Segregation properties of bovine papillomaviral plasmid DNA

Essentially complete segregation of replication-competent BPV-1 plasmid DNA species was observed in daughter subclones derived from primary co-transformed C127 cell lines. Thus, whereas primary co-transformants retained both of two distinguishable co-transfected plasmid species, subcloning experiments revealed that morphologically transformed daughter subclones derived from such co-transformed cell lines contained only one species of viral plasmid DNA. Similar results were obtained with each of two conveniently marked replication and transformation-competent mutants: one with a linker-insertion in the viral upstream regulatory region, and one with a 260 base-pair deletion within the L2 (late) gene, which has no recognized role in plasmid replication or stability. Morphological revertant cell clones that contained no detectable viral plasmid DNA genomes were also isolated at a surprisingly high frequency from clonal wild-type BPV-1 transformed cell lines and from cell lines transformed by various BPV-1 mutants. Further co-transfection experiments were done with a combination of transformation-competent and transformation-defective BPV-1 genomes to investigate a possible role for a viral oncogene in plasmid persistence. In this case, elimination of the transformation-defective mutant was observed after the initial establishment of both input genomes as replicating plasmids in cell clones morphologically transformed by the transformation-competent viral mutant with an intact E5 oncogene. No cell subclones were isolated that contained only the transformation-defective mutant, implying that it was defective in long-term plasmid persistence. Our results indicate that there is significant randomization in the processes of replication and/or partitioning of the BPV-1 genome in mouse C127 cells, and, in combination with previous observations, also suggest that BPV plasmid persistence in C127 cell lines may be the result of a selective proliferative advantage conferred on virus-infected cells by viral oncogene-induced cell growth transformation.