The A2L intermediate gene product is required for in vitro transcription from a vaccinia virus late promoter.

Previously, the in vitro late transcription system of vaccinia virus was resolved into four components: the 17- and 30-kDa products of the A1L and G8R intermediate genes, respectively, the viral DNA-dependent RNA polymerase, and an unmapped factor sedimenting at 32 to 38 kDa. Another protein, the 26-kDa product of the A2L open reading frame was predicted to be a late transcription factor on the basis of a transient-expression assay but was not recognized as being necessary for transcriptional activity in vitro. We now report that both the unmapped factor and the 26-kDa protein are required for transcription from a vaccinia virus late promoter in vitro. Since the 26-kDa protein has now been shown to be a trans-activator of late transcription and it is the product of a known gene, we suggest that it be designated VLTF-3.