Appearance of Low Molecular Weight DNA in a Rec^- Mutant of Escherichia coli K12 Irradiated with X-rays

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A Rec^- mutant of Escherichia coli K 12 (JC 1569 b) was irradiated with X-rays, lysed gently with lysozyme and centrifuged in alkaline sucrose gradients. The sedimentation profile for DNA demonstrated that X-rays produces low moleuclar weight DNA and that the sedimentation profile altered by irradiation with X-rays cannot be restored after incubation. In contrast, the altered profile for the original Rec⁺ bacteria was restored almost completely under the same conditions. These results suggested that X-rays yields single-strand breaks in the DNA of Rec^- and of Rec⁺ bacteria and that the break in the DNA of Rec⁺ bacteria can be repaired but not the break in Rec^- bacteria.