A possible mechanism for exonuclease 1-independent eukaryotic mismatch repair.
Mismatch repair contributes to genetic stability, and inactivation of the mammalian pathway leads to tumor development. Mismatch correction occurs by an excision-repair mechanism and has been shown to depend on the 5' to 3' hydrolytic activity exonuclease 1 (Exo1) in eukaryotic cells. However, genetic and biochemical studies have indicated that one or more Exo1-independent modes of mismatch repair also exist. We have analyzed repair of nicked circular heteroduplex DNA in extracts of Exo1-deficient mouse embryo fibroblast cells. Exo1-independent repair under these conditions is MutL alpha-dependent and requires functional integrity of the MutL alpha endonuclease metal-binding motif. In contrast to the Exo1-dependent reaction, we have been unable to detect a gapped excision intermediate in Exo1-deficient extracts when repair DNA synthesis is blocked. A possible explanation for this finding has been provided by analysis of a purified system comprised of MutS alpha, MutL alpha, replication factor C, proliferating cell nuclear antigen, replication protein A, and DNA polymerase delta that supports Exo1-independent repair in vitro. Repair in this system depends on MutL alpha incision of the nicked heteroduplex strand and dNTP-dependent synthesis-driven displacement of a DNA segment spanning the mismatch. Such a mechanism may account, at least in part, for the Exo1-independent repair that occurs in eukaryotic cells, and hence the modest cancer predisposition of Exo1-deficient mammalian cells.
Department of Biochemistry, Duke University Medical Center, Durham, NC 27710, USA.
Farid A
FA
Kadyrov
Jochen
J
Genschel
Yanan
Y
Fang
Elisabeth
E
Penland
Winfried
W
Edelmann
Paul
P
Modrich
D000818:Animals; D002460:Cell Line; D004247:DNA / Q000235:genetics / Q000378:metabolism; D053843:DNA Mismatch Repair / Q000235:genetics*; D005090:Exodeoxyribonucleases / Q000172:deficiency / Q000235:genetics / Q000378:metabolism; D006801:Humans; D051379:Mice; D018345:Mice, Knockout
D004247:DNA; D005090:Exodeoxyribonucleases; C028450:exodeoxyribonuclease I