Diversity of eukaryotic DNA replication origins revealed by genome-wide analysis of chromatin structure

Integrating high-throughput analysis to create an atlas of replication origins in Trypanosoma cruzi in the context of genome structure and variability

Trypanosoma cruzi is the etiologic agent of the most prevalent human parasitic disease in Latin America, Chagas disease. Its genome is rich in multigenic families that code for virulent antigens and are present in the rapidly evolving genomic compartment named Disruptive. DNA replication is a meticulous biological process in which flaws can generate mutations and changes in chromosomal and gene copy numbers. Here, integrating high-throughput and single-molecule analyses, we were able to identify Predominant, Flexible, and Dormant Orc1Cdc6-dependent origins as well as Orc1Cdc6-independent origins. Orc1Cdc6-dependent origins were found in multigenic family loci, while independent origins were found in the Core compartment that contains conserved and hypothetical protein-coding genes, in addition to multigenic families. In addition, we found that Orc1Cdc6 density is related to the firing of origins and that Orc1Cdc6-binding sites within fired origins are depleted of a specific class of nucleosomes that we previously categorized as dynamic. Together, these data suggest that Orc1Cdc6-dependent origins may contribute to the rapid evolution of the Disruptive compartment and, therefore, to the success of T. cruzi infection and that the local epigenome landscape is also involved in this process.IMPORTANCETrypanosoma cruzi, responsible for Chagas disease, affects millions globally, particularly in Latin America. Lack of vaccine or treatment underscores the need for research. Parasite's genome, with virulent antigen-coding multigenic families, resides in the rapidly evolving Disruptive compartment. Study sheds light on the parasite's dynamic DNA replication, discussing the evolution of the Disruptive compartment. Therefore, the findings represent a significant stride in comprehending T. cruzi's biology and the molecular bases that contribute to the success of infection caused by this parasite.

Chromatin's Influence on Pre-Replication Complex Assembly and Function

In all eukaryotes, the initiation of DNA replication requires a stepwise assembly of factors onto the origins of DNA replication. This is pioneered by the Origin Recognition Complex, which recruits Cdc6. Together, they bring Cdt1, which shepherds MCM2-7 to form the OCCM complex. Sequentially, a second Cdt1-bound hexamer of MCM2-7 is recruited by ORC-Cdc6 to form an MCM double hexamer, which forms a part of the pre-RC. Although the mechanism of ORC binding to DNA varies across eukaryotes, how ORC is recruited to replication origins in human cells remains an area of intense investigation. This review discusses how the chromatin environment influences pre-RC assembly, function, and, eventually, origin activity.

Identification of 1600 replication origins in S. cerevisiae

There are approximately 500 known origins of replication in the yeast genome, and the process by which DNA replication initiates at these locations is well understood. In particular, these sites are made competent to initiate replication by loading of the Mcm replicative helicase prior to the start of S phase; thus, 'a site that binds Mcm in G1' might be considered to provide an operational definition of a replication origin. By fusing a subunit of Mcm to micrococcal nuclease, we previously showed that known origins are typically bound by a single Mcm double hexamer, loaded adjacent to the ARS consensus sequence (ACS). Here, we extend this analysis from known origins to the entire genome, identifying candidate Mcm binding sites whose signal intensity varies over at least three orders of magnitude. Published data quantifying single-stranded DNA (ssDNA) during S phase revealed replication initiation among the most abundant 1600 of these sites, with replication activity decreasing with Mcm abundance and disappearing at the limit of detection of ssDNA. Three other hallmarks of replication origins were apparent among the most abundant 5500 sites. Specifically, these sites: (1) appeared in intergenic nucleosome-free regions flanked on one or both sides by well-positioned nucleosomes; (2) were flanked by ACSs; and (3) exhibited a pattern of GC skew characteristic of replication initiation. We conclude that, if sites at which Mcm double hexamers are loaded can function as replication origins, then DNA replication origins are at least threefold more abundant than previously assumed, and we suggest that replication may occasionally initiate in essentially every intergenic region. These results shed light on recent reports that as many as 15% of replication events initiate outside of known origins, and this broader distribution of replication origins suggest that S phase in yeast may be less distinct from that in humans than widely assumed.

Single molecule MATAC-seq reveals key determinants of DNA replication origin efficiency

Stochastic origin activation gives rise to significant cell-to-cell variability in the pattern of genome replication. The molecular basis for heterogeneity in efficiency and timing of individual origins is a long-standing question. Here, we developed Methylation Accessibility of TArgeted Chromatin domain Sequencing (MATAC-Seq) to determine single-molecule chromatin accessibility of four specific genomic loci. MATAC-Seq relies on preferential modification of accessible DNA by methyltransferases combined with Nanopore-Sequencing for direct readout of methylated DNA-bases. Applying MATAC-Seq to selected early-efficient and late-inefficient yeast replication origins revealed large heterogeneity of chromatin states. Disruption of INO80 or ISW2 chromatin remodeling complexes leads to changes at individual nucleosomal positions that correlate with changes in their replication efficiency. We found a chromatin state with an accessible nucleosome-free region in combination with well-positioned +1 and +2 nucleosomes as a strong predictor for efficient origin activation. Thus, MATAC-Seq identifies the large spectrum of alternative chromatin states that co-exist on a given locus previously masked in population-based experiments and provides a mechanistic basis for origin activation heterogeneity during eukaryotic DNA replication. Consequently, our single-molecule chromatin accessibility assay will be ideal to define single-molecule heterogeneity across many fundamental biological processes such as transcription, replication, or DNA repair in vitro and ex vivo.

Where and when to start: Regulating DNA replication origin activity in eukaryotic genomes

In eukaryotic genomes, hundreds to thousands of potential start sites of DNA replication named origins are dispersed across each of the linear chromosomes. During S-phase, only a subset of origins is selected in a stochastic manner to assemble bidirectional replication forks and initiate DNA synthesis. Despite substantial progress in our understanding of this complex process, a comprehensive 'identity code' that defines origins based on specific nucleotide sequences, DNA structural features, the local chromatin environment, or 3D genome architecture is still missing. In this article, we review the genetic and epigenetic features of replication origins in yeast and metazoan chromosomes and highlight recent insights into how this flexibility in origin usage contributes to nuclear organization, cell growth, differentiation, and genome stability.

