Figure 4. DNA polymerase errors reveal the division of labor between eukaryotic replicases.

A) Pols δ and ε variants Pol δ-L612M and Pol ε-M644G have wild type replication reduced and biased fidelity. For example, Pol δ-L612M makes T•dG errors more frequently than A•dC mispairs^{59, 85, 86} and Pol ε-M644G makes T•dT more frequently than A•dA⁵⁸. Mutation rates at the URA3 reporter gene, inserted adjacent to the ARS306 replication origin on S. cerevisiae chromosome 3. Example mutation hotspots are shown. In all cases, substitution rates indicated a division of labor between Pols δ and ε^{58, 60}. In a Pol δ-L612M strain, the AT→GC mutation rate was higher with a lagging strand template T and Pol ε-M644G AT→TA rate was higher with a leading strand template T. B) A schematic of converging replication forks. C–D) Thousands of mutations were accumulated in mismatch repair-deficient S. cerevisiae bearing either Pol δ-L612M or Pol ε-M644G⁶³. C) Where these variants possess similar in vitro mutation biases (denoted “≈”; e.g. T•dG>A•dC), opposite bias patterns exist between adjacent origins. D) Where in vitro mutation biases are opposite (denoted “≠”; e.g. T•dT>A•dA versus T•dT<A•dA), similar bias patterns exist. Thus Pols δ and ε must operate on different strands. The logic used at the URA3 locus, applied across the genome, indicates that Pols δ and ε primarily replicate the lagging and leading strands, respectively.