Impact of DNA ligase inhibition on the nick sealing of polβ nucleotide insertion products at the downstream steps of base excision repair pathway

DNA ligase (LIG) I and III alpha finalize base excision repair (BER) by sealing a nick product after nucleotide insertion by DNA polymerase (pol) beta at the downstream steps. We previously demonstrated that a functional interplay between pol beta and BER ligases is critical for efficient repair, and pol beta mismatch or oxidized nucleotide insertions confound the final ligation step. Yet, how targeting downstream enzymes with small molecule inhibitors could affect this coordination remains unknown. Here, we report that DNA ligase inhibitors, L67 and L82-G17, slightly enhance hypersensitivity to oxidative stress-inducing agent, KBrO3, in pol beta+/+ cells more than pol beta-/- null cells. We showed less efficient ligation after pol beta nucleotide insertions in the presence of the DNA ligase inhibitors. Furthermore, the mutations at the ligase inhibitor binding sites (G448, R451, A455) of LIG1 significantly affect nick DNA binding affinity and nick sealing efficiency. Finally, our results demonstrated that the BER ligases seal a gap repair intermediate by the effect of pol beta inhibitor that diminishes gap filling activity. Overall, our results contribute to understand how the BER inhibitors against downstream enzymes, pol beta, LIG1, and LIGIII alpha, could impact the efficiency of gap filling and subsequent nick sealing at the final steps leading to the formation of deleterious repair intermediates.