A CELLULAR PROTEIN BINDS VACCINIA VIRUS LATE PROMOTERS AND ACTIVATES TRANSCRIPTION IN-VITRO

Available evidence indicates that the transcription of the late class of vaccinia virus genes requires the participation of several virus en coded proteins in addition to the viral RNA polymerase, In this report we describe the identification of a protein present in extracts of un infected HeLa cells that binds avidly to viral late promoter DNA. The protein bound specifically to several different vaccinia virus late pr omoters but not an early nor an intermediate promoter. DNase I footpri nting localized the protein's binding site to nucleotides surrounding the transcriptional start site of the I1L promoter. Optimal promoter b inding required sequences in the highly conserved TAAAT motif at the t ranscriptional start site as well as sequences immediately upstream; h owever, one variation on the motifs sequence did not affect promoter b inding by the protein. Partially purified late promoter binding protei n (LPBP) was capable of stimulating the transcription activity of extr acts depleted of LPBP on a late promoter-driven template, establishing LPBP as a transcription activator in vitro. These results suggest tha t a cellular protein is responsible for targeting vaccinia virus late promoters for initiation of transcription.