Rescue of polyomavirus DNA after co-transfection of recombinant plasmids with viral DNA fragments.

Plasmid DNA bearing a single copy of the mouse polyomavirus (Py) genome (template A) was transfected into murine cells together with another DNA (template B) carrying intact the viral sequence interrupted in template A. Rescue of unit-length Py DNA including markers from both templates was observed as long as the viral DNA in B overlapped that split in A by one kbp or more. Such rescue was not detectably enhanced by linearizing either or both template(s), and occurred in the absence of template replication. These findings are suggestive of an intermolecular recombination process taking place soon after transfection and starting with homologous pairing between A and B. Such pairing would facilitate removal of vector DNA from one template (A), followed by closure of the resulting break or gap through recombination with the other template (B). Since B may consist of a PCR-synthesized DNA fragment, these observations could conceivably serve as the basis for a method of generating mutant viral genomes.