HeLa cells.

We have previously described a vaccinia virus late transcription factor, VLTF-X, which we found to be present in cells at early and late times in infection. In this study, transcription complementation assays were used to demonstrate that VLTF-X activity is also present in virion extracts and in the cytoplasm of uninfected HeLa cells. Mobility shift assays performed on various VLTF-X preparations revealed that a late promoter DNA-binding activity cochromatographed and cosedimented with VLTF-X activity. Competition experiments demonstrated that this binding was specific for the late promoter region of the probe and that late transcription was dramatically reduced by an oligonucleotide that blocked factor-DNA complex formation but was only minimally affected by an oligonucleotide that did not inhibit complex formation. These results suggest that a cellular factor may participate in vaccinia virus late transcription. These findings also confirm the requirement for VLTF-X and distinguish it from any of the previously described vaccinia virus late transcription factors, which have all been mapped to the viral genome. Finally, these studies also suggest that the biochemical role for VLTF-X may be in late promoter recognition.