RESEARCH ARTICLE Replicative DNA Polymerase δ but Not ε Proofreads Errors in Cis and in Trans

It is now well established that in yeast, and likely most eukaryotic organisms, initial DNA rep-lication of the leading strand is by DNA polymerase ε and of the lagging strand by DNA poly-merase δ. However, the role of Pol δ in replication of the leading strand is uncertain. In this work, we use a reporter system in Saccharomyces cerevisiae to measure mutation rates at specific base pairs in order to determine the effect of heterozygous or homozygous proof-reading-defective mutants of either Pol ε or Pol δ in diploid strains. We find that wild-type Pol εmolecules cannot proofread errors created by proofreading-defective Pol εmolecules, whereas Pol δ can not only proofread errors created by proofreading-defective Pol δmole-cules, but can also proofread errors created by Pol ε-defective molecules. These results suggest that any interruption in DNA synthesis on the leading strand is likely to result in completion by Pol δ and also explain the higher mutation rates observed in Pol δ-proofreading mutants compared to Pol ε-proofreading defective mutants. For strains revert-ing via AT!GC, TA!GC, CG!AT, and GC!AT mutations, we find in addition a strong ef-fect of gene orientation on mutation rate in proofreading-defective strains and demonstrat