The transposable drug resistance element Tn10 was employed as a region of homology to direct the insertion of Tn10-containing derivatives of F'ts114 lac into the chromosome of a Salmonella typhimurium strain that carries a Tn10 insertion in the histidine transport operon. Based on the direction of transfer of the resulting Hfr strains, the chromosomal Tn10 insertion was determined to be in orientation "A." New F' plasmids were selectively generated from one of the Hfr strains. The F' factors carry an intact dhuA hisJ portion of the histidine transport operon. A Southern hybridization revealed that one of the F' plasmids was formed by a type II excision event
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