The formation of Hfr strains inEscherichia coliK12

The isolation and characterization of an isogenic series of twelve Hfr strains is described. From their points of origin it was concluded that the sex factor integrates at a limited number of sites on the bacterial chromosome. Although the nutritional requirements of these Hfr strains were similar to those of the parent F + strain, it was observed that one group had a markedly slower growth rate. The relevance of these observations to theories on the nature of F + fertility and on the formation of Hfr strains is discussed.