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

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.