Mismatch repair involving localized DNA synthesis inextracts of Xenopus eggs (heteroduplex DNA/DNA-repair intermediates/mutagenesis/recomblnation)

ABSTRACT Repairofheteroduplex DNAcontaining GTor A-Cmismatchesor containing twotandemunpaired basesoccurred in vitro with Xenopusegg extracts as detected by aphysical assay. Therepair wasaccompanied by a mismatch-stimulated and mismatch-localized DNAsynthesis. Repairedmolecules,separatedfromunrepairedmolecules,showeda20- to 100-fold increase in DNAsynthesis in the region of themismatchcomparedtoregionsdistantfromthemismatch.Theremaining unrepaired heteroduplex DNAincluded moleculesthatalsodisplayedmismatch-stimulated DNAsynthesis in themismatch-proximal regions. These mayrepresent intermedi-ates in the repair process. The patterns of DNAsynthesissuggestthatrepairbeginsatsomedistancefromthemismatch andthat asmuch as 1 kilobaseormorecanbeinvolved in themismatch-stimulated synthesis.Mispaired or unpaired basesin DNA canarise in vivo byatleastthreedifferentmechanisms:(i) DNAreplication errors,(ii) deamination of5-methylcytosine to thymine creating aG-T mismatch, and (iii) events that result in the pairing ofhomologous but nonidentical sequences, such as in geneticrecombination or in the formation ofcruciform structureswith imperfect palindromes. Two global mismatch-repairmechanismshavebeenwellcharacterizedinEscherichiacoliand in Streptococcus pneumoniae: very short patch mis-match repair (VSPMR), involved in the repair of 5-methylcytosinedeamination,andlongpatchmismatchrepair(LPMR)(1-4).TheinvolvementofLPMRin the maintenance ofgeneticinformation duringDNAreplication andrecombination andtherole ofVSPMRintheconservationanddiversification ofgenetic information have beendescribed (5, 6). The size ofeukaryotic genomesand the density of5-methylcytosine in