Laser-UV-microirradiation of Chinese hamster cells: the influence of the distribution of photolesions on unscheduled DNA synthesis.

Fibroblastoid Chinese hamster cells synchronized by mitotic selection were microirradiated in G1, using a low power laser-UV-microbeam (2 = 257 nm). The incident e?ergy was either concentrated on a small part of the nucleus (mode 1) or distributed over the whole nucleus (mode 11). Using the same incident UV energy, the local UV fluences were estimated to differ by two orders sf magnitude. Following microirradiation the cells were incubated with C3H]-thymidine for 2 h and thereafter processed for autoradiography. Silver grains were concentrated over the microirradiated part after mode 1 and distributed over the whole nucleus after mode 11 irradiation. To quantify the amount of unscheduled DNA synthesis, the number of grains per nucleus was determined. It increased with the total incident energy, but was not or only slightly affected by the mode of microirradiation, if appropriate autoradiographic conditions were used. The findings suggest that within the investigated range of energy densities (2.7-1000 J/m2), the total amount of unscheduled DNA synthesis depends on the total number of pyrimidine dimers but not on their distribution in nuclear DNA.