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Order from clutter: selective interactions at mammalian replication origins

Key Points

  • Metazoan cells contain cis-acting DNA sequences, called replicators, that do not share a single consensus DNA sequence but are required for the initiation of DNA replication.

  • Instead of a single interaction with a consensus sequence, groups of replication origins exhibit multiple DNA–protein interactions and associate with distinct chromatin modifications that modulate the frequency of replication initiation.

  • Metazoans exhibit a flexible initiation programme, initiating replication intermittently from alternate replication origins.

  • Flexible initiation is essential for genomic stability in the face of perturbation of DNA replication and is facilitated by a surplus of pre-replication complexes.

  • Simulation studies can predict the order of replication initiation — the replication-timing programme — solely on the basis of the locations of active replication origins.

  • Cell-specific chromatin architecture is key for both replication origin activation and replication timing, and DNA synthesis can proceed in a consistent order without requiring specific interactions to dictate replication timing.

Abstract

Mammalian chromosome duplication progresses in a precise order and is subject to constraints that are often relaxed in developmental disorders and malignancies. Molecular information about the regulation of DNA replication at the chromatin level is lacking because protein complexes that initiate replication seem to bind chromatin indiscriminately. High-throughput sequencing and mathematical modelling have yielded detailed genome-wide replication initiation maps. Combining these maps and models with functional genetic analyses suggests that distinct DNA–protein interactions at subgroups of replication initiation sites (replication origins) modulate the ubiquitous replication machinery and supports an emerging model that delineates how indiscriminate DNA-binding patterns translate into a consistent, organized replication programme.

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Figure 1: Organization of replication origins, replicons and replication-timing domains.
Figure 2: Genetic and epigenetic features of replication origins.
Figure 3: Long-range chromatin interactions can regulate origin activation and replication timing.
Figure 4: Replication origin choice and origin dormancy.
Figure 5: Origin inhibition and activation in unperturbed cells and after replication stress.

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Acknowledgements

The authors thank H. Fu, K. Utani, S. Jang, J. Murai, F. E. Indig and A. B. Marks for critical reading of the manuscript, and would like to apologize to colleagues whose primary work could not be cited directly owing to space limitations. Work in the authors' laboratory is funded by the intramural programme of the Centre for Cancer Research, National Cancer Institute, US National Institutes of Health.

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Correspondence to Mirit I. Aladjem.

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PowerPoint slides

Glossary

Replication origins

Locations on chromatin from which replication forks emanate.

Replicons

Units of chromatin that are replicated from a single origin.

Replication-timing domain

A region on chromatin that exhibits a uniform replication time from multiple adjacent replicons.

Topologically associated domains

Discrete units of chromatin that tend to exhibit internal, rather than external, chromatin interactions that probably reflect distinct folding patterns.

Single-fibre analyses

Studies of chromosomal dynamics that are based on visualization of labelled, stretched single DNA molecules (for example, on microscope slides). Single-fibre replication analyses visualize the incorporation of nucleotide analogues into chromatin fibres.

Replicon model

A model proposing that replication initiates through an interaction between cis-acting elements (replicators) and trans-acting factors (initiators).

Replicators

Genetic elements that are required for the initiation of DNA replication from a particular chromosomal site.

Initiators

Proteins, or protein complexes, that interact with a replicator and are required for initiation of DNA replication.

CpG islands

Stretches of nucleotides (longer than 200 bp) that exhibit a high abundance of CpG dinucleotides.

G-quadruplex structures

Stacked, planar structures formed by intramolecular interactions among DNA sequences that contain multiple stretches of four guanines.

DNA triplexes

Non-B helix-type structures that contain a third strand.

Phased genomes

Assembled genome sequences that identify alleles of maternal and paternal chromosomes.

Negative selection

An evolutionary process by which genetic elements with potentially deleterious effects are removed from a population.

Constitutive origins

DNA replication origins that initiate replication in all cells.

Flexible origins

DNA replication origins that initiate replication in some cells but not in all cells.

Dormant origins

DNA replication origins that undergo replication licensing but do not initiate replication during a typical, unperturbed cell cycle.

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Aladjem, M., Redon, C. Order from clutter: selective interactions at mammalian replication origins. Nat Rev Genet 18, 101–116 (2017). https://doi.org/10.1038/nrg.2016.141

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