Gene replacement with linear DNA in electroporated wild-type Escherichia coli

Gene replacement using linear double-stranded DNA fragments in wild-type Escherichia coli transformation is generally inefficient due to exonucleolytic degradation of incoming DNA. Recombination-proficient strains, in which the exonucleolytic activity of RecBCD is inactivated, have been used as transformation recipients to overcome this difficulty. Here we report that gene replacements using linear double-stranded donor DNA can be achieved in wild-type E.coli if electrocompetent cells are used. Using a plasmid target, we obtained 10(2)-10(3) gene replacement events/microgram linear DNA. Using an independent chromosomal target, approximately 60 gene replacement events/microgram linear DNA were obtained. The presence of Chi sites on the linear DNA, which are known to block DNA degradation and stimulate recombination in E.coli, had no effect on gene replacement efficiency in either case. RecBCD-mediated exonucleolytic activity was found to be diminished in electroporated cells. Electrotransformation thus provides a simple way to perform gene replacements in many E.coli strains.