Stable replication of papillomavirus genomes in Saccharomyces cerevisiae

Papillomaviruses normally replicate in stratified squamous epithelial tissues of their mammalian hosts, in which the viral genome is found as a nuclear plasmid. Two viral proteins, E1, a helicase, and E2, a transcriptional activator and plasmid maintenance factor, are known to contribute to the episomal replication of the viral genome. Recently, our laboratory discovered that papillomaviruses can also replicate in an E1-independent manner in mammalian cells (K. Kim and P. F. Lambert, Virology, in press; K. Kim and P. F. Lambert, submitted for publication). In this study, we describe experiments investigating the capacity of the human papillomavirus type 16 (HPV16) genome to replicate in yeast (Saccharomyces cerevisiae). The full-length HPV16 genome, when linked in cis to a selectable yeast marker gene, either TRP1 or URA3, could replicate stably as an episome in yeast. The replication of papillomavirus genomes in yeast is not limited to HPV16. Bovine papillomavirus type 1 and HPV6b, -11, -16, -18, and -31 were all capable of replicating in short-term assays over a period of 20 cell doublings. The long-term persistence of viral episomes did not require any one viral gene, as mutant genomes defective in single genes also replicated episomally. These results indicate that the viral episome can replicate in the absence of the E1 DNA helicase. Similarly, E2 was also not required for replication in yeast, and E2 mutant viral genomes were stably maintained in the absence of selection, indicating the existence of an E2-independent mechanism for plasmid maintenance. The episomal replication of papillomavirus genomes in yeast provides a genetically manipulatable system in which to investigate cellular factors required for episomal replication and may provide a novel means for generating infectious papillomavirus.