Coupling of uracil DNA glycosylase activity to cell cycle progression

Uracil DNA glycosylase (UNG), an enzyme that removes uracil from DNA, varies characteristically through the cell cycle. The authors examined this cell cycle variation in V79-AO3 Chinese hamster cells and the non-ionizing/ionizing radiation sensitive subline, V79-UC. For V79-AO3 cells UNG activity is undetectable at mitosis, increases to a peak in G1, and after a brief decline increases to a second peak in mid-S-phase. This pattern is not affected by continuous exposure of G1 cells to either excess thymidine (Tdr, 7.5 mM) or hydroxyurea (HU, 2mM). That is, for both treated and control cells, the pattern of UNG activity was independent of normal DNA synthesis. In addition, when cells were treated with Tdr or HU in mid-S-phase (at the second peak in UNG activity) neither treatment affected the subsequent decrease in UNG activity. In comparison, V79-UC cells, which show increased survival after treatment with BrdUrd/near ultraviolet in mid-S-phase, also show increased UNG activity in mid-S-phase. For these cells, however, no pronounced peak of activity was observed in G1. Further, perturbation of normal DNA synthesis by excess Tdr or HU produces marked alterations in the UNG activity. The authors previously demonstrated that treatment with excess Tdr protects V79-AO3 cells in late-S-phase from BrdUrd/near ultraviolet light-induced killing. The present data indicate that this protection is not a result of the restoration of higher levels of UNG activity. Further the marked differences between the two cell lines show that UNG activity is not stringently coupled to DNA synthesis