Extended Data Figure 9. Model of Srs2/Exo1-mediated processing of a DNA nick induced by Top1 at an embedded ribonucleotide.

RNase H2 normally removes misincorporated rNMPs from DNA (not shown). In the absence of RNase H2, topoisomerase 1 can cleave at the embedded ribonucleotide, leaving 3′ cyclic phosphate and 5′ OH ends that cannot be ligated. Repair commences with unwinding of DNA from the 5′ OH side by Srs2, followed by DNA digestion by Exo1 via its exonuclease or 5′ FLAP endonuclease activity to result in a DNA gap, to be filled by a DNA polymerase. Our results show that Srs2 enhances the nuclease activities of Exo1. In the absence of Srs2, religation of ends processed by unknown nuclease activities leads to a deletion mutation.