The 3'-->5' exonuclease of DNA polymerase delta can substitute for the 5' flap endonuclease Rad27/Fen1 in processing Okazaki fragments and preventing genome instability

Proc Natl Acad Sci U S A. 2001 Apr 24;98(9):5122-7. doi: 10.1073/pnas.091095198. Epub 2001 Apr 17.


Many DNA polymerases (Pol) have an intrinsic 3'-->5' exonuclease (Exo) activity which corrects polymerase errors and prevents mutations. We describe a role of the 3'-->5' Exo of Pol delta as a supplement or backup for the Rad27/Fen1 5' flap endonuclease. A yeast rad27 null allele was lethal in combination with Pol delta mutations in Exo I, Exo II, and Exo III motifs that inactivate its exonuclease, but it was viable with mutations in other parts of Pol delta. The rad27-p allele, which has little phenotypic effect by itself, was also lethal in combination with mutations in the Pol delta Exo I and Exo II motifs. However, rad27-p Pol delta Exo III double mutants were viable. They exhibited strong synergistic increases in CAN1 duplication mutations, intrachromosomal and interchromosomal recombination, and required the wild-type double-strand break repair genes RAD50, RAD51, and RAD52 for viability. Observed effects were similar to those of the rad27-null mutant deficient in the removal of 5' flaps in the lagging strand. These results suggest that the 3'-->5' Exo activity of Pol delta is redundant with Rad27/Fen1 for creating ligatable nicks between adjacent Okazaki fragments, possibly by reducing the amount of strand-displacement in the lagging strand.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Alleles
  • Amino Acid Transport Systems*
  • Chromosomes, Fungal / genetics
  • DNA Polymerase III / genetics
  • DNA Polymerase III / metabolism*
  • DNA Repair / genetics
  • DNA Replication / genetics*
  • Endodeoxyribonucleases / deficiency
  • Endodeoxyribonucleases / genetics
  • Endodeoxyribonucleases / metabolism*
  • Exodeoxyribonucleases / deficiency
  • Exodeoxyribonucleases / genetics
  • Exodeoxyribonucleases / metabolism*
  • Flap Endonucleases
  • Fungal Proteins*
  • Gene Deletion
  • Gene Duplication
  • Genes, Lethal / genetics
  • Genetic Complementation Test
  • Genome, Fungal
  • Kinetics
  • Membrane Transport Proteins / genetics
  • Multienzyme Complexes / deficiency
  • Multienzyme Complexes / genetics
  • Multienzyme Complexes / metabolism
  • Mutagenesis / genetics*
  • Recombination, Genetic / genetics
  • Saccharomyces cerevisiae / enzymology*
  • Saccharomyces cerevisiae / genetics
  • Saccharomyces cerevisiae / growth & development
  • Saccharomyces cerevisiae Proteins*


  • Amino Acid Transport Systems
  • CAN1 protein, Candida albicans
  • Fungal Proteins
  • Membrane Transport Proteins
  • Multienzyme Complexes
  • Saccharomyces cerevisiae Proteins
  • DNA Polymerase III
  • Endodeoxyribonucleases
  • Exodeoxyribonucleases
  • Flap Endonucleases
  • exodeoxyribonuclease III
  • RAD27 protein, S cerevisiae