Mechanistic analysis of Xenopus EXO1's function in 5′-strand resection at DNA double-strand breaks

The processing of DNA double-strand breaks (DSBs) into 3' single-stranded tails is the first step of homology-dependent DSB repair. A key player in this process is the highly conserved eukaryotic exonuclease 1 (EXO1), yet its precise mechanism of action has not been rigorously determined. To address this issue, we reconstituted 5'-strand resection in cytosol derived from unfertilized interphase eggs of the frog Xenopus laevis. Xenopus EXO1 (xEXO1) was found to display strong 5'→3' dsDNA exonuclease activity but no significant ssDNA exonuclease activity. Depletion of xEXO1 caused significant inhibition of 5' strand resection. Co-depletion of xEXO1 and Xenopus DNA2 (xDNA2) showed that these two nucleases act in parallel pathways and by distinct mechanisms. While xDNA2 acts on ssDNA unwound mainly by the Xenopus Werner syndrome protein (xWRN), xEXO1 acts directly on dsDNA. Furthermore, xEXO1 and xWRN are required for both the initiation stage and the extension stage of resection. These results reveal important novel information on the mechanism of 5'-strand resection in eukaryotes.