In multicellular organisms, no consensus sequence for origin activity has been identified and replication initiates from flexible locations. In Saccharomyces cerevisiae, origins are genetically defined by specific DNA sequences ( Marahrens and Stillman, 1992). MCM-DHs that do not initiate replication are dislodged from DNA during replication.

During S phase, CDK2 and CDC7 kinase activities in conjunction with other origin-firing factors convert some MCM-DHs into pairs of active CDC45-MCM-GINS helicases that nucleate bidirectional replisome establishment ( Douglas et al., 2018 Moiseeva and Bakkenist, 2018). However, once MCM-DHs have been assembled, ORC does not maintain contact with the MCM-DH and neither ORC, nor Cdc6, nor Cdt1 are required for origin activation ( Fragkos et al., 2015 Hyrien, 2016 Powell et al., 2015 Remus et al., 2009 Rowles et al., 1999 Sun et al., 2014 Yeeles et al., 2015). A single ORC reiteratively loads multiple MCM-DHs. During this step, the origin recognition complex (ORC) binds DNA and, together with Cdt1 and Cdc6, loads minichromosome maintenance complexes (MCM), the core motor of the replicative helicase, as inactive head-to-head double hexamers (MCM-DHs) around double-stranded DNA ( Bell and Kaguni, 2013 Evrin et al., 2009 Remus and Diffley, 2009). Origin licensing, also called pre-replicative complex (pre-RC) formation, occurs in late mitosis and during the G1 phase of the cell cycle.

In human cells, DNA replication initiates from 20,000 to 50,000 replication origins selected from a five- to tenfold excess of potential or ‘licensed’ origins ( Moiseeva and Bakkenist, 2018 Papior et al., 2012). We discuss potential mechanisms specifying when and where replication initiates in human cells. Interestingly, H4K20me3, a histone modification proposed to facilitate late origin licensing, was enriched in late-replicating initiation zones and gene deserts of stochastic replication fork direction. Therefore, ORC/MCM density correlates with replication timing but does not solely regulate the probability of replication initiation. Surprisingly, after controlling these variables, no difference in ORC/MCM density is detected between initiation zones, termination zones, unidirectionally replicating regions, and randomly replicating regions. Both, the origin recognition complex (ORC) and the minichromosome maintenance complex (MCM) are significantly and homogeneously depleted from transcribed genes, enriched at gene promoters, and more abundant in early- than in late-replicating domains. Here, we compare ChIP-seq profiles of the licensing factors Orc2, Orc3, Mcm3, and Mcm7 with gene expression, replication timing, and fork directionality profiles obtained by RNA-seq, Repli-seq, and OK-seq. Claude Bernard, CNRS, Laboratoire de Physique, France Įukaryotic DNA replication initiates during S phase from origins that have been licensed in the preceding G1 phase.

Rheinische Friedrich-Wilhelms-Universität Bonn, Germany.

German Center for Neurodegenerative Diseases (DZNE e.V.), Germany.

Institute for Molecular Immunology, Monoclonal Antibody Core Facility, Helmholtz Zentrum München, German Research Center for Environmental Health (GmbH), Germany.

Laboratory for Functional Genome Analysis (LAFUGA), Gene Center of the Ludwig-Maximilians Universität (LMU) München, Germany.

Institut de Biologie de l'ENS (IBENS), Département de Biologie, Ecole Normale Supérieure, CNRS, Inserm, PSL Research University, France.

Research Unit Gene Vectors, Helmholtz Zentrum München (GmbH), German Research Center for Environmental Health and German Center for Infection Research (DZIF), Germany.

Research Unit Gene Vectors, Helmholtz Zentrum München (GmbH), German Research Center for Environmental Health, Germany.

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