Abstract
The reversion of lac::Tn5 insertion mutations was used to examine the control of excision of the kanamycin-resistance transposon Tn5 in Escherichia coli. Earlier work which showed that the fertility factor F enhances Tn5 excision had led another group to suggest that this is due to the product of a putative transposable element-specific “recombination” gene in the F factor which can act on Tn5 located anywhere in the genome. We show, however, that Tn 5 is excised from sites in the lac operon of F' lac plasmids several orders of magnitude more efficiently than from the same sites in the chromosomes of F -t-, F + or homozygous lac::Tn5[F lac::Tn5] strains. Thus F enhances Tn5 excision, but only if F and Tn5 are in cis in the same DNA molecule. Bacterial crosses showed that transfer of F lac::Tn5 plasmids by conjugation stimulates Tn5 excision, and that transfer is frequent even within F' populations. These results suggest that the ability of F to enhance excision is the consequence of DNA transfer in conjugation.
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