Sensitivity of excision repair in normal human, xeroderma pigmentosum variant and Cockayne's syndrome fibroblasts to inhibition by cytosine arabinoside

Inhibition of the gap‐filling, polymerizing step of excision repair by 1‐β‐D‐arabinofuranosylcytosine (ara‐C) after irradiation with ultraviolet light in human diploid fibroblasts resulted in the formation of persistent DNA strand breaks in G 1 , G 2 , and plateau phase cells, but not in S phase cells. Addition of hydroxyurea to ara‐C resulted in partial inhibition of repair in S phase cells. These observations can be explained either in terms of changing roles in repair for different DNA polymerases throughout the cell cycle or by the presence of a pool of deoxycytidine nucleotides during S phase equivalent to an external source of deoxycytidine at 50 μM concentration. A similar concentration dependence on ara‐C was observed for inhibition of repair in normal human, xeroderma pigmentosum (XP) variant, and Cockayne's syndrome cells. Ara‐C produced a similar number of breaks in normal and Cockayne's syndrome cells but slightly more in XP variant cells. Exonuclease III and S1 nuclease independently both degraded about 50% of the 3 H‐thymidine incorporated into repaired regions in the presence of ara‐C. Sequential digestion with both enzymes degraded nearly 90% of the repaired regions. These observations can be explained if excision repair proceeds by displacing the damaged strand so that both the 3 H‐labeled patch and the damaged region are still ligated to high molecular weight DNA and compete for the same complementary strand during in vitro incubation with the nucleases. The amount of 3 H‐thymidine incorporated in DNA by repair decreased with increasing concentrations of ara‐C and hydroxyurea, suggesting that the incomplete patches became shorter under these conditions. Extrapolation of the digestion kinetics with exonuclease III permits an estimate of the normal patch size of about 100 nucleotides, consistent with previous estimates.