Abstract
Hfr and F − bacteria were labeled before conjugation with radioactive isotopes and density isotopes and were mated in non-isotopic medium. Joint molecules were isolated as DNA that contained segments of both prelabeled (old) parental DNAs. The conditions of prelabeling and mating used here led to the production of larger quantities (~20-fold) of joint molecules as compared with previous experiments (Oppenheim & Riley, 1966). The molecules isolated in these experiments had a molecular weight of about 40 × 10 6. Old Hfr DNA segments in the molecules were heterogeneous in size, ranging from about 3 × 10 6 to 11 × 10 6 with a major fraction distributed around 9 × 10 6. Old F − DNA, old Hfr DNA and new DNA synthesized during mating each comprised from one quarter to one half of the joint molecules. Gaps in joint molecules that were capable of being filled by the incorporation of deoxynucleoside triphosphates by bacteriophage T4 DNA polymerases were, on the average, no greater than 450 nucleotides in length. No single-stranded old Hfr DNA was found in zygotes after 60 minutes mating. The old Hfr DNA that was transferred to F − bacteria during conjugation was replicated during the mating period. The capacity of three mutant recipients to produce joint molecules has been assessed. A RecA − mutant recipient formed joint molecules in amounts equivalent to Rec + recipients. A RecB − RecC − mutant recipient formed joint molecules in reduced amounts. A mutant carrying a deletion for the region homologous to the proximal end of the Hfr genome did not form joint molecules.
Published Version
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