Identification of 1600 replication origins in S. cerevisiae

There are approximately 500 known origins of replication in the yeast genome, and the process by which DNA replication initiates at these locations is well understood. In particular, these sites are made competent to initiate replication by loading of the Mcm replicative helicase prior to the start of S phase; thus, "a site to which MCM is bound in G1" might be considered to provide an operational definition of a replication origin. By fusing a subunit of Mcm to micrococcal nuclease, a technique referred to as "Chromatin Endogenous Cleavage", we previously showed that known origins are typically bound by a single Mcm double hexamer, loaded adjacent to the ARS consensus sequence (ACS). Here we extend this analysis from known origins to the entire genome, identifying candidate Mcm binding sites whose signal intensity varies over at least 3 orders of magnitude. Published data quantifying the production of ssDNA during S phase showed clear evidence of replication initiation among the most abundant 1600 of these sites, with replication activity decreasing in concert with Mcm abundance and disappearing at the limit of detection of ssDNA. Three other hallmarks of replication origins were apparent among the most abundant 5,500 sites. Specifically, these sites (1) appeared in intergenic nucleosome-free regions that were flanked on one or both sides by well-positioned nucleosomes; (2) were flanked by ACSs; and (3) exhibited a pattern of GC skew characteristic of replication initiation. Furthermore, the high resolution of this technique allowed us to demonstrate a strong bias for detecting Mcm double-hexamers downstream rather than upstream of the ACS, which is consistent with the directionality of Mcm loading by Orc that has been observed in vitro. We conclude that, if sites at which Mcm double-hexamers are loaded can function as replication origins, then DNA replication origins are at least 3-fold more abundant than previously assumed, and we suggest that replication may occasionally initiate in essentially every intergenic region. These results shed light on recent reports that as many as 15% of replication events initiate outside of known origins, and this broader distribution of replication origins suggest that S phase in yeast may be less distinct from that in humans than is widely assumed.

Changing protein-DNA interactions promote ORC binding site exchange during replication origin licensing

During origin licensing, the eukaryotic replicative helicase Mcm2-7 forms head-to-head double hexamers to prime origins for bidirectional replication. Recent single-molecule and structural studies revealed that one molecule of the helicase loader ORC can sequentially load two Mcm2-7 hexamers to ensure proper head-to-head helicase alignment. To perform this task, ORC must release from its initial high-affinity DNA binding site and "flip" to bind a weaker, inverted DNA site. However, the mechanism of this binding-site switch remains unclear. In this study, we used single-molecule Förster resonance energy transfer (sm-FRET) to study the changing interactions between DNA and ORC or Mcm2-7. We found that the loss of DNA bending that occurs during DNA deposition into the Mcm2-7 central channel increases the rate of ORC dissociation from DNA. Further studies revealed temporally-controlled DNA sliding of helicase-loading intermediates, and that the first sliding complex includes ORC, Mcm2-7, and Cdt1. We demonstrate that sequential events of DNA unbending, Cdc6 release, and sliding lead to a stepwise decrease in ORC stability on DNA, facilitating ORC dissociation from its strong binding site during site switching. In addition, the controlled sliding we observed provides insight into how ORC accesses secondary DNA binding sites at different locations relative to the initial binding site. Our study highlights the importance of dynamic protein-DNA interactions in the loading of two oppositely-oriented Mcm2-7 helicases to ensure bidirectional DNA replication.

Significance Statement

Bidirectional DNA replication, in which two replication forks travel in opposite directions from each origin of replication, is required for complete genome duplication. To prepare for this event, two copies of the Mcm2-7 replicative helicase are loaded at each origin in opposite orientations. Using single-molecule assays, we studied the sequence of changing protein-DNA interactions involved in this process. These stepwise changes gradually reduce the DNA-binding strength of ORC, the primary DNA binding protein involved in this event. This reduced affinity promotes ORC dissociation and rebinding in the opposite orientation on the DNA, facilitating the sequential assembly of two Mcm2-7 molecules in opposite orientations. Our findings identify a coordinated series of events that drive proper DNA replication initiation.

Establishment and function of chromatin organization at replication origins

The origin recognition complex (ORC) is essential for initiation of eukaryotic chromosome replication as it loads the replicative helicase-the minichromosome maintenance (MCM) complex-at replication origins1. Replication origins display a stereotypic nucleosome organization with nucleosome depletion at ORC-binding sites and flanking arrays of regularly spaced nucleosomes2-4. However, how this nucleosome organization is established and whether this organization is required for replication remain unknown. Here, using genome-scale biochemical reconstitution with approximately 300 replication origins, we screened 17 purified chromatin factors from budding yeast and found that the ORC established nucleosome depletion over replication origins and flanking nucleosome arrays by orchestrating the chromatin remodellers INO80, ISW1a, ISW2 and Chd1. The functional importance of the nucleosome-organizing activity of the ORC was demonstrated by orc1 mutations that maintained classical MCM-loader activity but abrogated the array-generation activity of ORC. These mutations impaired replication through chromatin in vitro and were lethal in vivo. Our results establish that ORC, in addition to its canonical role as the MCM loader, has a second crucial function as a master regulator of nucleosome organization at the replication origin, a crucial prerequisite for efficient chromosome replication.

The conserved histone chaperone Spt6 is strongly required for DNA replication and genome stability

Histone chaperones are an important class of proteins that regulate chromatin accessibility for DNA-templated processes. Spt6 is a conserved histone chaperone and key regulator of transcription and chromatin structure. However, its functions outside of these roles have been little explored. In this work, we demonstrate a requirement for S. cerevisiae Spt6 in DNA replication and, more broadly, as a regulator of genome stability. Depletion or mutation of Spt6 impairs DNA replication in vivo. Additionally, spt6 mutants are sensitive to DNA replication stress-inducing agents. Interestingly, this sensitivity is independent of the association of Spt6 with RNA polymerase II (RNAPII), suggesting that spt6 mutants have a transcription-independent impairment of DNA replication. Specifically, genomic studies reveal that spt6 mutants have decreased loading of the MCM replicative helicase at replication origins, suggesting that Spt6 promotes origin licensing. Our results identify Spt6 as a regulator of genome stability, at least in part through a role in DNA replication.

