Epithelial specific transcriptional regulation of the bovine papillomavirus 4 promoter by E2

Bovine papillomavirus 4 (BPV-4) is a mucosal epitheliotropic papillomavirus. It encodes a transcriptional regulator, E2, which acts on the BPV-4 transcriptional control region (the long control region or LCR) to regulate transcription. The distribution of E2 binding sites within the LCR Of BPV-4 is identical to that of the human papillomaviruses HPV-16 and HPV-18, indicating that the mechanism of transcriptional control by E2 of mucosal epitheliotropic papillomaviruses is conserved. In this study it has been shown that E2 activates transcription through the BPV-4 LCR promoter in primary bovine palate keratinocytes but not in primary bovine palate fibroblasts. The epithelial specific transcriptional activation of the BPV-4 LCR by E2 is promoter-specific because following binding to the BPV-4 LCR placed in an enhancer mode, E2 can activate transcription from heterologous promoters, such as SV40, in both keratinocytes and fibroblasts. Chimaeric VP16-E2 molecules suggest that the epithelial specific transcriptional activation of the BPV-4 LCR promoter is mediated by the E2 transactivation domain. Although low to intermediate levels of E2 can activate transcription from the BPV-4 LCR promoter, high levels of E2 result in down-regulation of transcription from this promoter in keratinocytes. Mutation of E2 binding site 1 (BS1), which is 3 bp upstream from the TATA box, abrogates down-regulation of transcription by high levels of E2. The results present a model system for studying transcriptional regulation of mucosal epitheiotropic papillomavirus LCRs by E2.