Stimulation of recombination between homologous sequences on plasmid DNA and chromosomal DNA in Escherichia coli by N-acetoxy-2-acetylaminofluorene

A plasmid containing a wild-type lac operon and a tetracycline-resistance gene was covalently modified by N-acetoxy-2-acetylaminofluorene and used to transform two series of Lac^- Escherichia coli cell types. Each set contained wild-type and repair-deficient mutants. One set of cells contained a lacY mutation and other a deletion of the entire lac operon. Survival and mutagenesis of the plasmid were measured as a function of the N-acetoxy-2-acetylaminofluorene concentration. The results indicate that when no homologous sequences are present in the chromosomal DNA, mutations occur at a low frequency: at 10% survival the frequency was 1-2 x 10^-4 mutants per transformant. When homologous sequences, the lacY allele, are present in the chromosomal DNA, Lac^- plasmids are found at a high frequency in a recA-dependent, lexA-independent fashion: at 10% survival the frequency was 5-10 x 10^-2 mutants per transformant. Southern blot analysis of the restriction enzyme profiles of the resulting plasmid and the host-cell DNA sequences showed recombinational transfer of host sequences to the N-acetoxy-2-acetylaminofluorene-treated plasmid had occurred. When the host chromosomes contained Lac⁺ homologous sequences no mutants were found, indicating that the results were not caused by error-prone recombination.