Single-copy locus proteomics of early- and late-firing DNA replication origins identifies a role of Ask1/DASH complex in replication timing control

The chromatin environment at origins of replication is thought to influence DNA replication initiation in eukaryotic genomes. However, it remains unclear how and which chromatin features control the firing of early-efficient (EE) or late-inefficient (LI) origins. Here, we use site-specific recombination and single-locus chromatin isolation to purify EE and LI replication origins in Saccharomyces cerevisiae. Using mass spectrometry, we define the protein composition of native chromatin regions surrounding the EE and LI replication start sites. In addition to known origin interactors, we find the microtubule-binding Ask1/DASH complex as an origin-regulating factor. Strikingly, tethering of Ask1 to individual origin sites advances replication timing (RT) of the targeted chromosomal domain. Targeted degradation of Ask1 globally changes RT of a subset of origins, which can be reproduced by inhibiting microtubule dynamics. Thus, our findings mechanistically connect RT and chromosomal organization via Ask1/DASH with the microtubule cytoskeleton.

Detection and characterization of constitutive replication origins defined by DNA polymerase epsilon

BACKGROUND

Despite the process of DNA replication being mechanistically highly conserved, the location of origins of replication (ORI) may vary from one tissue to the next, or between rounds of replication in eukaryotes, suggesting flexibility in the choice of locations to initiate replication. Lists of human ORI therefore vary widely in number and location, and there are currently no methods available to compare them. Here, we propose a method of detection of ORI based on somatic mutation patterns generated by the mutator phenotype of damaged DNA polymerase epsilon (POLE).

RESULTS

We report the genome-wide localization of constitutive ORI in POLE-mutated human tumors using whole genome sequencing data. Mutations accumulated after many rounds of replication of unsynchronized dividing cell populations in tumors allow to identify constitutive origins, which we show are shared with high fidelity between individuals and tumor types. Using a Smith-Waterman-like dynamic programming approach, we compared replication origin positions obtained from multiple different methods. The comparison allowed us to define a consensus set of replication origins, identified consistently by multiple ORI detection methods. Many DNA features co-localized with the consensus set of ORI, including chromatin loop anchors, G-quadruplexes, S/MARs, and CpGs. Among all features, the H2A.Z histone exhibited the most significant association.

CONCLUSIONS

Our results show that mutation-based detection of replication origins is a viable approach to determining their location and associated sequence features.

Origin recognition complex harbors an intrinsic nucleosome remodeling activity

Nucleosomes package the entire eukaryotic genome, yet enzymes need access to the DNA for numerous metabolic activities, such as replication and transcription. Eukaryotic origins of replication in Saccharomyces cerevisiae are AT rich and are generally nucleosome free for the binding of ORC (origin recognition complex). However, the nucleosome-free region often undergoes expansion during G1/S phase, presumably to make room for MCM double-hexamer formation that nucleates the 11-subunit helicase, CMG (Cdc45, Mcm2–7, Cdc45). While nucleosome remodelers could perform this function, in vitro studies indicate that nucleosome remodeling may be intrinsic to the replication machinery. Indeed, we find here that ORC contains an intrinsic nucleosome remodeling activity that is capable of ATP-stimulated removal of H2A-H2B from nucleosomes.

Eukaryotic DNA replication is initiated at multiple chromosomal sites known as origins of replication that are specifically recognized by the origin recognition complex (ORC) containing multiple ATPase sites. In budding yeast, ORC binds to specific DNA sequences known as autonomously replicating sequences (ARSs) that are mostly nucleosome depleted. However, nucleosomes may still inhibit the licensing of some origins by occluding ORC binding and subsequent MCM helicase loading. Using purified proteins and single-molecule visualization, we find here that the ORC can eject histones from a nucleosome in an ATP-dependent manner. The ORC selectively evicts H2A-H2B dimers but leaves the (H3-H4)₂ tetramer on DNA. It also discriminates canonical H2A from the H2A.Z variant, evicting the former while retaining the latter. Finally, the bromo-adjacent homology (BAH) domain of the Orc1 subunit is essential for ORC-mediated histone eviction. These findings suggest that the ORC is a bona fide nucleosome remodeler that functions to create a local chromatin environment optimal for origin activity.

Impact of Chromosomal Context on Origin Selection and the Replication Program

Eukaryotic DNA replication is regulated by conserved mechanisms that bring about a spatial and temporal organization in which distinct genomic domains are copied at characteristic times during S phase. Although this replication program has been closely linked with genome architecture, we still do not understand key aspects of how chromosomal context modulates the activity of replication origins. To address this question, we have exploited models that combine engineered genomic rearrangements with the unique replication programs of post-quiescence and pre-meiotic S phases. Our results demonstrate that large-scale inversions surprisingly do not affect cell proliferation and meiotic progression, despite inducing a restructuring of replication domains on each rearranged chromosome. Remarkably, these alterations in the organization of DNA replication are entirely due to changes in the positions of existing origins along the chromosome, as their efficiencies remain virtually unaffected genome wide. However, we identified striking alterations in origin firing proximal to the fusion points of each inversion, suggesting that the immediate chromosomal neighborhood of an origin is a crucial determinant of its activity. Interestingly, the impact of genome reorganization on replication initiation is highly comparable in the post-quiescent and pre-meiotic S phases, despite the differences in DNA metabolism in these two physiological states. Our findings therefore shed new light on how origin selection and the replication program are governed by chromosomal architecture.

Cell-Cycle-Dependent Chromatin Dynamics at Replication Origins

Origins of DNA replication are specified by the ordered recruitment of replication factors in a cell-cycle–dependent manner. The assembly of the pre-replicative complex in G1 and the pre-initiation complex prior to activation in S phase are well characterized; however, the interplay between the assembly of these complexes and the local chromatin environment is less well understood. To investigate the dynamic changes in chromatin organization at and surrounding replication origins, we used micrococcal nuclease (MNase) to generate genome-wide chromatin occupancy profiles of nucleosomes, transcription factors, and replication proteins through consecutive cell cycles in Saccharomyces cerevisiae. During each G1 phase of two consecutive cell cycles, we observed the downstream repositioning of the origin-proximal +1 nucleosome and an increase in protected DNA fragments spanning the ARS consensus sequence (ACS) indicative of pre-RC assembly. We also found that the strongest correlation between chromatin occupancy at the ACS and origin efficiency occurred in early S phase, consistent with the rate-limiting formation of the Cdc45–Mcm2-7–GINS (CMG) complex being a determinant of origin activity. Finally, we observed nucleosome disruption and disorganization emanating from replication origins and traveling with the elongating replication forks across the genome in S phase, likely reflecting the disassembly and assembly of chromatin ahead of and behind the replication fork, respectively. These results provide insights into cell-cycle–regulated chromatin dynamics and how they relate to the regulation of origin activity.

