Transcriptional termination between bovine papillomavirus type 1 (BPV-1) early and late polyadenylation sites blocks late transcription in BPV-1-transformed cells.

Bovine papillomavirus type 1 (BPV-1) is a small DNA tumor virus which induces fibropapillomas in cattle and transforms rodent cells in culture. Transcripts are derived from a single strand of the circular viral genome, which has multiple promoters and two polyadenylation sites. In the transformed cell, the first (early) polyadenylation site is utilized exclusively and, therefore, only the early region is expressed. Transcription of the late genes, which requires use of the second (late) polyadenylation site, is seen only in the fully differentiated keratinocytes of the fibropapilloma. In this study, nascent RNA chain analysis of BPV-1-transformed C127 cells was used to demonstrate that at least 90% of the RNA polymerases which transcribe past the early polyadenylation site terminate transcription within the late region before reaching the late polyadenylation site. Therefore, transcription termination is at least partially responsible for the absence of late transcription in the BPV-1-transformed cell and is likely to be an important mechanism for regulation of papillomavirus late transcription during keratinocyte differentiation.