Abstract
Insertion of foreign DNA containing the E. coli gpt marker by homologous recombination in the pigeonpox virus (PPV) thymidine kinase (TK) gene and selection for the presence of this DNA in the viral genome produced unstable recombinants after 3 plaque purifications. We highlight the persistence of duplicated TK DNA sequences arising from single crossing over, due to the growth advantage of TK+ virus. Restoration of the TK function by coinsertion of the vaccinia virus TK gene led to stable TK+ recombinants arising from double crossing over.
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