Disruption of origin chromatin structure by helicase activation in the absence of DNA replication

Prior to initiation of DNA replication, the eukaryotic helicase, Mcm2-7, must be activated to unwind DNA at replication start sites in early S phase. To study helicase activation within origin chromatin, we constructed a conditional mutant of the polymerase α subunit Cdc17 (or Pol1) to prevent priming and block replication. Recovery of these cells at permissive conditions resulted in the generation of unreplicated gaps at origins, likely due to helicase activation prior to replication initiation. We used micrococcal nuclease (MNase)-based chromatin occupancy profiling under restrictive conditions to study chromatin dynamics associated with helicase activation. Helicase activation in the absence of DNA replication resulted in the disruption and disorganization of chromatin, which extends up to 1 kb from early, efficient replication origins. The CMG holohelicase complex also moves the same distance out from the origin, producing single-stranded DNA that activates the intra-S-phase checkpoint. Loss of the checkpoint did not regulate the progression and stalling of the CMG complex but rather resulted in the disruption of chromatin at both early and late origins. Finally, we found that the local sequence context regulates helicase progression in the absence of DNA replication, suggesting that the helicase is intrinsically less processive when uncoupled from replication.

Interaction of replication factor Sld3 and histone acetyl transferase Esa1 alleviates gene silencing and promotes the activation of late and dormant replication origins

DNA replication in eukaryotes is a multi-step process that consists of three main reactions: helicase loading (licensing), helicase activation (firing), and nascent DNA synthesis (elongation). Although the contributions of some chromatin regulatory factors in the licensing and elongation reaction have been determined, their functions in the firing reaction remain elusive. In the budding yeast Saccharomyces cerevisiae, Sld3, Sld7, and Cdc45 (3-7-45) are rate-limiting in the firing reaction and simultaneous overexpression of 3-7-45 causes untimely activation of late and dormant replication origins. Here, we found that 3-7-45 overexpression not only activated dormant origins in the silenced locus, HMLα, but also exerted an anti-silencing effect at this locus. For these, interaction between Sld3 and Esa1, a conserved histone acetyltransferase, was responsible. Moreover, the Sld3-Esa1 interaction was required for the untimely activation of late origins. These results reveal the Sld3-Esa1 interaction as a novel level of regulation in the firing reaction.

Who gets a license: DNA synthesis in quiescent cells re-entering the cell cycle

The precise regulation of the entry into S phase is critical for preventing genome instability. The first step in the initiation of eukaryotic DNA synthesis occurs in G1 phase cells and involves the loading of the conserved MCM helicase onto multiple origins of replication in a process known as origin licensing. In proliferating metazoan cells, an origin-licensing checkpoint delays initiation until high levels of MCM loading occur, with excess origins being licensed. One function of this checkpoint is to ensure that S phase can be completed in the face of replication stress by activation of dormant MCM bound origins. However, when both metazoan and yeast cells enter S phase from quiescence or G0 phase, a non-growing but reversible cell cycle state, origins are significantly under-licensed. In metazoan cells, under-licensing is the result of a compromised origin-licensing checkpoint. In budding yeast, our study has revealed that under-licensing can be attributed to the chromatin structure at a class of origins that is inhibitory to the binding of MCM. Thus, defects in multiple pathways may contribute to the failure to fully license origins in quiescent cells re-entering the cell cycle, thereby promoting a higher risk of genome instability.

Nucleosome Positioning and Spacing: From Mechanism to Function

Eukaryotes associate their genomes with histone proteins, forming nucleosome particles. Nucleosomes regulate and protect the genetic information. They often assemble into evenly spaced arrays of nucleosomes. These regular nucleosome arrays cover significant portions of the genome, in particular over genes. The presence of these evenly spaced nucleosome arrays is highly conserved throughout the entire eukaryotic domain. Here, we review the mechanisms behind the establishment of this primary structure of chromatin with special emphasis on the biogenesis of evenly spaced nucleosome arrays. We highlight the roles that transcription, nucleosome remodelers, DNA sequence, and histone density play towards the formation of evenly spaced nucleosome arrays and summarize our current understanding of their cellular functions. We end with key unanswered questions that remain to be explored to obtain an in-depth understanding of the biogenesis and function of the nucleosome landscape.

Chromatin structure restricts origin utilization when quiescent cells re-enter the cell cycle

Quiescent cells reside in G0 phase, which is characterized by the absence of cell growth and proliferation. These cells remain viable and re-enter the cell cycle when prompted by appropriate signals. Using a budding yeast model of cellular quiescence, we investigated the program that initiated DNA replication when these G0 cells resumed growth. Quiescent cells contained very low levels of replication initiation factors, and their entry into S phase was delayed until these factors were re-synthesized. A longer S phase in these cells correlated with the activation of fewer origins of replication compared to G1 cells. The chromatin structure around inactive origins in G0 cells showed increased H3 occupancy and decreased nucleosome positioning compared to the same origins in G1 cells, inhibiting the origin binding of the Mcm4 subunit of the MCM licensing factor. Thus, quiescent yeast cells are under-licensed during their re-entry into S phase.

Humanizing the yeast origin recognition complex

The Origin Recognition Complex (ORC) is an evolutionarily conserved six-subunit protein complex that binds specific sites at many locations to coordinately replicate the entire eukaryote genome. Though highly conserved in structure, ORC’s selectivity for replication origins has diverged tremendously between yeasts and humans to adapt to vastly different life cycles. In this work, we demonstrate that the selectivity determinant of ORC for DNA binding lies in a 19-amino acid insertion helix in the Orc4 subunit, which is present in yeast but absent in human. Removal of this motif from Orc4 transforms the yeast ORC, which selects origins based on base-specific binding at defined locations, into one whose selectivity is dictated by chromatin landscape and afforded with plasticity, as reported for human. Notably, the altered yeast ORC has acquired an affinity for regions near transcriptional start sites (TSSs), which the human ORC also favors.

