Incorporation of Parental Dna Into Genetic Recombinants of E. Coli

Two kinds of mechanisms have been considered to account for genetic recombination in bacteriophage and bacteria. One involves exchange of pre-existing genetic material between homologous chromosomes (breakage and reunion), and the other implies exchange of genetic information without exchange of pre-existing genetic material (copy choice). Experiments by Meselson and Weigle2 and Kellenberger, Zichichi, and Weigle have shown that when a strain of bacteriophage X containing labeled DNA is crossed with an unlabeled and genetically distinguishable strain, the genetic recombinants arising from such a mating contain discrete amounts of the parental DNA. We present evidence which indicates that recombinants arising from the mating of an Hfr with an F - strain of Escherichia coli inherit labeled DNA from the F- parent. Bacterial conjugation involves a progressive transfer of genetic markers from the Hfr to the recipient F - . There is a concomitant transfer of DNA from the Hfr to the F - . The recipient subsequently gives rise to recombinants. The present procedure for detecting the incorporation of parental DNA into recombinants involves the mating of a T6-resistant Hfr to a T6-sensitive F - carrying specifically labeled DNA. When the mated cells are exposed to a high multiplicity of the phage T6, the sensitive cells are rapidly lysed. The T6-resistant F - recombinants can then be separated by filtration and examined for the presence of the parental label.