ATM promotes reversed fork processing during DNA interstrand cross-link repair

During replication-coupled DNA interstrand cross-link (ICL) repair, fork reversal is thought to enable the Fanconi anemia (FA) pathway to resolve the ICL through nucleolytic incisions. Subsequent fork restoration then allows nascent DNA strand extension past the lesion. Although these fork remodeling events are crucial for ICL repair, their regulation remains poorly understood. Here, we use cell-free Xenopus egg extracts to investigate fork dynamics during ICL repair by the FA pathway. We find that the ataxia telangiectasia-mutated (ATM) kinase is activated concomitantly with fork reversal and promotes resection of the reversed fork intermediate. This resection depends on the coordinated activities of the EXO1 and DNA2 nucleases. Our data indicate that EXO1 initiates 5' to 3' resection of nascent lagging strands in the regressed arm, while DNA2 performs 5' to 3' resection of recessed lagging strands. We further show that the inhibition of protein Phosphatase 2A (PP2A) during ICL repair results in ATM hyperactivation, reversed fork over-resection, and formation of aberrant end-joining products, indicating that PP2A counteracts ATM signaling to constrain reversed fork resection. Taken together, this work implicates reversed forks as substrates for ATM activation and reveals a phospho-regulatory circuit that governs reversed fork processing during ICL repair.