In most model yeast species the Origin Recognition Complex (ORC) binds defined and species-specific base sequences while in humans what determines the binding appears to be more complex. Here the authors reveal that the yeast’s ORC complex binding specificity is dependent on a 19-amino acid insertion helix in the Orc4 subunit which is lost in human.

Bioinformatical dissection of fission yeast DNA replication origins

Replication origins in eukaryotes form a base for assembly of the pre-replication complex (pre-RC), thereby serving as an initiation site of DNA replication. Characteristics of replication origin vary among species. In fission yeast Schizosaccharomyces pombe, DNA of high AT content is a distinct feature of replication origins; however, it remains to be understood what the general molecular architecture of fission yeast origin is. Here, we performed ChIP-seq mapping of Orc4 and Mcm2, two representative components of the pre-RC, and described the characteristics of their binding sites. The analysis revealed that fission yeast efficient origins are associated with two similar but independent features: a ≥15 bp-long motif with stretches of As and an AT-rich region of a few hundred bp. The A-rich motif was correlated with chromosomal binding of Orc, a DNA-binding component in the pre-RC, whereas the AT-rich region was associated with efficient binding of the DNA replicative helicase Mcm. These two features, in combination with the third feature, a transcription-poor region of approximately 1 kb, enabled to distinguish efficient replication origins from the rest of chromosome arms with high accuracy. This study, hence, provides a model that describes how multiple functional elements specify DNA replication origins in fission yeast genome.

Superresolution imaging reveals spatiotemporal propagation of human replication foci mediated by CTCF-organized chromatin structures

Mammalian DNA replication is initiated at numerous replication origins, which are clustered into thousands of replication domains (RDs) across the genome. However, it remains unclear whether the replication origins within each RD are activated stochastically or preferentially near certain chromatin features. To understand how DNA replication in single human cells is regulated at the sub-RD level, we directly visualized and quantitatively characterized the spatiotemporal organization, morphology, and in situ epigenetic signatures of individual replication foci (RFi) across S-phase at superresolution using stochastic optical reconstruction microscopy. Importantly, we revealed a hierarchical radial pattern of RFi propagation dynamics that reverses directionality from early to late S-phase and is diminished upon caffeine treatment or CTCF knockdown. Together with simulation and bioinformatic analyses, our findings point to a "CTCF-organized REplication Propagation" (CoREP) model, which suggests a nonrandom selection mechanism for replication activation at the sub-RD level during early S-phase, mediated by CTCF-organized chromatin structures. Collectively, these findings offer critical insights into the key involvement of local epigenetic environment in coordinating DNA replication across the genome and have broad implications for our conceptualization of the role of multiscale chromatin architecture in regulating diverse cell nuclear dynamics in space and time.

FORK-seq: replication landscape of the Saccharomyces cerevisiae genome by nanopore sequencing

Genome replication mapping methods profile cell populations, masking cell-to-cell heterogeneity. Here, we describe FORK-seq, a nanopore sequencing method to map replication of single DNA molecules at 200-nucleotide resolution. By quantifying BrdU incorporation along pulse-chased replication intermediates from Saccharomyces cerevisiae, we orient 58,651 replication tracks reproducing population-based replication directionality profiles and map 4964 and 4485 individual initiation and termination events, respectively. Although most events cluster at known origins and fork merging zones, 9% and 18% of initiation and termination events, respectively, occur at many locations previously missed. Thus, FORK-seq reveals the full extent of cell-to-cell heterogeneity in DNA replication.

MRX Increases Chromatin Accessibility at Stalled Replication Forks to Promote Nascent DNA Resection and Cohesin Loading

The recovery of stalled replication forks depends on the controlled resection of nascent DNA and on the loading of cohesin. These processes operate in the context of nascent chromatin, but the impact of nucleosome structure on a fork restart remains poorly understood. Here, we show that the Mre11-Rad50-Xrs2 (MRX) complex acts together with the chromatin modifiers Gcn5 and Set1 and the histone remodelers RSC, Chd1, and Isw1 to promote chromatin remodeling at stalled forks. Increased chromatin accessibility facilitates the resection of nascent DNA by the Exo1 nuclease and the Sgs1 and Chl1 DNA helicases. Importantly, increased ssDNA promotes the recruitment of cohesin to arrested forks in a Scc2-Scc4-dependent manner. Altogether, these results indicate that MRX cooperates with chromatin modifiers to orchestrate the action of remodelers, nucleases, and DNA helicases, promoting the resection of nascent DNA and the loading of cohesin, two key processes involved in the recovery of arrested forks.

Origins of DNA replication

In all kingdoms of life, DNA is used to encode hereditary information. Propagation of the genetic material between generations requires timely and accurate duplication of DNA by semiconservative replication prior to cell division to ensure each daughter cell receives the full complement of chromosomes. DNA synthesis of daughter strands starts at discrete sites, termed replication origins, and proceeds in a bidirectional manner until all genomic DNA is replicated. Despite the fundamental nature of these events, organisms have evolved surprisingly divergent strategies that control replication onset. Here, we discuss commonalities and differences in replication origin organization and recognition in the three domains of life.

Nascent chromatin occupancy profiling reveals locus- and factor-specific chromatin maturation dynamics behind the DNA replication fork

Proper regulation and maintenance of the epigenome is necessary to preserve genome function. However, in every cell division, the epigenetic state is disassembled and then reassembled in the wake of the DNA replication fork. Chromatin restoration on nascent DNA is a complex and regulated process that includes nucleosome assembly and remodeling, deposition of histone variants, and the re-establishment of transcription factor binding. To study the genome-wide dynamics of chromatin restoration behind the DNA replication fork, we developed nascent chromatin occupancy profiles (NCOPs) to comprehensively profile nascent and mature chromatin at nucleotide resolution. Although nascent chromatin is inherently less organized than mature chromatin, we identified locus-specific differences in the kinetics of chromatin maturation that were predicted by the epigenetic landscape, including the histone variant H2AZ, which marked loci with rapid maturation kinetics. The chromatin maturation at origins of DNA replication was dependent on whether the origin underwent initiation or was passively replicated from distal-originating replication forks, suggesting distinct chromatin assembly mechanisms surrounding activated and disassembled prereplicative complexes. Finally, we identified sites that were only occupied transiently by DNA-binding factors following passage of the replication fork, which may provide a mechanism for perturbations of the DNA replication program to shape the regulatory landscape of the genome.

