FIG. 2.

Schema for excision of the BAC vector from the viral genome in eukaryotic cells. (A) The MCMV BAC genome has an overlength due to the BAC vector insertion. Results obtained with other DNA viruses (2, 3) suggest that there is a relatively tight constraint on the length of DNA that can be encapsidated into virions. The BAC-containing genomes probably exceed this limit and are poorly packaged. (B) Insertion of the duplicate sequence d* should lead to excision of the BAC vector since the duplicated sequences (short identical viral sequences) (hatched boxes) serve as a target site for homologous recombination. The resulting unit length genomes will be preferentially packaged. This will lead to accumulation of a virus progeny with the wt genome.