Summary: The sensitivity to γ-radiation of the chromosome of a Hfr strain of was six times greater when assayed by donor ability in conjugation than when assayed by survival in unmated bacteria. With a Hfr strain sensitivities were comparable when assayed by the two methods, and were similar to the sensitivity of donor ability in the strain. We suggest that the greater resistance of survival in the strain may be due to a post-replicational repair process which does not occur in the strain and which is prevented from occurring when a chromosome of either strain is involved in conjugation. When a Hfr and a F- strain were given 8·5 krads γ-radiation, enough to inhibit completely normal DNA synthesis, the DNA synthesis associated with conjugation still occurred. This suggests that the lesions which prevent recombinant formation may act not by inhibiting chromosome transfer, but by affecting a later stage in the process of integration.


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