(A) Domain map (top) and graphical map (bottom) of Exo1 and known interacting partners. Exo1 contains at least two distinct domains: a structured N-terminal nuclease domain (NTD) and an unstructured C-terminal domain (CTD). Previous studies have hypothesized an auto-inhibitory interaction between the NTD and CTD (). (B) DNA Curtains assay for studying Exo1 activity and regulation. Reproduced with permission from (). (C) Kymograph and particle tracking (top) of a single Exo1 molecule resecting from a DNA end. Rate and processivity measurements are indicated. Boxplots (bottom) show the velocity and processivity of Exo1 molecules from the lambda DNA end (magenta), nicked DNA (orange), or with the nuclease-dead Exo1(D78A/D173A) (black). (D) Kymographs of Exo1 lifetime upon injection of RPA (left) or SOSS1 (right). White arrow indicates dissociation of Exo1. Lifetimes (bottom) of Exo1 upon no injection (left), RPA (middle), or SOSS1 (right). (E) Paramagnetic bead assay for monitoring Exo1 resection. Reproduced with permission from (). Briefly, a paramagnetic bead attached to nicked DNA is tethered to a microscope slide surface via a biotin-streptavidin interaction. In the presence of flow, the DNA is extended and Exo1 is added. As Exo1 resects the DNA, it generates single-stranded DNA, which extends differently than dsDNA, resulting in a change in bead location. This change is monitored over time to determine the rate and processivity of Exo1 (right). In the presence of SSB, the bead does not move. However, MSH2-6 is able to overcome SSB inhibition of Exo1 activity. (F) Model for Exo1 resection in the presence of RPA. In the absence of a processivity factor (left), Exo1 is physically removed by RPA, which binds the single-stranded DNA generated by resection. However, processivity factors (green ring) may promote Exo1 degradation in the presence of RPA (right).