Electroporation-mediated gene transfer efficiency is reduced by linear plasmid carrier DNAs.

Carrier DNA has generally been found to stimulate DNA-mediated gene transfer of Chinese hamster ovary (CHO) cells by calcium phosphate coprecipitation. In studies employing electroporation, however, we observed that linear plasmid DNA was inhibitory to the transfection of CHO cells. This unexpected result prompted us to explore the effects of various types and forms of plasmid, cosmid, and chromosomal DNAs on transfection efficiencies. Both carrier DNA form and type were found to influence transfection efficiencies. Circular and linear forms of plasmid carrier DNA had opposite effects: circular plasmids increased and linear plasmids decreased transfection efficiencies. These effects were independent of homology with the selected plasmid and are probably independent of homologous recombination mechanisms. Bacterial genomic DNA failed to stimulate transfection, while calf thymus and cosmid DNA consisting primarily of human sequences stimulated transfection significantly. Our results have importance for plasmid-based experiments in mammalian cells such as those involving the induction of interplasmid homologous recombination.