Evidence that DNA polymerase δ contributes to initiating leading strand DNA replication in Saccharomyces cerevisiae

To investigate nuclear DNA replication enzymology in vivo, we have studied Saccharomyces cerevisiae strains containing a pol2-16 mutation that inactivates the catalytic activities of DNA polymerase epsilon (Pol epsilon). Although pol2-16 mutants survive, they present very tiny spore colonies, increased doubling time, larger than nor mal cells, aberrant nuclei, and rapid acquisition of suppressor mutations. These phenotypes reveal a severe growth defect that is distinct from that of strains that lack only Pol epsilon proofreading (pol2-4), consistent with the idea that Pol epsilon is the major leading-strand polymerase used for unstressed DNA replication. Ribonucleotides are incorporated into the pol2-16 genome in patterns consistent with leading-strand replication by Pol delta when Pol epsilon is absent. More importantly, ribonucleotide distributions at replication origins suggest that in strains encoding all three replicases, Pol delta contributes to initiation of leading-strand replication. We describe two possible models.