NEW PATTERNS OF BULK DNA-REPAIR IN ULTRAVIOLET-IRRADIATED MOUSE EMBRYO CARCINOMA-CELLS FOLLOWING DIFFERENTIATION

Mouse embryocarcinoma stem cells differentiate in culture, given the a ppropriate induction. We examined whether these cells could provide in formation about the regulation of nucleotide excision repair in relati on to differentiation by measuring the rate-limiting incision step, th e removal of cyclobutane dimers and (6-4) photoproducts from the genom e as a whole and the effect of the bacteriophage T4 endonuclease (denV ) gene on repair in differentiated cells. It was found that differenti ation is accompanied by a marked decline in the early incision ability after UV irradiation (sixfold for P19, fourfold for PCC7 and twofold for F9), and we measured, in parallel, the loss of two common UV photo products [cyclobutane dimers and (6-4) photoproducts] from P19 cells. After differentiation, the excellent overall cyclobutane dimer repair capacity of proliferating cells (84% removal in 24 h) is lost (no remo val in 24 h), while removal of (6-4) photoproducts, although normal at 24 h (94%), is m ch slower than in undifferentiated P19 at 3 h (no re moval versus 64%). The presence of the denV gene greatly stimulates th e repair of cyclobutane dimers in undifferentiated P19 cells (94% remo val at 3 h versus 40%) and also in differentiated cells (50% removal a t 24 h versus no removal). The denV gene also stimulates the early rep air of (6-4) photoproducts in both differentiated and undifferentiated cells.