iNuc-ext-PseTNC: an efficient ensemble model for identification of nucleosome positioning by extending the concept of Chou's PseAAC to pseudo-tri-nucleotide composition

Nucleosome is a central element of eukaryotic chromatin, which composes of histone proteins and DNA molecules. It performs vital roles in many eukaryotic intra-nuclear processes, for instance, chromatin structure and transcriptional regulation formation. Identification of nucleosome positioning via wet lab is difficult; so, the attention is diverted towards the accurate intelligent automated prediction. In this regard, a novel intelligent automated model "iNuc-ext-PseTNC" is developed to identify the nucleosome positioning in genomes accurately. In this predictor, the sequences of DNA are mathematically represented by two different discrete feature extraction techniques, namely pseudo-tri-nucleotide composition (PseTNC) and pseudo-di-nucleotide composition. Several contemporary machine learning algorithms were examined. Further, the predictions of individual classifiers were integrated through an evolutionary genetic algorithm. The success rates of the ensemble model are higher than individual classifiers. After analyzing the prediction results, it is noticed that iNuc-ext-PseTNC model has achieved better performance in combination with PseTNC feature space, which are 94.3%, 93.14%, and 88.60% of accuracies using six-fold cross-validation test for the three benchmark datasets S₁, S₂, and S₃, respectively. The achieved outcomes exposed that the results of iNuc-ext-PseTNC model are prominent compared to the existing methods so far notifiable in the literature. It is ascertained that the proposed model might be more fruitful and a practical tool for rudimentary academia and research.

Recent advances in understanding DNA replication: cell type-specific adaptation of the DNA replication program

DNA replication is an essential process occurring prior to cell division. Cell division coupled to proliferation ensures the growth and renewal of a large variety of specialized cell types generated during embryonic development. Changes in the DNA replication program occur during development. Embryonic undifferentiated cells show a high replication rate and fast proliferation, whereas more differentiated cells are characterized by reduced DNA synthesis and a low proliferation rate. Hence, the DNA replication program must adapt to the specific features of cells committed to different fates. Recent findings on DNA synthesis regulation in different cell types open new perspectives for developing efficient and more adapted therapies to treat various diseases such as genetic diseases and cancer. This review will put the emphasis on recent progress made in this field.

Evidence for DNA Sequence Encoding of an Accessible Nucleosomal Array across Vertebrates

Nucleosome-depleted regions around which nucleosomes order following the "statistical" positioning scenario were recently shown to be encoded in the DNA sequence in human. This intrinsic nucleosomal ordering strongly correlates with oscillations in the local GC content as well as with the interspecies and intraspecies mutation profiles, revealing the existence of both positive and negative selection. In this letter, we show that these predicted nucleosome inhibitory energy barriers (NIEBs) with compacted neighboring nucleosomes are indeed ubiquitous to all vertebrates tested. These 1 kb-sized chromatin patterns are widely distributed along vertebrate chromosomes, overall covering more than a third of the genome. We have previously observed in human deviations from neutral evolution at these genome-wide distributed regions, which we interpreted as a possible indication of the selection of an open, accessible, and dynamic nucleosomal array to constitutively facilitate the epigenetic regulation of nuclear functions in a cell-type-specific manner. As a first, very appealing observation supporting this hypothesis, we report evidence of a strong association between NIEB borders and the poly(A) tails of Alu sequences in human. These results suggest that NIEBs provide adequate chromatin patterns favorable to the integration of Alu retrotransposons and, more generally to various transposable elements in the genomes of primates and other vertebrates.

Major Determinants of Nucleosome Positioning

The compact structure of the nucleosome limits DNA accessibility and inhibits the binding of most sequence-specific proteins. Nucleosomes are not randomly located on the DNA but positioned with respect to the DNA sequence, suggesting models in which critical binding sites are either exposed in the linker, resulting in activation, or buried inside a nucleosome, resulting in repression. The mechanisms determining nucleosome positioning are therefore of paramount importance for understanding gene regulation and other events that occur in chromatin, such as transcription, replication, and repair. Here, we review our current understanding of the major determinants of nucleosome positioning: DNA sequence, nonhistone DNA-binding proteins, chromatin-remodeling enzymes, and transcription. We outline the major challenges for the future: elucidating the precise mechanisms of chromatin opening and promoter activation, identifying the complexes that occupy promoters, and understanding the multiscale problem of chromatin fiber organization.

Spectrum structures and biological functions of 8-mers in the human genome

The spectra of k-mer frequencies can reveal the structures and evolution of genome sequences. We confirmed that the trimodal spectrum of 8-mers in human genome sequences is distinguished only by CG2, CG1 and CG0 8-mer sets, containing 2,1 or 0 CpG, respectively. This phenomenon is called independent selection law. The three types of CG 8-mers were considered as different functional elements. We conjectured that (1) nucleosome binding motifs are mainly characterized by CG1 8-mers and (2) the core structural units of CpG island sequences are predominantly characterized by CG2 8-mers. To validate our conjectures, nucleosome occupied sequences and CGI sequences were extracted, then the sequence parameters were constructed through the information of the three CG 8-mer sets respectively. ROC analysis showed that CG1 8-mers are more preference in nucleosome occupied segments (AUC > 0.7) and CG2 8-mers are more preference in CGI sequences (AUC > 0.99). This validates our conjecture in principle.

