Structure of the polymerase ε holoenzyme and atomic model of the leading strand replisome

The eukaryotic leading strand DNA polymerase (Pol) epsilon contains 4 subunits, Pol2, Dpb2, Dpb3 and Dpb4. Pol2 is a fusion of two B-family Pols; the N-terminal Pol module is catalytic and the C-terminal Pol module is non-catalytic. Despite extensive efforts, there is no atomic structure for Pol epsilon holoenzyme, critical to understanding how DNA synthesis is coordinated with unwinding and the DNA path through the CMG helicase-Pol epsilon -PCNA clamp. We show here a 3.5-angstrom cryo-EM structure of yeast Pol epsilon revealing that the Dpb3-Dpb4 subunits bridge the two DNA Pol modules of Pol2, holding them rigid. This information enabled an atomic model of the leading strand replisome. Interestingly, the model suggests that an OB fold in Dbp2 directs leading ssDNA from CMG to the Pol epsilon active site. These results complete the DNA path from entry of parental DNA into CMG to exit of daughter DNA from PCNA. DNA polymerase epsilon (Pol epsilon) is responsible for leading strand synthesis during DNA replication. Here the authors use Cryo-EM to describe the architecture of the Pol epsilon holoenzyme and to provide an atomic model for the leading strand replisome.