Unscheduled DNA Synthesis after Ultraviolet Microirradiation of the Cell Nucleus

Ultraviolet (uv) microirradiation of the cell nucleus was used to study the unscheduled DNA synthesis. Xeroderma pigmentosum (XP) fibroblasts and KB cells in tissue culture were microirradiated over one or more nuclear areas of 5 μm in diameter with uv doses of the order of 10, 100, 1000, and $2000\ {\rm ergs}/{\rm mm}^{2}$. After the irradiation, the cells were labeled with ^{3}{\rm H}\text{-thymidine}\ ({}^{3}{\rm HTdR})$ for 3 hr and autoradiographed. In XP cells not in S phase, no ^{3}{\rm HTdR}$ uptake was detected under these conditions, but in KB cells an incorporation localized over the irradiated area(s) of the nucleus was observed. XP and KB cells in S phase during the labeling time, microirradiated with doses of the order of 10 or $100\ {\rm ergs}/{\rm mm}^{2}$, were labeled throughout the nucleus, but cells irradiated with a dose of the order of $1000\ {\rm ergs}/{\rm mm}^{2}$ showed a lower incorporation at the irradiated area than in the nonirradiated part of the nucleus. In addition, when the irradiated area of the KB cell nucleus was increased by irradiating several spots (two and four) with the same uv dose per unit area, the average number of grains per irradiated area decreased. No modification of the unscheduled DNA synthesis was detected after partial irradiation of the cytoplasm.