A novel silencer element in the bovine papillomavirus type 4 promoter represses the transcriptional response to papillomavirus E2 protein.

The long control regions (LCRs) of mucosal epitheliotropic papillomaviruses have similar organizations: a promoter region, an enhancer region, and a highly conserved distribution of E2 DNA binding sites (C. Desaintes and C. Demeret, Semin. Cancer Biol. 7:339--347, 1996). The enhancer of these viruses is epithelial cell specific, as it fails to activate transcription from heterologous promoters in nonepithelial cell types (B. Gloss, H. U. Bernard, K. Seedorf, and G. Klock, EMBO J. 6:3735--3743, 1987). Using the bovine papillomavirus type 4 (BPV-4) LCR and a bovine primary cell system, we have shown previously that a level of epithelial specificity resides in a papillomavirus promoter region. The BPV-4 promoter shows an enhanced response to transcriptional activators in epithelial cells compared with that of fibroblasts (K. W. Vance, M. S. Campo, and I. M. Morgan, J. Biol. Chem. 274:27839--27844, 1999). A chimeric lcr/tk promoter suggests that the upstream BPV-4 promoter region determines the cell-type-selective response of this promoter in fibroblasts and keratinocytes. Promoter deletion analysis identified two novel repressor elements that are, at least in part, responsible for mediating the differential response of this promoter to upstream activators in fibroblasts and keratinocytes. One of these elements, promoter repressor element 2 (PRE-2), is conserved in position and sequence in the related mucosal epitheliotropic papillomaviruses, BPV-3 and BPV-6. PRE-2 functions in cis to repress the basal activity of the simian virus 40 promoter and binds a specific protein complex. We identify the exact nucleotides necessary for binding and correlate loss of binding with loss of transcriptional repression. We also incorporate these mutations into the BPV-4 promoter and demonstrate an enhanced response of the mutated promoter to E2 in fibroblasts. The DNA binding protein in the detected complex is shown to have a molecular mass of approximately 50 kDa. The PRE-2 binding protein represents a novel transcriptional repressor and regulator of papillomavirus transcription.