Hfr FORMATION DIRECTED BY Tn10

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Abstract
The transposable drug-resistance element, TnlO, can serve as a region of homology to direct the insertion of an F′tsll4 lac plasmid into the chromosome of Salmonella typhimurium. Derivatives of F′tsl14 lac were constructed that carry TnlO insertions; these plasmids were transferred to strains having a TnlO insertion in the chromosome. Under these circumstances, Hfr formation requires homologous recombination between plasmid-borne and Chromosomal TnlO elements. The process is dependent on recA function and on the presence of both TnlO elements. All Hfr's isolated from a given merodiploid show the same direction of transfer. Depending on the orientation of TnlO in the F′ plasmid, Hfr's transferring in either direction can be obtained from any chromosomal TnlO insertion. Since TnlO insertions can be generated in any region of the chromosome, this method permits the isolation of Hfr's with either direction of transfer having their origin at almost any predetermined site. The Hfr's constructed by this method are sufficiently stable for standard genetic mapping crosses, and they have also been used to generate new F′ plasmids. Implicit in the results above is the possibility of determining the orientation of any chromosomal TnlO insertion by constructing an Hfr using a standard F′ TnlO plasmid and determining the direction of chromosome transfer. The general approaches described here are applicable to other transposable elements and other bacterial systems.