Human DNA ligases I and IIIa as determinants of accuracy and efficiency of base excision DNA repair

Mammalian Base Excision Repair (BER) DNA ligases I and IIIa (LigI, LigIIIa) are major determinants of DNA repair fidelity, alongside with DNA polymerases. Here we compared activities of human LigI and LigIIIa on specific and nonspecific substrates representing intermediates of distinct BER sub-pathways. The enzymes differently discriminate mismatches in the nicked DNA, depending on their identity and position, but are both more selective against the 3'-end non-complementarity. LigIIIa is less active than LigI in premature ligation of one-nucleotide gapped DNA and more efficiently discriminates misinsertion products of DNA polymerase 13-catalyzed gap filling, that reinforces a leading role of LigIIIa in the accuracy of short-patch BER. LigI and LigIIIa reseal the intermediate of long-patch BER containing an incised synthetic AP site (F) with different efficiencies, depending on the DNA sequence context, 3'-end mismatch presence and coupling of the ligation reaction with DNA repair synthesis. Processing of this intermediate in the absence of fiap endonuclease 1 generates non-canonical DNAs with bulged F site, which are very inefficiently repaired by AP endonuclease 1 and represent potential mutagenic repair products. The extent of conversion of the 5'-adenylated intermediates of specific and nonspecific substrates is revealed to depend on the DNA sequence context; a higher sensitivity of LigI to the sequence is in line with the enzyme structural feature of DNA binding. LigIIIa exceeds LigI in generation of potential abortive ligation products, justifying importance of XRCC1-mediated coordination of LigIIIa and aprataxin activities for the efficient DNA repair. (c) 2023 Elsevier B.V. and Societe Francaise de Biochimie et Biologie Moleculaire (SFBBM). All rights reserved.