Chinese hamster V79 cells (subline M12G) were exposed repeatedly to fractionated doses of germicidal 254 nm light (far-uv) at <tex-math>$6\ {\rm J}\cdot {\rm m}^{-2}/\text{fraction}/\text{day}$</tex-math> or sunlight-simulating 290-330 nm (mid-uv) at <tex-math>$150\ {\rm J}\cdot {\rm m}^{-2}/\text{fraction}/\text{day}$</tex-math> and sensitivities to cell killing action and mutation of far-uv and mid-uv were examined. As the number of exposure fractions increased, the cell cultures became resistant to cell killing induced by both far-uv and mid-uv. Increases in both D0 and <tex-math>$D_{{\rm q}}$</tex-math> were observed. Treatment with exposures of <tex-math>$6\ {\rm J}\cdot {\rm m}^{-2}$</tex-math> far-uv is more efficient in yielding cell cultures that are resistant than exposures of <tex-math>$150\ {\rm J}\cdot {\rm m}^{-2}$</tex-math> mid-uv. In contrast to the cells exposed to repeated far-uv, the cells exposed to repeated mid-uv were relatively more resistant to cell killing effects of mid-uv than far-uv, suggesting a possible role of photolesions other than pyrimidine dimers. When mutants resistant to 6-thioguanine were assayed during repeated exposure to far- or mid-uv light, the yield was initially linear with accumulating dose. At high total accumulated doses, the frequency decreased gradually (<tex-math>$6\ {\rm J}\cdot {\rm m}^{-2}$</tex-math> mid-uv) or reached a plateau (<tex-math>$150\ {\rm J}\cdot {\rm m}^{-2}$</tex-math> mid-uv). The sensitivity of N80 cells (exposed to 80 fractions of mid-uv) to mutation induction by uv light is higher than that of the original M12G cells, whereas U81 cells (exposed to 81 fractions of far-uv) have a sensitivity similar to that of the original cells. Although an initial decrease in resistance to cell killing was observed, resistant cells retained their characteristics after 100 days in culture without further exposure. Cross-resistance to X rays was not shown. The data in this paper suggest that the capacity for repair of photolesions in DNA by repair processes was enhanced in cell cultures by repeated exposure to far-uv or mid-uv and that this altered the cells' ability to cope with lethal and mutagenic lesions. It remains to be seen if these changes in cell sensitivity were brought about by selective or inductive processes or a combination of both.

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