The effect of deoxynucleosides on repair of DNA breaks in UVC-irradiated human lymphocytes.

UVC irradiation of mammalian cells induces DNA lesions, which can give rise to transient DNA breaks at subsequent incubation of the cells. The yield of these transient DNA breaks depends on the incision rate as well as on the polymerase and ligation rates. It has previously been shown that the yield of transient DNA breaks is drastically lowered in human lymphocytes if the 4 deoxynucleosides are added to the culture medium during the repair period after UVC irradiation. The present results show that addition of the combination dAdo dGuo dThd or addition of the combination dAdo dThd also efficiently reduces the yield of transient DNA breaks during a repair period of 3 h after the UVC irradiation. Other combinations of deoxynucleosides are less efficient or not efficient at all. This indicates that the pool sizes of dATP and dTTP affect the yield of transient DNA breaks in human lymphocytes. However, the present data also indicate that the number of processed repair sites does not increase during the repair period after UVC irradiation, if the combination dAdo dThd is present in the culture medium during the repair period. Therefore, it is proposed that the presence of dAdo dThd affects the rate of insertion of repair patches but not the total amount of synthesized and inserted patches.