Localization and analysis of bovine papillomavirus type 1 transforming functions

Bovine papillomavirus type 1 (BPV-1) or cloned BPV-1 DNA can transform susceptible rodent cells, and the viral DNA remains as a stable extrachromosomal plasmid in the transformed cells. The transforming region of the BPV-1 genome has previously been localized to a specific fragment comprising 69% of the genome, which also contains the elements sufficient for extrachromosomal plasmid maintenance. To define more precisely the viral DNA sequences which are involved in cellular transformation, we have tested the ability of defined deletion mutants of BPV-1 DNA to morphologically transform mouse C127 cells. Cells containing the mutated DNAs have been examined for anchorage independence and tumorigenicity in nude mice. Several distinct regions of the BPV-1 genome were found to influence expression of the viral transformation functions. A transcriptional regulatory region located in the noncoding region 5' to the early open reading frames is essential for transcriptional activity and transformation. A transcriptional enhancer element, located 3' to the polyadenylation site for the viral RNAs expressed in transformed cells, has previously been shown to be essential for transformation (Lusky et al., Mol. Cell. Biol., 3:1108-1122, 1983). Deletion mutants affecting the E2 open reading frame, particularly the NH2 half, are significantly impaired in their ability to transform, suggesting that the E2 gene product is an important transforming protein of BPV-1. Mutants lacking the E6 and E7 open reading frames are still able to induce transformation but at a lowered efficiency, and the transformants have altered characteristics. Mutations localized within the E1 open reading frame do not significantly affect the transforming functions but result in the integration of the viral genome in the transformed cells, implicating the E1 gene product in stable plasmid replication and maintenance.