Division of labor at the eukaryotic replication fork

DNA polymerase delta (Pol delta) and DNA polymerase epsilon (Pol epsilon) are both required for efficient replication of the nuclear genome, yet the division of labor between these enzymes has remained unclear for many years. Here we investigate the contribution of Pol 8 to replication of the leading and lagging strand templates in Saccharomyces cerevisiae using a mutant Pol 8 allele (pol3-L6121W) whose error rate is higher for one mismatch (e.g., T center dot dGTP) than for its complement (A center dot dCTP). We find that strand-specific mutation rates strongly depend on the orientation of a reporter gene relative to an adjacent replication origin, in a manner implying that > 90% of Pol delta replication is performed using the lagging strand template. When combined with recent evidence implicating Pol E in leading strand replication, these data support a model of the replication fork wherein the leading and lagging strand templates are primarily copied by Pol epsilon an Pol delta, respectively.