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' or 5' to a mismatch cause an overall 3-fold stimulation of the repair of the m ismatch in circular heteroduplex DNA molecules. The increase in mismat ch repair is almost entirely due to an increase in repair of the nicke d strand, which is stimulated 5-fold. Repair synthesis is centered to the mismatch site, decreases symmetrically on both sides, and its posi tion is not significantly altered by the presence of the nick. Therefo re, it appears that in the Xenopus germ cells, the mismatch repair sys tem utilizes nicks as signals for the induction and direction of misma tch repair, but not as the start or end point for excision and resynth esis.