Excision of F plasmid sequences by recombination at directly repeated insertion sequence 2 elements: involvement of recA.

The DNA of the F plasmid is joined to bacterial DNA sequences in the F' ORF203 by directly repeated insertion sequence 2 (IS2) elements. The rate of excision of the F plasmid form this F' (presumably by recombination at the directly repeated IS2s) has been estimated in both recA+ and recA- strains. Normal F is produced in the recA+ strain, but is not detected in recA-. The autonomous plasmids produced in the recA- background were F's having deletions. F excision in this particular recA+ case is specific in the sense that the directly repeated IS2s appear to be more active in recombination than similarly disposed IS3 direct repetitions in this F'.