Stability of the human polymerase δ holoenzyme and its implications in lagging strand DNA synthesis

In eukaryotes, DNA polymerase delta (pol delta) is responsible for replicating the lagging strand template and anchors to the proliferating cell nuclear antigen (PCNA) sliding clamp to form a holoenzyme. The stability of this complex is integral to every aspect of lagging strand replication. Most of our understanding comes from Saccharomyces cerevisae where the extreme stability of the pol delta holoenzyme ensures that every nucleobase within an Okazaki fragment is faithfully duplicated before dissociation but also necessitates an active displacement mechanism for polymerase recycling and exchange. However, the stability of the human pol delta holoenzyme is unknown. We designed unique kinetic assays to analyze the processivity and stability of the pol delta holoenzyme. Surprisingly, the results indicate that human pol delta maintains a loose association with PCNA while replicating DNA. Such behavior has profound implications on Okazaki fragment synthesis in humans as it limits the processivity of pol delta on undamaged DNA and promotes the rapid dissociation of pol delta from PCNA on stalling at a DNA lesion.