Delivering nonidentical twins


1 primed, single-stranded DNA mimicking lagging-strand replication, Pol δ synthesized DNA more quickly and processively than Pol ε (also ref. 4), and when Pol δ and Pol ε were placed in competition for PCNA binding, Pol δ was by far the favored partner. These results strongly imply that CMG selectively recruits Pol ε over Pol δ for leading-strand replication, whereas PCNA selectively recruits Pol δ over Pol ε for lagging-strand replication2 (Fig. 2). These observations provide key insights into how eukaryotic cells deliver two nonidentical polymerase siblings to synthesize the nascent leading and lagging strands. As impressive as it already is in size and complexity, the reconstituted leading-strand apparatus carries out replication about ten-fold other proteins, including the RFC clamp loader, the DNA-encircling PCNA clamp and the single-strand DNA–binding protein RPA. In a tour de force display of protein biochemistry, O’Donnell and colleagues2 purified more than two dozen Saccharomyces cerevisiae replication proteins and reconstituted two distinct replication reactions. In CMG-dependent reactions mimicking leading-strand replication, Pol ε synthesized DNA more processively and ten times faster than Pol δ, the latter of which is distributive with CMG. When the two polymerases were placed in competition, Pol ε was selectively recruited over Pol δ, and when Pol δ was loaded in a complex with CMG, it was displaced by Pol ε. In complementary studies of PCNAand RPA-dependent synthesis of Among the many DNA polymerases used to replicate and maintain eukaryotic genomes, two have the major responsibility for replicating nuclear DNA1. DNA polymerase (Pol) ε synthesizes most of the nascent leading strand in a largely continuous manner, and Pol δ synthesizes most of the nascent lagging strand as a series of ~200-bp Okazaki fragments. Pol ε and Pol δ are members of the same family of B polymerases and catalyze the same 5′-polymerization and 3′-exonucleolyticproofreading reactions, yet they differ in structure, subunit composition, biochemical properties and protein partnerships (Fig. 1). Although these differences undoubtedly reflect their strand-specific roles in replication, uncertainty remains regarding how they are differentially delivered to the two strands. An exciting study by O’Donnell and colleagues2 now offers new insights into the mechanisms responsible for this asymmetric polymerase targeting. Replication requires the CMG complex, which is composed of Cdc45, Mcm2–7 and GINS. Assembly of this 11-protein complex at replication origins activates the helicase activity of Mcm2–7, a six-member ring that encircles leading-strand DNA3. A replisome then forms, which contains Pols α, δ and ε as well as many Delivering nonidentical twins

DOI: 10.1038/nsmb.2852

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Cite this paper

@article{Kunkel2014DeliveringNT, title={Delivering nonidentical twins}, author={Thomas Kunkel and P. M. J. Burgers}, journal={Nature Structural &Molecular Biology}, year={2014}, volume={21}, pages={649-651} }