Transformation by iontophoretic microinjection of DNA: multiple integrations without tandem insertions.

DNA transformations of mouse tissue culture cells and mouse embryos were carried out by iontophoretic microinjection of DNA. Iontophoresis involves the use of an externally applied electric current to expel DNA molecules from the injection micropipette into the impaled cell (microelectrophoresis). Restriction analysis of transformants obtained by using this procedure demonstrated that this method gave strikingly different results from those previously obtained by others with transformants generated by pressure injection. First, tandem insertions were never observed in any of our transformants. Second, despite the lack of tandem insertions, we have obtained transformants which have integrated many copies of the injected DNA sequences, probably via a comparably large number of independent integration events. And third, in one transformant, an injected BamHI restriction fragment was found to have been integrated in its entirety; this included the preservation of the terminal BamHI recognition sequence. Based on these observations, we discuss the potential usefulness of iontophoretic DNA microinjections in DNA transformation studies that are focused either on the analysis of the regulation of gene expression or on the targeting of DNA sequences into the eucaryotic cell genome.