Addition of T4 endonuclease V, which is reported to complement defects of XP cells, stimulated DNA repair synthesis by the 293 cell extract, and also complemented the defects of all XP cell extracts.
When this endonuclease V was microinjected into the cytoplasm of xeroderma pigmentosum (XP) cells of complementation group A, B, C, D, F, G or H, unscheduled DNA synthesis (UDS) above the residual level was detected in all the cells at a dose of about 10(3) molecules following UV irradiation.
The UV-induced unscheduled DNA synthesis (UDS) in cultured human fibroblasts of repair-deficient xeroderma pigmentosum complementation groups A and C was assayed after injection of identical activities of either Uvr excinuclease (UvrA, B, C and D) from Escherichia coli or endonuclease V from phage T4.