DNA polymerization-independent functions of DNA polymerase epsilon in assembly and progression of the replisome in fission yeast

DNA polymerase epsilon (Pol epsilon) synthesizes the leading strands, following the CMG (Cdc45, Mcm2-7, and GINS [Go-Ichi-Nii-San]) helicase that translocates on the leading-strand template at eukaryotic replication forks. Although Pol epsilon is essential for the viability of fission and budding yeasts, the N-terminal polymerase domain of the catalytic subunit, Cdc20/Pol2, is dispensable for viability, leaving the following question: what is the essential role(s) of Pol epsilon? In this study, we investigated the essential roles of Pol epsilon using a temperature-sensitive mutant and a recently developed protein-depletion (off-aid) system in fission yeast. In cdc20-ct1 cells carrying mutations in the C-terminal domain of Cdc20, the CMG components, RPA, Pol alpha, and Pol delta were loaded onto replication origins, but Cdc45 did not translocate from the origins, suggesting that Pol epsilon is required for CMG helicase progression. In contrast, depletion of Cdc20 abolished the loading of GINS and Cdc45 onto origins, indicating that Pol epsilon is essential for assembly of the CMG complex. These results demonstrate that Pol epsilon plays essential roles in both the assembly and progression of CMG helicase.