COOPERATIVE ACTIVATION OF TRANSCRIPTION BY BOVINE PAPILLOMAVIRUS TYPE-1 E2 CAN OCCUR OVER A LARGE DISTANCE

The viral transcriptional factors encoded by the E2 open reading frame bind to the specific DNA sequence elements ACCGNNNNCGGT, allowing activation or repression of transcription. We have analyzed bovine papillomavirus type 1 E2 transactivation using recombinant genes containing E2-binding sites inserted at either 3' or 5' positions relative to the heterologous transcriptional initiation site of the herpes simplex virus thymidine kinase gene. In these hybrid plasmids, strong transactivation required the presence of a minimum of two E2-binding sites in close proximity to the promoter or five binding sites at a distance. The presence of a single E2-binding motif 5', close to the initiation site, increased the efficiency of E2 transactivation from a distance in a more-than-additive manner. Since each E2-binding site bound a dimer of the E2 protein, these experiments suggest that transactivation by E2 requires the interaction between several E2 dimers with other essential transcription factors. This interaction may be facilitated by DNA looping, which would bring E2 molecules close to the promoter.