The functional BPV-1 E2 trans-activating protein can act as a repressor by preventing formation of the initiation complex.
The products encoded by the E2 open reading frame of the papillomaviruses are DNA-binding transcription factors involved in the positive or negative regulation of multiple viral promoters. To further understand the mechanisms by which the same transcription factor may act differentially, the full-length BPV-1 E2 protein was expressed and purified from yeast and assayed in vitro for its capacity to modulate transcription. E2 stimulated transcription of the HSV thymidine kinase (TK) promoter when E2-binding sites were positioned in an enhancer configuration approximately 100 bp upstream of the promoter start site. In contrast, the same full-length E2 protein repressed transcription of the HPV-18 E6/E7 P105 promoter. This repression was mediated through binding to the E2 DNA-binding site immediately upstream of the P105 promoter TATA box and could be abrogated by preincubation of the HPV-18 P105 promoter template with the nuclear extract allowing the formation of the preinitiation complex. In vitro DNA-binding experiments with purified E2 and TFIID showed that binding of E2 to its DNA target placed at different positions with respect to the TATA box differentially affects binding of TFIID to its cognate site. In these respects, E2 is similar to the bacteriophage lambda repressor, which can act either as a repressor or an activator of transcription depending on the position of its binding sites relative to the promoter sequences.