The Yield and Repair of X-Ray-Induced Single-Strand Breaks in the DNA of Escherichia coli K-12 Cells

The yield of single-strand breaks present in chromosomal DNA of Escherichia coli cells after aerobic x irradiation was determined using alkaline sucrose gradient techniques. Experimental conditions were used that avoided a centrifugation speed dependence effect on DNA sedimentation, and allowed either minimal or complete enzymatic repair to occur. The results confirm the qualitative conclusions made in earlier reports from this laboratory, but indicate that the extent of DNA strand breakage was previously underestimated by factors of 3.4 to 7.6, depending on the strain and repair conditions used. Technical and interpretive difficulties in the measurement of DNA single-strand breakage using alkaline sucrose gradient techniques are discussed briefly. In the present experiments, the initial yield of DNA single-strand breaks was found to be 32.4/genome krad/sup -1/ (9.0 eV/break).