ROLE OF THE TK+ PHENOTYPE IN THE STABILITY OF PIGEONPOX VIRUS RECOMBINANT

Insertion of foreign DNA containing the E. coli gpt marker by homologo us recombination in the pigeonpox virus (PPV) thymidine kinase (TK) ge ne and selection for the presence of this DNA in the viral genome prod uced unstable recombinants after 3 plaque purifications. We highlight the persistence of duplicated TK DNA sequences arising from single cro ssing over, due to the growth advantage of TK+ virus. Restoration of t he TK function by coinsertion of the vaccinia virus TK gene led to sta ble TK+ recombinants arising from double crossing over.