Repair of single‐strand DNA interruptions by redundant pathways and its implication in cellular sensitivity to DNA‐damaging agents

Single‐strand DNA interruptions (SSIs) are produced during the process of base excision repair (BER). Through biochemical studies, two SSI repair subpathways have been identified: a pathway mediated by DNA polymerase β (Pol β) and DNA ligase III (Lig III), and a pathway mediated by DNA polymerase δ/ϵ (Pol δ/ϵ) and DNA ligase I (Lig I). In addition, the existence of another pathway, mediated by Pol β and DNA Lig I, has been suggested. Although each pathway may play a unique role in cellular DNA damage response, the functional implications of SSI repair by these three pathways are not clearly understood. To obtain a better understanding of the functional relevance of SSI repair by these pathways, we investigated the involvement of each pathway by monitoring the utilization of DNA ligases in cell‐free extracts. Our results suggest that the majority of SSIs produced during the repair of alkylated DNA bases are repaired by the pathway mediated by Pol β and either Lig I or Lig III, although some SSIs are repaired by Pol δ/ϵ and Lig I. At a cellular level, we found that Lig III over‐expression increased the resistance of cells to DNA‐damaging agents, while Lig I over‐expression had little effect. Thus, repair pathways mediated by Lig III may have a role in the regulation of cellular sensitivity to DNA‐damaging agents.