Separable recruitment of DNA Polymerase α for leading- and lagging-strand replication initiation

Abstract

During eukaryotic DNA replication, DNA polymerase alpha/primase (Pol ) initiates synthesis on both the leading and lagging strands. It is unknown whether leading- and lagging-strand priming are mechanistically identical, and whether Pol associates processively or distributively with the replisome. Here, we titrate cellular levels of Pol in S. cerevisiae and analyze Okazaki fragments to study both replication initiation and ongoing lagging-strand synthesis in vivo. We observe that both Okazaki fragment initiation and the productive firing of replication origins are sensitive to Pol abundance, and that both processes are disrupted at similar Pol concentrations. When the replisome adaptor protein Ctf4 is absent or cannot interact with Pol , lagging-strand initiation is impaired at Pol concentrations that still support normal origin firing. Additionally, we observe that activation of the checkpoint becomes essential for viability upon severe depletion of Pol . Using strains in which the Pol -Ctf4 interaction is disrupted, we demonstrate that this checkpoint requirement is not solely caused by reduced lagging-strand priming. Our results suggest that Pol recruitment for replication initiation and ongoing lagging-strand priming are distinctly sensitive to the presence of Ctf4. We propose that the global changes we observe in Okazaki fragment length and origin firing efficiency are consistent with distributive association of Pol at the replication fork, at least when Pol is limiting. Author summaryHalf of each eukaryotic genome is replicated continuously as the leading strand, while the other half is synthesized discontinuously as Okazaki fragments on the lagging strand. The bulk of DNA replication is completed by DNA polymerases {varepsilon} and {delta} on the leading and lagging strand respectively, while synthesis on each strand is initiated by DNA polymerase -primase (Pol ). Using the model eukaryote S. cerevisiae, we modulate cellular levels of Pol and interrogate the impact of this perturbation on both replication initiation on DNA synthesis and cellular viability. We observe that Pol can associate dynamically at the replication fork for initiation on both strands. Although the initiation of both strands is widely thought to be mechanistically similar, we determine that Ctf4, a hub that connects proteins to the replication fork, stimulates lagging-strand priming to a greater extent than leading-strand initiation. We also find that decreased leading-strand initiation results in a checkpoint response that is necessary for viability when Pol is limiting. Because the DNA replication machinery is highly conserved from budding yeast to humans, this research provides insights into how DNA replication is accomplished throughout eukaryotes.