Intermolecular homologous recombination between transfected sequences in mammalian cells is primarily nonconservative

Intermolecular recombination in mammalian cells was studied by coinfecting African green monkey cells in culture with two shuttle vector plasmids, each carrying an incomplete but overlapping portion of the gene for neomycin resistance. The region of homology between the two plasmids was about 0.6 kilobases. Recombination between the homology regions could reconstruct the neomycin resistance gene, which was monitored by analysis of progeny plasmids in bacteria. The individual plasmids carried additional markers which, in combination with restriction analysis, allowed the determination of the frequency of formation of the heterodimeric plasmid which would be formed in a conservative recombination reaction between the homologous sequences. Reconstruction of the neomycin resistance gene was readily observed, but only 1 to 2% of the neomycin resistance plasmids had the structure of the conservative heterodimer. Treatment of the plasmids which enhanced the frequency of the neomycin resistance gene reconstruction reaction did not significantly increase the relative frequency of conservative product plasmids. The results support nonconservative models for recombination of these sequences.