Identification of factors specific for transcription of the late class of vaccinia virus genes.

Cytoplasmic extracts made from HeLa cells that have been harvested late after infection with vaccinia virus are capable of specifically transcribing templates containing vaccinia virus late-gene promoters. We applied such an extract to a phosphocellulose column and eluted the proteins with a series of buffers containing successively higher concentrations of NaCl. None of three column fractions alone was capable of specific transcription of a late-gene template. However, specific transcriptase activity could be reconstituted by mixing column fractions, with maximal activity seen when all three fractions were present. The activities present in all fractions were heat labile, resistant to micrococcal nuclease, and present only in extracts from vaccinia virus-infected cells. A quantitative complementation assay was used to further purify one factor, named VLTF-1, over subsequent columns of DEAE-cellulose and hydroxylapatite. VLTF-1 was separated from endogenous RNA polymerase, was a late-promoter-specific transcription factor, and had a sedimentation rate consistent with an apparent Mr of 45,000. The RNA polymerase-containing fraction was not only necessary for transcription with a late-promoter template but alone was capable of specifically transcribing a vaccinia virus early-gene promoter. A further difference between early and late gene transcription in this system was in the ability of the ATP analog beta-8-imidoadenosine-5'-triphosphate (AMP-PNP) to substitute for ATP in supporting specific transcription of only the late-promoter template. The system reconstituted from the various fractions retained the ability to produce the novel poly(A) sequence found on the 5' end of vaccinia virus late messages.