Schematic for ssDNA curtain assay. Briefly, ssDNA curtains were formed by flowing 1 nM RPA-eGFP for 3 minutes. Then 5 nM RPA-mCherry pre-mixed with or without 5 nM Rtt105 wild-type (WT) or E171L172AA (EL) mutant proteins were injected into the flow cell. Data were acquired with a TIRF microscope (Nikon, Inverted Microscope Eclipse Ti-E). Lasers were shuttered at 2-second intervals, and the exposure time was 100 ms. Each kymogram with two channels, in which green and red represent eGFP and mCherry signal, respectively. 