Transforming activity of plasmid and chromosomal DNA in Escherichia coli.

An auxotrophic strain of Escherichia coli with the recB recC sbcB genotype was transformed by chromosomal DNA of the prototrophic strain and by plasmid DNA carrying genes for antibiotic resistance (R1 drd19). The donor plasmid DNA obtained by cell lysis in the presence of Triton X-100 and subsequent centrifugation in a caesium chloride-ethidium bromide gradient was shown to have a circular molecule and to retain its completeness after penetration into the recipient. Experiments with mixtures or plasmid and chromosomal DNA indicate a competition between these two DNA types during the transformation reaction in the given system.