Bovine papillomavirus E2 trans-activating gene product binds to specific sites in papillomavirus DNA

Enhancers are cis-acting elements that activate transcription in higher eukaryotes independently of their position or orientation relative to the promoter that they activate. The mechanisms by which enhancers activate transcription are poorly understood, in part because, with the exception of the glucocorticoid receptor, the proteins that directly interact with enhancers have not been purified, nor have the genes encoding them been cloned. The upstream regulatory region (URR) that immediately precedes the early genes of the bovine papillomavirus type 1 genome (BPV) has enhancer activity when it is activated by a trans-acting gene product of the BPV E2 open reading frame (ORF) (Fig. 1). It is not known whether this enhancement represents a direct or indirect effect of E2 on the URR. We have used an E2 peptide expressed in bacteria and a DNA-protein complex immunoprecipitation assay to study E2-mediated enhancement of transcription by the URR. We show here that this peptide directly binds to four specific sites in the BPV URR, and to one site in the human papillomavirus (H)PV16 URR. All the binding sites contain a related sequence of nucleotides; a 23 base pair (bp) fragment containing this sequence can specifically prevent binding of the E2 protein to the BPV URR. The BPV E2-URR enhancer interaction may therefore represent a useful model system for studying the mechanism of transcriptional enhancement, as both an effector protein and its target enhancer can be purified and genetically manipulated.