Both Viral E2 Protein and the Cellular Factor PEBP2 Regulate Transcription Via E2 Consensus Sites Within the Bovine Papillomavirus Type 4 Long Control Region

The bovine papillomavirus type 4 (BPV4) long control region (LCR) contains three consensus binding sites, E2(1), E2(2), and E2(3) (ACCN6GGT), for the viral E2 transcription factor and a fourth degenerate site, dE2 (ATCN6GGT), which lies 3 bp upstream of E2(3). The E2(2) site was found to bind the cellular transcription factor PEBP2, and mutations at this site reduced basal promoter activity by as much as 60%, indicating an important role for PEBP2 in LCR function. Mutation of the E2(3) or dE2 site slightly decreased basal promoter activity, but the cellular proteins binding these sites have not yet been characterized. E2 protein was found to have considerable influence upon LCR promoter activity in primary bovine palate keratinocytes. Thus, when high levels of BPV1 E2 were present, almost complete repression of the BPV4 LCR was observed, whereas smaller amounts of BPV1 or BPV4 E2 led to transactivation. Mutational analysis indicated that E2(1) and dE2 mediated transactivation by E2, whereas E2(2) and E2(3) were responsible for repression by E2. In vitro complexes of binding sites E2(1) and E2(2) with E2 protein demonstrated much greater stability than complexes formed by the E2(3) and dE2 sites. These data suggest that the four E2 sites in the BPV4 LCR each perform different functions in the control of transcription and that competition between cellular transcription factors and viral E2 proteins is essential in regulating the level of viral gene expression during papilloma development.