Super-resolution visualization of distinct stalled and broken replication fork structures

Abstract Endogenous genotoxic stress occurs in healthy cells due to competition between DNA replication machinery, and transcription and topographic relaxation processes. This causes replication fork (RF) stalling and regression, which can further collapse to form single-ended double strand breaks (seDSBs). To avoid mutagenesis, these breaks require repair via Homologous Recombination (HR). Here we apply multicolor single molecule super resolution microscopy to visualize individual RFs under mild stress from the trapping of Topoisomerase I cleavage complexes, a damage induction which closely mimics endogenous replicative stress. We identify RAD51 and RAD52, alongside RECQ1, as the first responder proteins to stalled but unbroken forks, whereas Ku and MRE11 are initially recruited to seDSBs. Ku loads directly onto the DSB end whereas MRE11 associates with nascent DNA away from the break, and both proteins colocalize contemporaneously with a single seDSB. We are thus able to discern closely related RF stress motifs and their repair pathways in vivo, uncovering mechanistic insights into the nature of RF damage and repair.