| Abstract: | The isolation of a rec(-) Hfr strain of Escherichia coli K-12 is described. The method used consisted of mating AB2463 F(-) Rec(-) His(-) Lac(-) with P4X6 Hfr Rec(+) His(+) Lac(+), selecting Rec(-) His(-) Lac(+) recombinants, and searching for Hfr strains. One Hfr rec(-) strain, no. 12, was used as donor in crosses with Rec(+) and Rec(-) recipients. Crosses with Rec(+) recipients are fertile, and those with Rec(-) recipients are almost infertile, the frequency of recombinants being 10(-2) to 10(-3) that found with Rec(+) recipients. The Rec(-) mutant marker is transfered to and integrated into Rec(+) recipients. Zygotic induction of prophage lambda is observed in crosses between two Rec(-) strains. In crosses of F(-) Rec(-) with Hfr Rec(-), the gradient of integration frequencies for markers progressively more distant from the origin is steeper than in the Rec(+) x Rec(-) or the Rec(-) x Rec(+) crosses. |
