I have used a plasmid containing two copies of the Saccharmyces cerevisiae his3 gene to study intramolecular homologous recombination in vaccina virus-infected cells. Recombination of the plasmid was monitored by restriction enzyme digestion and Southern blot hybridization in cells infected with representatives from each of 32 complementation groups of temperature-sensitive mutants ts42 and ts17 did not replicate nor detectably recombine the input plasmid. All except one of the mutants that synthesized normal amounts of viral DNA and protein replicated and recombined the plasmid in a manner indistinguishable from wild-type virus. The remaining mutant, ts13, only poorly replicated and recombined the input plasmid. Thus, the processes of replication and recombination could not be separated by using this battery of mutants. Viral mutants defective in late protein synthesis were unable to resolve the vaccinia virus concatemer junction in plasmids but carried out intramolecular homologous recombination with plasmids as efficiently as did wild-type virus at the conditionally lethal temperature. This result distinguishes homologous recombination, which requires early gene products, from resolution of concatemer junctions, which requires additional late gene products
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