Abstract
DNA replication origins strikingly differ between eukaryotic species and cell types. Origins are localized and can be highly efficient in budding yeast, are randomly located in early fly and frog embryos, which do not transcribe their genomes, and are clustered in broad (10-100 kb) non-transcribed zones, frequently abutting transcribed genes, in mammalian cells. Nonetheless, in all cases, origins are established during the G1-phase of the cell cycle by the loading of double hexamers of the Mcm 2-7 proteins (MCM DHs), the core of the replicative helicase. MCM DH activation in S-phase leads to origin unwinding, polymerase recruitment, and initiation of bidirectional DNA synthesis. Although MCM DHs are initially loaded at sites defined by the binding of the origin recognition complex (ORC), they ultimately bind chromatin in much greater numbers than ORC and only a fraction are activated in any one S-phase. Data suggest that the multiplicity and functional redundancy of MCM DHs provide robustness to the replication process and affect replication time and that MCM DHs can slide along the DNA and spread over large distances around the ORC. Recent studies further show that MCM DHs are displaced along the DNA by collision with transcription complexes but remain functional for initiation after displacement. Therefore, eukaryotic DNA replication relies on intrinsically mobile and flexible origins, a strategy fundamentally different from bacteria but conserved from yeast to human. These properties of MCM DHs likely contribute to the establishment of broad, intergenic replication initiation zones in higher eukaryotes.
Highlights
We review here recent progress in understanding how MCM proteins, which form the core of the eukaryotic replicative helicase, are loaded onto chromatin and redistributed along the genome to specify the location and activation time of eukaryotic DNA replication initiation sites
Replication errors due to endogenous or exogenous causes can lead to cancer or genetic diseases[11,12,13], and several DNA replication proteins including MCMs are used as cancer biomarkers[14]
Multiple mechanisms that involve various DNA translocation and nucleosome eviction machineries probably contribute to this spreading
Summary
We review here recent progress in understanding how MCM proteins, which form the core of the eukaryotic replicative helicase, are loaded onto chromatin and redistributed along the genome to specify the location and activation time of eukaryotic DNA replication initiation sites. Studies in budding yeast[79] and metazoans[80,81,82] revealed that MCM proteins are bound to chromatin in G1-phase at levels that far exceed (by a factor of 10 to 50) the number of active replication origins and ORC This raises questions about the loading mechanism and the location and function of these abundant. A single MCM DH per origin is detected in these experiments[105], additional MCM DHs may escape detection if they are not complexed with nucleosomes and translocate off DNA during MNase digestion or if they are too heterogeneously scattered to form ChIP-seq peaks These results may be reconciled with the large body of evidence for an excess of chromatin-bound MCMs to ORCs. Dispersive MCM DH loading and non-canonical budding yeast origins In ChIP-seq experiments, most MCM peaks coincide with ORC peaks and with ARSs35,100,101,107. Given that cohesin appears to have functions beyond sister chromatid cohesion by entrapping DNA segments of the same chromosome[118], it is interesting to consider, in addition to collision and pushing by RNAP, that MCM DHs may be loaded away from ORC via cohesin-mediated chromatin looping
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