Viral Plasmids in Mammalian Cells

Abstract

Like bacteria and yeast, mammalian cells can harbor plasmids. These double-stranded circular DNA plasmids or episomes are the genomes of DNA viruses. The genomes of Epstein-Barr virus, the related Kaposi's sarcoma-associated (virus human herpesvirus 8), and papillomavirus can persist indefinitely in latently infected cells due to their ability to replicate and stably segregate during cell division, and this chapter focuses on these viruses. Efficient replication from the dyad symmetry (DS) element requires all four Epstein-Barr nuclear antigen (EBNA)1-binding sites as well as the nonamer repeats that flank the EBNA1 sites. EBNA1 is the only viral protein required to replicate and maintain oriP plasmids and EBV episomes and does so through interactions with the 18-bp palindromic sequences present in the family of repeats (FR) and DS elements of oriP. Replication from oriP requires EBNA1 binding to the DS element, but this interaction alone does not activate the origin, as EBNA1 is bound to the DS throughout most of the cell cycle. The transient replication of bovine papillomavirus (BPV) genomes is dependent on the viral E1 and E2 proteins, and no other viral proteins are required. On the basis of this finding, a mouse cell line was developed that stably expressed E1 and E2 and was used to map the cis-acting requirements for BPV plasmid replication. The latent genomes of several different DNA viruses are stably maintained in mammalian cells as low-copy-number plasmids.