Characteristics of DNA synthesis following ultraviolet light irradiation in mouse L cells: Postreplication repair

Abstract

Mouse L cells partially synchronized as to DNA synthesis were tested for ability to bypass UV-induced DNA photoproducts during postirradiation DNA synthesis. The amount of 3H-thymidine incorporated into the DNA of irradiated cells indicates that dimers remaining unexcised in the DNA, block DNA synthesis, though not permanently. Alkaline sucrose sedimentation has revealed that daughter DNA newly synthesized for 2 h in irradiated cells is discontinuous due to a gap presumably opposite each dimer in parental DNA. Such smaller daughter strands exhibit a dose-dependent reduction in the rate of sedimentation and differ from normally replicating short segments. Taking into account the replication rate reduced by UV irradiation, these gaps in daughter DNA seem to be almost closed by the time the synthesis of the involved DNA units is completed, as the sedimentation rate of daughter DNA approaches that of parental DNA during the subsequent chase incubation of irradiated cells. These results provide evidence for the bypass of photoproducts remaining unexcised in DNA, by which mouse L cells may recover.