We studied in vivo recombination between the plasmid pHSl, a temperature-sensitive replication mutant carrying tetracycline resistance, and pSMl, a small plasmid carrying one copy of the insertion element IS1. Recombinant plasmids were found by selection for tetracycline resistance at 420C. Their formation was independent of recA function. Analysis of the physical structures of various recombinant DNA molecules with electron microscopy and restriction endonucleases revealed that Recombination between two different genomes independent of recA function is an event frequently seen in vivo in the Escherichia coli system. The integration of phages such as X and 080 into a host chromosome is a typical example of such recombination, which generally occurs at specific sites on a phage and host chromosome and is catalyzed by an integration enzyme of the phages (1-3). Several lines of evidence indicate that there is another type of recombination, which is mediated by translocatable DNA elements responsible for insertion and transposition of particular DNA segments. The integration of the plasmid F into a host chromosome may be one example. Heteroduplex studies demonstrated that translocatable DNA elements such as insertion sequence 2 (IS2), insertion sequence 3 (IS3), or -y3 sequences appear at the recombination junctions between F and chromosomal DNA sequences (4-7). Gill et al. (8) reported that in a recombination between two plasmids, one of which carries a mutant Tn3, a transposable DNA element responsible for ampicillin resistance, the recombinant had Tn3 sequences at the junctions of the parental plasmids. Genetic analysis of fusion between a bacterial chromosome lysogenized by phage Mu and a defective X-gal demonstrated that Mu, itself a translocatable element, mediated the formation of a cointegrated genome in which Mu genomes sandwiched the X-gal genome (9).In this paper, we report studies on recombination between two different plasmid genomes, one of which carries a copy of an insertion DNA element IS1 (10-12). We developed a system to score cells having recombinant plasmids. Analysis of the recombinant molecules with electron microscopy and restriction enzyme cleavage demonstrated that they were formed by the transposition of the plasmid containing the IS element into the second plasmid such that the IS element appears as a direct repeat at both junctions of the two plasmid sequences. Based on the results obtained by riucleotide sequence analysis, models for the formation of these recombinants will be presented.
MATERIALS AND METHODSBacterial and Plasmid Strains. The bacterial strains used were JE5507 and JE5519, which is a recombination-deficient (recA-) mutant of JE5507; both were provided by K. Nakamura. pHS1 and pSM1, the plasmids (13-15) used in recombination experiments, as well as the recombinants between them, named pMZ, are described in the Results section. Covalently closed circular DNA was isolated-according to the method described by Ohtsubo et al. (16).Bacterial strains carrying both p...
