The stability and structure of the products of recombination in a fowlpox virus (FPV) system using the thymidine kinase (TK) gene as the insertion site were examined. A 4.6 kb chimeric DNA fragment from the pUV1 expression vector, containing the bacterial gene and the vaccinia virus P7.5 promoter, was ligated into the I site of the FPV TK gene. The resulting vector, pFTKlacZb, was transfected into chicken embryo fibroblast cultures infected with FPV at an m.o.i. of 0.1. Recombinants were screened for the expression of β-galactosidase. Five recombinants were isolated and plaque-purified to 80 to 90% for expression of β-glucosidase. Serial cell culture passage of the recombinants led to the gradual reappearance of the non-recombinant parental phenotype. Southern hybridization analysis of RI fragments from all five recombinants indicated that a single cross-over homologous recombination had occurred between either the 5′ or the 3′ end fragments of the TK gene, generating unstable intermediate recombinants incorporating the entire pFTKlacZb vector. Secondary intermolecular or intramolecular recombination of intergenic repetitive sequences within the intermediate recombinants appears to have resulted in frequent regeneration of the parental genotype and an infrequent generation of more stable recombinants. A method was developed to select stable recombinants by passage of the intermediate recombinants in chicken embryo fibroblast cultures treated with 5-bromo-2′-deoxyuridine.


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