REPAIRABLE AND NONREPAIRABLE DNA STRAND BREAKS INDUCED BY X-IRRADIATION IN CHO K1 CELLS AND THE RADIOSENSITIVE MUTANTS XRS1 AND XRS5

Repair kinetics of X-ray-induced DNA lesions was measured for CHO K1, xrs1 and xrs5 cells using the alkaline unwinding technique. Cells were irradiated on ice with X-ray doses of 9, 30 and 90 Gy followed by rep air incubation at 37-degrees-C. Repair was studied for up to 60 h to c over the kinetics of reparable and non-reparable DNA damage. The kinet ics of single-strand break (ssb) repair was found to be the same in CH O K1 and xrs cells. In contrast, the number of double-strand breaks (d sbs) was shown to decline with half-times that were longer for xrs1 an d xrs5 as compared with CHO K1 cells (12.2 and 15.0 h versus 4.9 h for 90 Gy). These differences are primarily due to differences in the fra ction of non-reparable dsbs which was higher by a factor of 3-4 for xr s1 and xrs5 cells as compared with KI cells (3.2 and 3.7% versus 1.0% for 90 Gy). However, the kinetics of those dsbs which were actually re paired did not show any significant differences for the three cell lin es studied and were rejoined at a mean half-time of 3.8 h. It is sugge sted that the higher number of non-reparable breaks in xrs cells may n ot be attributable to a deficient enzymatic repair system. It is sugge sted that alterations in chromatin conformation might prevent rejoinin g of a portion of dsbs.