Replication origin-flanking roadblocks reveal origin-licensing dynamics and altered sequence dependence

In eukaryotes, DNA replication initiates from multiple origins of replication for timely genome duplication. These sites are selected by origin licensing, during which the core enzyme of the eukaryotic DNA replicative helicase, the Mcm2-7 (minichromosome maintenance) complex, is loaded at each origin. This origin licensing requires loading two Mcm2-7 helicases around origin DNA in a head-to-head orientation. Current models suggest that the origin-recognition complex (ORC) and cell-division cycle 6 (Cdc6) proteins recognize and encircle origin DNA and assemble an Mcm2-7 double-hexamer around adjacent double-stranded DNA. To test this model and assess the location of Mcm2-7 initial loading, we placed DNA-protein roadblocks at defined positions adjacent to the essential ORC-binding site within Saccharomyces cerevisiae origin DNA. Roadblocks were made either by covalent cross-linking of the HpaII methyltransferase to DNA or through binding of a transcription activator-like effector (TALE) protein. Contrary to the sites of Mcm2-7 recruitment being precisely defined, only single roadblocks that inhibited ORC-DNA binding showed helicase loading defects. We observed inhibition of helicase loading without inhibition of ORC-DNA binding only when roadblocks were placed on both sides of the origin to restrict sliding of a helicase-loading intermediate. Consistent with a sliding helicase-loading intermediate, when either one of the flanking roadblocks was eliminated, the remaining roadblock had no effect on helicase loading. Interestingly, either origin-flanking nucleosomes or roadblocks resulted in helicase loading being dependent on an additional origin sequence known to be a weaker ORC-DNA-binding site. Together, our findings support a model in which sliding helicase-loading intermediates increase the flexibility of the DNA sequence requirements for origin licensing.

[G-quadruplex: key controllers of human genome duplication]

The correct duplication of the human genome is under the control of a spatiotemporal program that determines where and when replication forks start. This regulation thus mainly operates on replication start sites named replication origins. During the S-phase, about 50 000 origins fire in one human cell. However, the normal or perturbed progression of replication forks also strongly impacts on replication. Recently, several studies have put forward the role of a noncanonical DNA structure, the G-quadruplex, in the control of genome duplication. In this review, we describe the major impact of this structure on starting points and on the progression of replication forks.

Chromosome Duplication in Saccharomyces cerevisiae

The accurate and complete replication of genomic DNA is essential for all life. In eukaryotic cells, the assembly of the multi-enzyme replisomes that perform replication is divided into stages that occur at distinct phases of the cell cycle. Replicative DNA helicases are loaded around origins of DNA replication exclusively during G₁ phase. The loaded helicases are then activated during S phase and associate with the replicative DNA polymerases and other accessory proteins. The function of the resulting replisomes is monitored by checkpoint proteins that protect arrested replisomes and inhibit new initiation when replication is inhibited. The replisome also coordinates nucleosome disassembly, assembly, and the establishment of sister chromatid cohesion. Finally, when two replisomes converge they are disassembled. Studies in Saccharomyces cerevisiae have led the way in our understanding of these processes. Here, we review our increasingly molecular understanding of these events and their regulation.

Nucleosomes influence multiple steps during replication initiation

Eukaryotic replication origin licensing, activation and timing are influenced by chromatin but a mechanistic understanding is lacking. Using reconstituted nucleosomal DNA replication assays, we assessed the impact of nucleosomes on replication initiation. To generate distinct nucleosomal landscapes, different chromatin-remodeling enzymes (CREs) were used to remodel nucleosomes on origin-DNA templates. Nucleosomal organization influenced two steps of replication initiation: origin licensing and helicase activation. Origin licensing assays showed that local nucleosome positioning enhanced origin specificity and modulated helicase loading by influencing ORC DNA binding. Interestingly, SWI/SNF- and RSC-remodeled nucleosomes were permissive for origin licensing but showed reduced helicase activation. Specific CREs rescued replication of these templates if added prior to helicase activation, indicating a permissive chromatin state must be established during origin licensing to allow efficient origin activation. Our studies show nucleosomes directly modulate origin licensing and activation through distinct mechanisms and provide insights into the regulation of replication initiation by chromatin.

Mechanisms and regulation of DNA replication initiation in eukaryotes

Cellular DNA replication is initiated through the action of multiprotein complexes that recognize replication start sites in the chromosome (termed origins) and facilitate duplex DNA melting within these regions. In a typical cell cycle, initiation occurs only once per origin and each round of replication is tightly coupled to cell division. To avoid aberrant origin firing and re-replication, eukaryotes tightly regulate two events in the initiation process: loading of the replicative helicase, MCM2-7, onto chromatin by the origin recognition complex (ORC), and subsequent activation of the helicase by its incorporation into a complex known as the CMG. Recent work has begun to reveal the details of an orchestrated and sequential exchange of initiation factors on DNA that give rise to a replication-competent complex, the replisome. Here, we review the molecular mechanisms that underpin eukaryotic DNA replication initiation - from selecting replication start sites to replicative helicase loading and activation - and describe how these events are often distinctly regulated across different eukaryotic model organisms.

Historical Perspective of Eukaryotic DNA Replication

The replication of the genome of a eukaryotic cell is a complex process requiring the ordered assembly of multiprotein replisomes at many chromosomal sites. The process is strictly controlled during the cell cycle to ensure the complete and faithful transmission of genetic information to progeny cells. Our current understanding of the mechanisms of eukaryotic DNA replication has evolved over a period of more than 30 years through the efforts of many investigators. The aim of this perspective is to provide a brief history of the major advances during this period.

Regulation and Function of Cdt1; A Key Factor in Cell Proliferation and Genome Stability

Successful cell proliferation requires efficient and precise genome duplication followed by accurate chromosome segregation. The Cdc10-dependent transcript 1 protein (Cdt1) is required for the first step in DNA replication, and in human cells Cdt1 is also required during mitosis. Tight cell cycle controls over Cdt1 abundance and activity are critical to normal development and genome stability. We review here recent advances in elucidating Cdt1 molecular functions in both origin licensing and kinetochore–microtubule attachment, and we describe the current understanding of human Cdt1 regulation.

