Processing of a psoralen DNA interstrand cross-link by XPF-ERCC1 complex in vitro

The processing of stalled forks caused by DNA interstrand cross-links (ICLs) has been proposed to be an important step in initiating mammalian ICL repair. To investigate a role of the XPF-ERCC1 complex in this process, we designed a model substrate DNA with a single psoralen ICL at a three-way junction (Y-shaped DNA), which mimics a stalled fork structure. We found that the XPF-ERCC1 complex makes an incision 5′ to a psoralen lesion on Y-shaped DNA in a damage-dependent manner. Furthermore, the XPF-ERCC1 complex generates an ICL-specific incision on the 3′-side of an ICL. The ICL-specific 3′-incision, along with the 5′-incision, on the cross-linked Y-shaped DNA resulted in the separation of the two cross-linked strands (the unhooking of the ICL) and the induction of a double strand break near the cross-linked site. These results implicate the XPF-ERCC1 complex in initiating ICL repair by unhooking the ICL, which simultaneously induces a double strand break at a stalled fork.