Bovine papillomavirus type 1 genome in hamster sarcoma cells in vivo and in vitro: variation in the level of transcription.

The physical state and expression of the bovine papillomavirus type 1 (BPV-1) genome were analysed in two transplantable hamster sarcomas (HT1 and HT2) after a low number of transplantations, in two tumourigenic cell lines derived from the first transplant of HT2 sarcoma and another transplantable sarcoma (HT3) and in tumours obtained by grafting these cells. Blot hybridization experiments indicated the presence of multiple free copies of the whole viral genome in HT1 and HT2 tumour transplants (100 and 25 copies/cell respectively), irrespective of the number of in vivo or in vitro passages. In contrast, HT3 cells, and tumours induced by these cells, at early and late passages in vitro contained viral sequences probably integrated in the cell genome, in addition to the free viral DNA sequences. Polyadenylated viral transcripts were easily detected in HT1, HT2 and HT3 tumours obtained at early passages in vivo and in vitro, with electrophoretic mobilities corresponding to 1200 to 1400 bases (HT1, HT2 and HT3), 1750 bases (HT1) and 2500 bases (HT2). Homologous sequences of the transcripts were localized in the transforming BamHI-HindIII fragment of BPV-1 DNA, mainly in the BamHI-EcoRI fragment (0.31 to 0.602 map units). In contrast, almost no viral transcription was detected in HT2 and HT3 cells after 50 subcultures and in the tumours induced by these cells. This suggests that the tumourigenicity of the HT2 and HT3 cells is compatible with a very low level of expression of the transforming region of the BPV-1 genome.