Context based computational analysis and characterization of ARS consensus sequences (ACS) of Saccharomyces cerevisiae genome

Genome-wide experimental studies in Saccharomyces cerevisiae reveal that autonomous replicating sequence (ARS) requires an essential consensus sequence (ACS) for replication activity. Computational studies identified thousands of ACS like patterns in the genome. However, only a few hundreds of these sites act as replicating sites and the rest are considered as dormant or evolving sites. In a bid to understand the sequence makeup of replication sites, a content and context-based analysis was performed on a set of replicating ACS sequences that binds to origin-recognition complex (ORC) denoted as ORC-ACS and non-replicating ACS sequences (nrACS), that are not bound by ORC. In this study, DNA properties such as base composition, correlation, sequence dependent thermodynamic and DNA structural profiles, and their positions have been considered for characterizing ORC-ACS and nrACS. Analysis reveals that ORC-ACS depict marked differences in nucleotide composition and context features in its vicinity compared to nrACS. Interestingly, an A-rich motif was also discovered in ORC-ACS sequences within its nucleosome-free region. Profound changes in the conformational features, such as DNA helical twist, inclination angle and stacking energy between ORC-ACS and nrACS were observed. Distribution of ACS motifs in the non-coding segments points to the locations of ORC-ACS which are found far away from the adjacent gene start position compared to nrACS thereby enabling an accessible environment for ORC-proteins. Our attempt is novel in considering the contextual view of ACS and its flanking region along with nucleosome positioning in the S. cerevisiae genome and may be useful for any computational prediction scheme.

Prediction of nucleosome positioning by the incorporation of frequencies and distributions of three different nucleotide segment lengths into a general pseudo k-tuple nucleotide composition

Motivation

Nucleosome positioning plays important roles in many eukaryotic intranuclear processes, such as transcriptional regulation and chromatin structure formation. The investigations of nucleosome positioning rules provide a deeper understanding of these intracellular processes.

Results

Nucleosome positioning prediction was performed using a model consisting of three types of variables characterizing a DNA sequence—the number of five-nucleotide sequences, the number of three-nucleotide combinations in one period of a helix, and mono- and di-nucleotide distributions in DNA fragments. Using recently proposed stringent benchmark datasets with low biases for Saccharomyces cerevisiae, Homo sapiens, Caenorhabditis elegans and Drosophila melanogaster, the present model was shown to have a better prediction performance than the recently proposed predictors. This model was able to display the common and organism-dependent factors that affect nucleosome forming and inhibiting sequences as well. Therefore, the predictors developed here can accurately predict nucleosome positioning and help determine the key factors influencing this process.

Supplementary information

Supplementary data are available at Bioinformatics online.

Evidence of selection for an accessible nucleosomal array in human

BACKGROUND

Recently, a physical model of nucleosome formation based on sequence-dependent bending properties of the DNA double-helix has been used to reveal some enrichment of nucleosome-inhibiting energy barriers (NIEBs) nearby ubiquitous human "master" replication origins. Here we use this model to predict the existence of about 1.6 millions NIEBs over the 22 human autosomes.

RESULTS

We show that these high energy barriers of mean size 153 bp correspond to nucleosome-depleted regions (NDRs) in vitro, as expected, but also in vivo. On either side of these NIEBs, we observe, in vivo and in vitro, a similar compacted nucleosome ordering, suggesting an absence of chromatin remodeling. This nucleosomal ordering strongly correlates with oscillations of the GC content as well as with the interspecies and intraspecies mutation profiles along these regions. Comparison of these divergence rates reveals the existence of both positive and negative selections linked to nucleosome positioning around these intrinsic NDRs. Overall, these NIEBs and neighboring nucleosomes cover 37.5 % of the human genome where nucleosome occupancy is stably encoded in the DNA sequence. These 1 kb-sized regions of intrinsic nucleosome positioning are equally found in GC-rich and GC-poor isochores, in early and late replicating regions, in intergenic and genic regions but not at gene promoters.

CONCLUSION

The source of selection pressure on the NIEBs has yet to be resolved in future work. One possible scenario is that these widely distributed chromatin patterns have been selected in human to impair the condensation of the nucleosomal array into the 30 nm chromatin fiber, so as to facilitate the epigenetic regulation of nuclear functions in a cell-type-specific manner.

Establishment of a promoter-based chromatin architecture on recently replicated DNA can accommodate variable inter-nucleosome spacing

Nucleosomes, the fundamental subunits of eukaryotic chromatin, are organized with respect to transcriptional start sites. A major challenge to the persistence of this organization is the disassembly of nucleosomes during DNA replication. Here, we use complimentary approaches to map the locations of nucleosomes on recently replicated DNA. We find that nucleosomes are substantially realigned with promoters during the minutes following DNA replication. As a result, the nucleosomal landscape is largely re-established before newly replicated chromosomes are partitioned into daughter cells and can serve as a platform for the re-establishment of gene expression programmes. When the supply of histones is disrupted through mutation of the chaperone Caf1, a promoter-based architecture is generated, but with increased inter-nucleosomal spacing. This indicates that the chromatin remodelling enzymes responsible for spacing nucleosomes are capable of organizing nucleosomes with a range of different linker DNA lengths.

A deformation energy-based model for predicting nucleosome dyads and occupancy

Nucleosome plays an essential role in various cellular processes, such as DNA replication, recombination, and transcription. Hence, it is important to decode the mechanism of nucleosome positioning and identify nucleosome positions in the genome. In this paper, we present a model for predicting nucleosome positioning based on DNA deformation, in which both bending and shearing of the nucleosomal DNA are considered. The model successfully predicted the dyad positions of nucleosomes assembled in vitro and the in vitro map of nucleosomes in Saccharomyces cerevisiae. Applying the model to Caenorhabditis elegans and Drosophila melanogaster, we achieved satisfactory results. Our data also show that shearing energy of nucleosomal DNA outperforms bending energy in nucleosome occupancy prediction and the ability to predict nucleosome dyad positions is attributed to bending energy that is associated with rotational positioning of nucleosomes.

The Chromatin Remodelling Enzymes SNF2H and SNF2L Position Nucleosomes adjacent to CTCF and Other Transcription Factors

Within the genomes of metazoans, nucleosomes are highly organised adjacent to the binding sites for a subset of transcription factors. Here we have sought to investigate which chromatin remodelling enzymes are responsible for this. We find that the ATP-dependent chromatin remodelling enzyme SNF2H plays a major role organising arrays of nucleosomes adjacent to the binding sites for the architectural transcription factor CTCF sites and acts to promote CTCF binding. At many other factor binding sites SNF2H and the related enzyme SNF2L contribute to nucleosome organisation. The action of SNF2H at CTCF sites is functionally important as depletion of CTCF or SNF2H affects transcription of a common group of genes. This suggests that chromatin remodelling ATPase's most closely related to the Drosophila ISWI protein contribute to the function of many human gene regulatory elements.