OVEREXPRESSION, PURIFICATION, AND LATE TRANSCRIPTION FACTOR ACTIVITY OF THE 17-KILODALTON PROTEIN ENCODED BY THE VACCINIA VIRUS A1L-GENE

The A1L, A2L, and G8R open reading frames (ORFs) were previously shown by transfection assays to encode transactivators of late gene express ion. We now present evidence that the 17-kDa protein product of the A1 L gene can function in vitro as a transcription factor. Simultaneous o verexpression of the transactivators was achieved by coinfecting HeLa cells with one recombinant vaccinia virus that encodes the bacteriopha ge T7 RNA polymerase and three recombinant vaccinia viruses that conta in copies of A1L, A2L, and G8R ORFs regulated by T7 promoters. Extract s from the recombinant virus-infected cells exhibited greatly enhanced late in vitro transcription activity and served as a source of factor s. The 17-kDa product of the A1L ORF represented approximately 8% of t he ammonium sulfate-precipitated cell protein and copurified with a la te transcription factor activity. The transcription factor activity co uld be specifically immunodepleted with immobilized antibody to the ba cterially expressed A1L-encoded protein, providing additional evidence for its identity and role. A sequence encoding six consecutive histid ines was added to the AIL ORF, which was then incorporated into the ge nome of a baculovirus expression vector. The 17-kDa protein, synthesiz ed in insect cells and purified by binding to an Ni2+-chelating affini ty column, could replace the vaccinia virus-overexpressed 17-kDa prote in in transcription assays. In addition to the 17-kDa product of the A 1L gene, which was named vaccinia virus late transcription factor 2, t he proteins that stimulate specific transcription of late promoter-reg ulated templates included the viral multisubunit RNA polymerase, vacci nia virus late transcription factor 1 (the product of the G8R ORF), an d at least one other partially purified protein.