Cloning the vaccinia virus genome as a bacterial artificial chromosome in Escherichia coli and recovery of infectious virus in mammalian cells

The ability to manipulate the vaccinia virus (VAC) genome, as a plasmid in bacteria, would greatly facilitate genetic studies and provide a powerful alternative method of making recombinant viruses. VAC, like other poxviruses, has a linear, double-stranded DNA genome with covalently closed hairpin ends that are resolved from transient head-to-head and tail-to-tail concatemers during replication in the cytoplasm of infected cells. Our strategy to construct a nearly 200,000-bp VAC-bacterial artificial chromosome (BAC) was based on circularization of head-to-tail concatemers of VAC DNA. Cells were infected with a recombinant VAC containing inserted sequences for plasmid replication and maintenance in Escherichia coli; DNA concatemer resolution was inhibited leading to formation and accumulation of head-to-tail concatemers, in addition to the usual head-to-head and tail-to-tail forms; the concatemers were circularized by homologous or Cre–loxP-mediated recombination; and E. coli were transformed with DNA from the infected cell lysates. Stable plasmids containing the entire VAC genome, with an intact concatemer junction sequence, were identified. Rescue of infectious VAC was consistently achieved by transfecting the VAC–BAC plasmids into mammalian cells that were infected with a helper nonreplicating fowlpox virus.