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The viral DNA sequences in mouse C127 cells transformed by bovine papillomavirus type 1 (BPV-1) virions, by full-length linear BPV-1 DNA, or by a defined transforming subgenomic DNA segment of BPV-1 were examined by reassociation kinetics and blot hybridization. In all cases, the transformed cells contained multiple copies of BPV-1 DNA, present exclusively as supercoiled or nicked circular extrachromosomal molecules or as a slowly migrating complex of circular viral DNA molecules. In the transformed cell lines established from cells transfected with full-length linear BPV-1 DNA, there was recircularization of the input DNA which in some cases resulted in the loss of the restriction site used in the linearization of the DNA. In the transformed cell lines established with the defined subgenomic segment there was circularization of the DNA accompanied by the acquisition of new sequences or duplication and rearrangement of the BPV-1 sequences. In contrast to other well-studied virus transformation systems, no integration of the BPV-1 genome into the host chromosome could be detected under conditions sensitive enough to detect 0.1-0.2 viral genome equivalent. It was concluded that maintenance of transformation may be mediated by nonintegrated viral DNA.

The effect of mouse L-cell interferon on bovine papillomavirus type 1 (BPV-1) transformation of murine cells was examined. Mouse interferon reduced the level of BPV-1-induced transformation of mouse C127 cells by 95%. Long-term treatment of established BPV-1-transformed mouse cell clones with mouse L-cell interferon led to a decrease in the average number of the plasmid viral genomes present in these cells to 1/3 to 1/8. Although revertant lines could not be isolated from these lines in the absence of treatment with interferon, flat revertants were easily selected from two independent clonal transformed lines carried for 60 generations in the continued presence of 200 units of interferon per ml. These flat revertants had the biological characteristics of nontransformed C127 cells and could be retransformed by BPV-1. Southern blot hybridization failed to detect BPV-1 DNA in any of eight independent revertant lines examined under conditions that could detect 0.2 copies per cell. We conclude that interferon treatment has resulted in a selective reduction of the amount of extrachromosomal BPV-1 DNA in transformed cells and has cured some treated cells completely of their viral DNA.