Replication of the bovine papillomavirus type 1 genome; antisense transcripts prevent episomal replication

A subgenomic fragment, representing 69% of the bovine papillomavirus type 1 (BPV-1) genome, has the capacity to transform mouse C127 cells in vitro and to replicate episomally in these cells. In the present study we have cloned this BPV-1 fragment between two retrovirus-derived long terminal repeats (LTRs) in the two possible orientations. The constructs were designated pMR and pML. The pMR construct contained the BPV-1 genome in the same transcriptional orientation as that of the LTRs whereas the pML construct contained the BPV-1 fragment in the opposite orientation. Both types of construct were capable of transforming mouse C127 cells with approximately the same efficiency. Analysis of the intracellular DNA from cells transformed with pMR and pML revealed that both cell types contained a large number of viral DNA copies. However, whereas the pMR-transformed cell clones contained episomally replicating BPV-1 DNA, the pML-transformed cell clones all contained integrated viral genomes. Analysis of RNA from the transformed cells revealed that the pML-transformed cells, unlike the pMR-transformed cells, contained large amounts of antisense BPV-1 transcripts which presumably interfere with the mRNAs that are required for episomal BPV-1 replication. The results are of importance for the future design of BPV-1 vector constructs.