Sequence-nonspecific replication of transfected plasmid DNA in poxvirus-infected cells

A system in which transfected plasmid DNA replicates in the cytoplasm of poxvirus-infected cells is described. A variety of recombinant plasmids was introduced into poxvirus-infected cells by transfection, and replication of input plasmid DNA was monitored by (i) digestion with restriction enzymes that discriminate between input methylated plasmid DNA and unmethylated DNA produced by replication in mammalian cells; (ii) amplification of intracellular plasmid DNA; and (iii) density shift analysis in the presence of BrdUrd. Replication of plasmid DNA was observed in the cytoplasm of cells infected with the tumorigenic leporipoxviruses Shope fibroma virus (SFV) and myxoma, and less extensively with the orthopoxvirus vaccinia, but not in uninfected cells. Unexpectedly, all input plasmids tested, including pBR322, pUC13, polyoma, PM2 phi X174 replicative form (RF), and M13 RF, replicated with equal efficiency in SFV-infected cells, indicating that no specific replication origin sequence is required. The transfected plasmid DNA was replicated concomitantly with the infecting poxviral DNA and by 24 hr post-transfection, it resided predominantly in high molecular weight Dpn I-resistant head-to-tail tandem repeats. The failure to detect unreplicated Dpn I-sensitive plasmid concatemers early in replication together with the absence of significant levels of integrated plasmid sequences in the poxviral genome suggest that replication of the transfected plasmid DNA is not the consequence of nonhomologous recombination of concatemeric plasmid DNA into the poxvirus genome, but rather of an autonomous process that is dependent on trans-acting replication factors produced during virus infection, and that does not require a specific origin sequence on the substrate plasmid DNA.