FIDELITY OF HOMOLOGOUS RECOMBINATION IN VACCINIA VIRUS-DNA

The fidelity of homologous recombination in vaccinia virus (VV) DNA wa s examined by constructing a viral recombinant whose genome contained a copy of the Escherichia coli lac z gene in which the central third o f the gene was repeated on either side of the W thymidine kinase (tk) gene. In this virus, homologous DNA recombination and consequent excis ion of the tk gene were necessary to restore the open reading frame fo r beta-galactosidase and thereby to confer a lac z(+) phenotype. Impre cise recombination was predicted to increase the frequency of lac z(-) virus. However, after several passages during which almost every vira l genome underwent homologous recombination, the frequency of lac z(-) plaques was indistinguishable from that of a control virus that could express beta-galactosidase without prior recombination. We conclude t hat homologous recombination in W DNA occurs with perfect fidelity at least 99% of the time. (C) 1995 Academic Press, Inc.