Evidence for Dark-Reactivation of Ultraviolet Light Damage in Mouse L Cells

Abstract

The survival of colony-forming ability of mouse L cells after ultraviolet light (UVL) irradiation can be changed drastically by immediately incubating the cells in growth medium to which caffeine has been added. The nature of this effect is similar to previous results obtained by other workers for UVL-irradiated bacteria containing a dark-reactivation system. The nature of these similarities is as follows The effect of caffeine on L cells is concentration-dependent in the range of 0.3 to 5 m[image], and it is purely a postirradiation phenomenon, being dependent on the time after irradiation that caffeine is added to the cells. Cells incubated for one generation time in the absence of caffeine show no decreased colony-forming ability when incubated in its presence. The caffeine effect is not seen if the cells are exposed to ionizing radiation instead of UVL irradiation. Substitution of some of the thymine moieties of L cell DNA with BUdR [bromo-deoxy-uridine] sensitizes the cell to UVL, but when such cells are plated in the presence of caffeine no further decrease in colony-forming ability is seen. These results can be interpreted in terms of L cells containing a dark-reactivation system capable of acting on UVL damage.