BPV-4 induces amplification and activation of 'silent' BPV-1 in a sub-line of C127 cells.

BPV-4-induced malignant transformation of C127 mouse fibroblasts in vitro is the result of a 'hit and run' mechanism. Viral DNA is lost on continued subculture of transformed cell lines without loss of any malignant characteristics. The DNA from these cells harbours specific amplified sequences. Two such amplified fragments of approximately 10 kb and 12 kb were molecularly cloned and designated HL-10 and HL-12 respectively. HL-10 transformed C127 cells efficiently and therefore encodes a transforming function, whereas the 12 kb clone did not. Heteroduplexes showed that HL-12 was homologous to HL-10 except for two additional tandem copies of an approximately 1.7 kb sequence. Sequence analysis of HL-10 revealed that the clone contained a 5.2 kb region from BPV-1 including the transforming ORFs. Transformation studies have shown differences between HL-10 and BPV-1, indicating that the host flanking sequences may contribute to the transforming potential of the BPV-1 ORFs. The BPV-1 DNA was associated with sequences homologous to murine autonomously replicating sequences (ARS) implicated in the establishment of multiple tandem DNA repeats. As the parental cells contain the set of sequences amplified in transformed cells in single copy and show none of the characteristics of transformed cells, we conclude that BPV-4 has activated these sequences by amplification and rearrangement. These phenomena may be mediated through an interaction between BPV-4 proteins and the BPV-1 origin of DNA replication or via the ARS region.