Abstract
Base Excision Repair (BER), a predominant pathway to repair small base damages, includes two sub-pathways, short-patch and long-patch BER. Since it has been suggested that long-patch BER is dependent on PCNA, it is possible that this sub-pathway is more effective in S phase than in other phases. In this study, to compare the mode of BER in G1 and S phase cells, we analyzed the numbers of methyl methansulfonate (MMS)-induced AP sites in G1 and S phase cells. We first synchronized HeLa RC355 cells in G0/G1 phase by serum starvation/mitotic selection method. After releasing the synchronized cells, we examined the numbers of apparent MMS-induced AP sites in G1 and S phase cells using the ARP method. We found no significant difference between the numbers of cellular AP sites in both phases. Next, we have directly analyzed intracellular AP sites using the FARP-1 method and a flow cytometer. After MMS treatment (2.5 and 5 mM), more AP sites were detected in S phase. The result is consistent with that of ARP assay, since more DNA should be included in a single S phase cell than in a G1 cell. Thus, These results suggest that both G1 and S phase cells efficiently repair the methylated bases by BER.
