Mismatch Repair in Xenopus Egg Extracts: DNA Strand Breaks Act as Signals Rather than Excision Points

In Xenopus egg extracts, DNA strand breaks (nicks) located 3$^{\prime}$ or 5$^{\prime}$ to a mismatch cause an overall 3-fold stimulation of the repair of the mismatch in circular hetero-duplex DNA molecules. The increase in mismatch repair is almost entirely due to an increase in repair of the nicked strand, which is stimulated 5-fold. Repair synthesis is centered to the mismatch site, decreases symmetrically on both sides, and its position is not significantly altered by the presence of the nick. Therefore, it appears that in the Xenopus germ cells, the mismatch repair system utilizes nicks as signals for the induction and direction of mismatch repair, but not as the start or end point for excision and resynthesis.