Journal Article

Bidirectional eukaryotic DNA replication is established by quasi-symmetrical helicase loading

Diffley, John F. X., Coster, Gideon

2017

Overview

Chromosomal DNA replication initiates bidirectionally by loading two ring-shaped helicases onto DNA in opposite orientations. How this symmetry is achieved has been puzzling because replication initiation sites contain only one essential binding site for the initiator, the origin recognition complex (ORC). Coster and Diffley now show that both helicases are loaded by a similar mechanism. Efficient loading requires binding of two ORC complexes to two ORC binding sites in opposite orientations. Natural origins were found to be partially symmetrical, containing functionally relevant secondary ORC sites. Sites can be flexibly spaced, but introducing an intervening “roadblock” prevented loading, suggesting that individual helicases translocate toward each other on DNA to form a stable double ring. Science , this issue p. 314 The minichromosome maintenance double hexamer, a precursor of the replicative DNA helicase, results from a symmetrical loading reaction. Bidirectional replication from eukaryotic DNA replication origins requires the loading of two ring-shaped minichromosome maintenance (MCM) helicases around DNA in opposite orientations. MCM loading is orchestrated by binding of the origin recognition complex (ORC) to DNA, but how ORC coordinates symmetrical MCM loading is unclear. We used natural budding yeast DNA replication origins and synthetic DNA sequences to show that efficient MCM loading requires binding of two ORC molecules to two ORC binding sites. The relative orientation of these sites, but not the distance between them, was found to be critical for MCM loading in vitro and origin function in vivo. We propose that quasi-symmetrical loading of individual MCM hexamers by ORC and directed MCM translocation into double hexamers acts as a unifying mechanism for the establishment of bidirectional replication in archaea and eukaryotes.

• Publisher: The American Association for the Advancement of Science
• Subject: Archaea / Binding Sites / Deoxyribonucleic acid / DNA / DNA biosynthesis / DNA helicase / DNA Helicases - metabolism / DNA Replication / Eukaryotes / Gene sequencing / Hexamers / Minichromosome Maintenance Proteins - metabolism / Nucleotide sequence / Origin recognition complex / Origin Recognition Complex - metabolism / Origins / Recognition / Replication / Replication initiation / Replication Origin / Replication origins / Saccharomyces cerevisiae - genetics / Saccharomyces cerevisiae Proteins - metabolism / Translocation / Yeast