Eukaryotic replication [1, 2] begins at origins and on the lagging strand with RNA-primed DNA synthesis of a few nucleotides by polymerase α, which lacks proofreading activity. A polymerase switch then allows chain elongation by proofreading-proficient pol δ and pol ε. Pol δ and pol ε are essential, but their roles in replication are not yet completely defined . Here, we investigate their roles by using yeast pol α with a Leu868Met substitution . L868M pol α copies DNA in vitro with normal activity and processivity but with reduced fidelity. In vivo, the pol1-L868M allele confers a mutator phenotype. This mutator phenotype is strongly increased upon inactivation of the 3′ exonuclease of pol δ but not that of pol ε. Several nonexclusive explanations are considered, including the hypothesis that the 3′ exonuclease of pol δ proofreads errors generated by pol α during initiation of Okazaki fragments. Given that eukaryotes encode specialized, proofreading-deficient polymerases with even lower fidelity than pol α , such intermolecular proofreading could be relevant to several DNA transactions that control genome stability.
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