High frequency of homologous recombination in mammalian cells between endogenous and introduced SV40 genomes.

We have detected a high frequency of homologous recombination between introduced and chromosomal DNA in mammalian cells. Linear enhancerless SV40 DNA has been transfected into monkey cells that have either one (COS1 cells) or five to seven (COS7 cells) copies of the SV40 early region stably integrated into their genome. Enhancer-containing wild-type SV40 DNA is formed as a result of homologous recombination of the introduced DNA with chromosomal DNA. Up to 25% of the successfully transfected cells produce wild-type virus within 48 hr after transfection. The highest levels of wild-type virus were produced from transfections of molecules that contained a double-strand break at positions of uninterrupted homology with the chromosomal template. This SV40/COS cell system provides a rapid assay for recombination between introduced and genomic DNA.