Transformed mouse cell lines that consist predominantly of cells maintaining bovine papilloma virus at high copy number.

Rare cells that contain large amounts of bovine papilloma virus (BPV) DNA have been observed in populations of BPV-transformed mouse ID13 cells. The viral DNA molecules in these "jackpot cells" have been thought to have switched from the controlled replication typical of latent BPV infection to the uncontrolled "runaway" prelytic replication characteristic of terminal stage infection of bovine epidermal cells. By sequential subcloning of high-BPV derivatives of ID13, we isolated stable cell lines enriched more than 1000-fold for cells showing large amounts of BPV by fluorescence in situ hybridization analysis. High-BPV subclones contained a variant plasmid as well as wild-type BPV DNA; analysis of the BPV variants in two independently isolated subclones that showed the high-BPV phenotype in 50 to 80% of cells in the population indicated that both variants had undergone tandem duplication of the BPV long control region, which contains the viral origin of replication and transcription enhancer sequences. Transfer of the high-copy-number phenotype by transfection of DNA from high-BPV cells was accompanied by transfer of the variant plasmid. Density gradient analysis of BPV plasmid replication in high-BPV subclones showed the random-choice mode of replication observed in the parental ID13 population, rather than the rapid BPV replication found in epidermal cells destined for lysis and death. Our results indicate that high-BPV cells in actively dividing ID13 populations are not produced by uncontrolled replication of viral DNA and suggest that they may result instead from abnormal plasmid partitioning.