Hfr's can form by homologous recombination between an IS element on the F-plasmid and the same IS element on the host chromosome.
Multiple IS insertions are present in many bacterial chromosomes. For example, wild-type E. coli K-12 has 8 IS1 insertions, 6 IS2 insertions, and 5 IS3 insertions. Because IS elements are transposable, each strain may have different numbers of IS insertions. The locations of some of the IS1, IS2, and IS3 insertions on the E. coli K-12 chromosome and the orientation of the insertions are shown below.
Thus, Hfr's can be isolated at many sites in E. coli and in different orientations relative to the chromosome. A few examples of Hfr insertions that have been isolated in E. coli are shown in the figure below. Note that the E. coli chromosome is shown linearized in the figure. The numbers at the top of the figure represent minutes (or "centisomes") on the E. coli genetic map.
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Last modified October 17, 2000