DNA ligase I (LIG1) completes the base excision repair (BER) pathway at the last nick-sealing step after DNA polymerase (pol) β gap-filling DNA synthesis. However, the mechanism by which LIG1 fidelity mediates the faithful substrate–product channeling and ligation of repair intermediates at the final steps of the BER pathway remains unclear. We previously reported that pol β 8-oxo-2'-deoxyribonucleoside 5'-triphosphate insertion confounds LIG1, leading to the formation of ligation failure products with a 5'-adenylate block. Here, using reconstituted BER assays in vitro , we report the mutagenic ligation of pol β 8-oxo-2'-deoxyribonucleoside 5'-triphosphate insertion products and an inefficient ligation of pol β Watson–Crick–like dG:T mismatch insertion by the LIG1 mutant with a perturbed fidelity (E346A/E592A). Moreover, our results reveal that the substrate discrimination of LIG1 for the nicked repair intermediates with preinserted 3'-8-oxodG or mismatches is governed by mutations at both E346 and E592 residues. Finally, we found that aprataxin and flap endonuclease 1, as compensatory DNA-end processing enzymes, can remove the 5'-adenylate block from the abortive ligation products harboring 3'-8-oxodG or the 12 possible noncanonical base pairs. These findings contribute to the understanding of the role of LIG1 as an important determinant in faithful BER and how a multiprotein complex (LIG1, pol β, aprataxin, and flap endonuclease 1) can coordinate to prevent the formation of mutagenic repair intermediates with damaged or mismatched ends at the downstream steps of the BER pathway.

DNA ligase I (LIG1) completes base excision repair (BER) pathway at the last nick sealing step following DNA polymerase (pol) β gap filling DNA synthesis. We previously reported that pol β 8-oxo-2'-deoxyribonucleoside 5'-triphosphate (8-oxodGTP) insertion confounds LIG1 leading to the formation of ligation failure products with 5'-adenylate (AMP) block. Here, we report the mutagenic ligation of pol β 8-oxodGTP insertion products and an inefficient substrate-product channeling from pol β Watson-Crick like dG:T mismatch insertion to DNA ligation by LIG1 mutant with perturbed fidelity (E346A/E592A) in vitro. Moreover, our results revealed that the substrate discrimination of LIG1 for the nicked repair intermediates with preinserted 3'-8-oxodG or mismatches is governed by the mutations at both E346 and E592 residues. Finally, we found that Aprataxin (APTX) and Flap Endonuclease 1 (FEN1), as compensatory DNA-end processing enzymes, can remove 5'-AMP block from the abortive ligation products with 3'-8-oxodG or all possible 12 non-canonical base pairs. These findings contribute to understand the role of LIG1 as an important determinant of faithful BER, and how a multi-protein complex (LIG1, pol β, APTX and FEN1) can coordinate to hinder the formation of mutagenic repair intermediates with damaged or mismatched ends at the downstream steps of the BER pathway.