Poxvirus DNA Replication

Poxviruses, of which vaccinia is the prototype, are complex DNA viruses that display an unusual degree of physical and genetic autonomy from the host cell. The vaccinia genome encodes most, if not all, of the functions required for three temporally regulated phases of transcription and for DNA replication (Moss 1990a,b, 1994; Traktman 1990a,b, 1991). In addition, the 192-kb genome encodes numerous proteins that interact with and modulate the inflammatory and immune responses of the host (Smith 1993). The virus also encodes two protein kinases and a protein phosphatase (Traktman et al. 1989a; Guan et al. 1991; Banham and Smith 1992; Lin et al. 1992; Rempel and Traktman 1992; Lin and Broyles 1994), suggesting that its life cycle, like that of eukaryotic cells, is regulated by dynamic networks of protein phosphorylation. Infection is initiated by the attachment of virions to an unknown receptor on the cell surface. After direct fusion of cellular and viral membranes, the inner viral core is delivered into the cytoplasmic compartment, where it remains intact for several hours. Early gene expression initiates immediately and is directed by the encapsidated transcriptional machinery. This virally encoded apparatus includes a heterodimeric transcription factor (vETF), a seven-subunit RNA polymerase, the RAP94 polymerase accessory protein, the heterodimeric capping enzyme/ termination protein, and the heterodimeric poly(A) polymerase, one of whose subunits also functions as a methyltransferase during mRNA cap maturation (Moss 1990a). An encapsidated RNA helicase is also essential for transcription (Shuman 1992), and two other encapsidated NTPases are reported to affect transcription in vivo and/or in vitro (Kunzi and Traktman 1989; Kahn and Esteban 1990; Diaz-Guerra and Esteban 1993; Simpson and Condit 1994; Bayliss and Condit 1995). Early mRNAs are extruded from the core and translated on host polysomes. A secondary uncoating event then ensues, which releases the viral genome into the cytoplasm where it is accessible to the newly synthe-