An element binding a C/EBP-related transcription factor contributes to negative regulation of the bovine papillomavirus type 4 long control region

Deletion of the NR2 element of the long control region (LCR) of bovine papillomavirus type 4 (BPV-4) was observed previously to lead to a fivefold increase in enhancer activity of a subfragment of the LCR. Further characterization of this element indicates that mutations in NR2 lead to increased enhancer activity in both mouse CT3 fibroblasts and in a transformed bovine epithelial cell line derived from an alimentary canal papilloma/in situ carcinoma, but not in primary bovine keratinocytes. Since similar oligonucleotide-nuclear factor complexes were obtained in electrophoretic mobility shift assays (EMSA) for all three cell types, the observed difference in negative activity may result from variation in the NR2-binding factor itself between primary and established/transformed cell lines, or from the involvement of other factors that vary between the lines. Characterization of the NR2-binding factor by heat stability and antibody supershifts in EMSA indicate that the factor is related to the CCAAT/enhancer-binding protein (C/EBP) family, and that one component of the complexes may be C/EBP beta.