Resolution of AP sites via the multiple-nucleotide patch replacement pathway

While the single nucleotide replacement pathway appears to facilitate the repair of most damaged bases, an alternative BER pathway is evoked when the structure of the 5'-terminal sugar phosphate is such that it cannot be cleaved through the AP lyase activity of DNA polymerase beta (POLB). Under these circumstances, a short stretch of residues containing the abasic site is excised and replaced (Dianov et al., 1999). Following DNA glycosylase-mediated cleavage of the damaged base, the endonuclease APEX1 is recruited to the site of damage where it cleaves the 5' side of the abasic deoxyribose residue, as in the single nucleotide replacement pathway. However, POLB then synthesizes the first replacement residue without prior cleavage of the 5'-terminal sugar phosphate, hence displacing this entity. Long-patch BER can be completed by continued POLB-mediated DNA strand displacement synthesis in the presence of PARP1 or PARP2, FEN1 and DNA ligase I (LIG1) (Prasad et al. 2001). When the PCNA-containing replication complex is available, as is the case with cells in the S-phase of the cell cycle, DNA strand displacement synthesis is catalyzed by DNA polymerase delta (POLD) or DNA polymerase epsilon (POLE) complexes, in the presence of PCNA, RPA, RFC, APEX1, FEN1 and LIG1 (Klungland and Lindahl 1997, Dianova et al. 2001). In both POLB-dependent and PCNA-dependent DNA displacement synthesis, the displaced DNA strand containing the abasic sugar phosphate creates a flap structure that is recognized and cleaved by the flap endonuclease FEN1. The replacement residues added by POLB or POLD/POLE are then ligated by the DNA ligase I (LIG1) (Klungland and Lindahl, 1997; Matsumoto et al., 1999).<br><br>

尽管单核苷酸替换途径似乎促进了大多数受损碱基的修复，但当5'-端糖磷酸的结构使得其无法通过DNA多聚酶β（POLB）的AP裂解酶活性进行切割时，则会激活一种替代的碱基切除修复（BER）途径。在此情形下，包含无碱基位点的短序列被切除并替换（Dianov等，1999年）。在DNA糖苷酶介导的受损碱基切割之后，内切酶APEX1被招募至损伤位点，在那里它切割无碱基脱氧核糖的5'侧，正如在单核苷酸替换途径中。然而，POLB随后在不先切割5'-端糖磷酸的情况下合成第一个替换残基，从而取代了该实体。长片段BER可通过在PARP1或PARP2、FEN1和DNA连接酶I（LIG1）存在的情况下，由POLB介导的DNA链置换合成来完成（Prasad等，2001年）。当含有PCNA的复制复合体可用时，例如细胞周期S期的细胞，DNA链置换合成由DNA多聚酶δ（POLD）或DNA多聚酶ε（POLE）复合体在PCNA、RPA、RFC、APEX1、FEN1和LIG1的存在下催化（Klungland和Lindahl，1997年，Dianova等，2001年）。在POLB依赖型和PCNA依赖型的DNA置换合成中，含有无碱基糖磷酸的置换DNA链形成了一个由 flap内切酶FEN1识别并切割的错配结构。随后，由POLB或POLD/POLE添加的替换残基由DNA连接酶I（LIG1）连接（Klungland和Lindahl，1997年；Matsumoto等，1999年）。