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

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 ' fro 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 fro I, fro II, and fro 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 fro I and fro II motifs. However, rad27-p Pol delta fro 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 ' fro 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.