DNA replication origins in the Schizosaccharomyces pombe genome

High Nucleotide Skew Palindromic DNA Sequences Function as Potential Replication Origins due to their Unzipping Propensity

Locations of DNA replication initiation in prokaryotes, called "origins of replication", are well-characterized. However, a mechanistic understanding of the sequence dependence of the local unzipping of double-stranded DNA, the first step towards replication initiation, is lacking. Here, utilizing a Markov chain model that was created to address the directional nature of DNA unzipping and replication, we model the sequence dependence of local melting of double-stranded linear DNA segments. We show that generalized palindromic sequences with high nucleotide skews have a low kinetic barrier for local melting near melting temperatures. This allows for such sequences to function as potential replication origins. We support our claim with evidence for high-skew palindromic sequences within the replication origins of mitochondrial DNA, bacteria, archaea and plasmids.

Unraveling the complexity of asymmetric DNA replication: Advancements in ribonucleotide mapping techniques and beyond

DNA replication is a fundamental process for cell proliferation, governed by intricate mechanisms involving leading and lagging strand synthesis. In eukaryotes, canonical DNA replication occurs during the S phase of the cell cycle, facilitated by various components of the replicative machinery at sites known as replication origins. Leading and lagging strands exhibit distinct replication dynamics, with leading strand replication being relatively straightforward compared to the complex synthesis of lagging strands involving Okazaki fragment maturation. Central to DNA synthesis are DNA polymerases, with Polα, Polε, and Polδ playing pivotal roles, each specializing in specific tasks during replication. Notably, leading and lagging strands are replicated by different polymerases, contributing to the division of labor in DNA replication. Understanding the enzymology of asymmetric DNA replication has been challenging, with methods relying on ribonucleotide incorporation and next-generation sequencing techniques offering comprehensive insights. These methodologies, such as HydEn-seq, PU-seq, ribose-seq, and emRiboSeq, offer insights into polymerase activity and strand synthesis, aiding in understanding DNA replication dynamics. Recent advancements include novel conditional mutants for ribonucleotide excision repair, enzymatic cleavage alternatives, and unified pipelines for data analysis. Further developments in adapting techniques to different organisms, studying non-canonical polymerases, and exploring new sequencing platforms hold promise for expanding our understanding of DNA replication dynamics. Integrating strand-specific information into single-cell studies could offer novel insights into enzymology, opening avenues for future research and applications in repair and replication biology.

Origins of DNA replication in eukaryotes

Errors occurring during DNA replication can result in inaccurate replication, incomplete replication, or re-replication, resulting in genome instability that can lead to diseases such as cancer or disorders such as autism. A great deal of progress has been made toward understanding the entire process of DNA replication in eukaryotes, including the mechanism of initiation and its control. This review focuses on the current understanding of how the origin recognition complex (ORC) contributes to determining the location of replication initiation in the multiple chromosomes within eukaryotic cells, as well as methods for mapping the location and temporal patterning of DNA replication. Origin specification and configuration vary substantially between eukaryotic species and in some cases co-evolved with gene-silencing mechanisms. We discuss the possibility that centromeres and origins of DNA replication were originally derived from a common element and later separated during evolution.

Ribonucleotide Incorporation by Eukaryotic B-family Replicases and Its Implications for Genome Stability

Our current view of how DNA-based genomes are efficiently and accurately replicated continues to evolve as new details emerge on the presence of ribonucleotides in DNA. Ribonucleotides are incorporated during eukaryotic DNA replication at rates that make them the most common noncanonical nucleotide placed into the nuclear genome, they are efficiently repaired, and their removal impacts genome integrity. This review focuses on three aspects of this subject: the incorporation of ribonucleotides into the eukaryotic nuclear genome during replication by B-family DNA replicases, how these ribonucleotides are removed, and the consequences of their presence or removal for genome stability and disease. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Genome-wide oscillations in G + C density and sequence conservation

Eukaryotic genomes typically show a uniform G + C content among chromosomes, but on smaller scales, many species have a G + C density that fluctuates with a characteristic wavelength. This oscillation is evident in many insect species, with wavelengths ranging between 700 bp and 4 kb. Measures of evolutionary conservation oscillate in phase with G + C content, with conserved regions having higher G + C. Loci with large regulatory regions show more regular oscillations; coding sequences and heterochromatic regions show little or no oscillation. There is little oscillation in vertebrate genomes in regions with densely distributed mobile repetitive elements. However, species with few repeats show oscillation in both G + C density and sequence conservation. These oscillations may reflect optimal spacing of cis-regulatory elements.

Conservation of a DNA Replication Motif among Phylogenetically Distant Budding Yeast Species

Eukaryotic DNA replication begins at genomic loci termed origins, which are bound by the origin recognition complex (ORC). Although ORC is conserved across species, the sequence composition of origins is more varied. In the budding yeast Saccharomyces cerevisiae, the ORC-binding motif consists of an A/T-rich 17 bp “extended ACS” sequence adjacent to a B1 element composed of two 3-bp motifs. Similar sequences occur at origins in closely related species, but it is not clear when this type of replication origin arose and whether it predated a whole-genome duplication that occurred around 100 Ma in the budding yeast lineage. To address these questions, we identified the ORC-binding sequences in the nonduplicated species Torulaspora delbrueckii. We used chromatin immunoprecipitation followed by sequencing and identified 190 ORC-binding sites distributed across the eight T. delbrueckii chromosomes. Using these sites, we identified an ORC-binding motif that is nearly identical to the known motif in S. cerevisiae. We also found that the T. delbrueckii ORC-binding sites function as origins in T. delbrueckii when cloned onto a plasmid and that the motif is required for plasmid replication. Finally, we compared an S. cerevisiae origin with two T. delbrueckii ORC-binding sites and found that they conferred similar stabilities to a plasmid. These results reveal that the ORC-binding motif arose prior to the whole-genome duplication and has been maintained for over 100 Myr.

Replication initiation: Implications in genome integrity

In every cell cycle, billions of nucleotides need to be duplicated within hours, with extraordinary precision and accuracy. The molecular mechanism by which cells regulate the replication event is very complicated, and the entire process begins way before the onset of S phase. During the G1 phase of the cell cycle, cells prepare by assembling essential replication factors to establish the pre-replicative complex at origins, sites that dictate where replication would initiate during S phase. During S phase, the replication process is tightly coupled with the DNA repair system to ensure the fidelity of replication. Defects in replication and any error must be recognized by DNA damage response and checkpoint signaling pathways in order to halt the cell cycle before cells are allowed to divide. The coordination of these processes throughout the cell cycle is therefore critical to achieve genomic integrity and prevent diseases. In this review, we focus on the current understanding of how the replication initiation events are regulated to achieve genome stability.

Orc4 spatiotemporally stabilizes centromeric chromatin

The establishment of centromeric chromatin and its propagation by the centromere-specific histone CENPA is mediated by epigenetic mechanisms in most eukaryotes. DNA replication origins, origin binding proteins, and replication timing of centromere DNA are important determinants of centromere function. The epigenetically regulated regional centromeres in the budding yeast Candida albicans have unique DNA sequences that replicate earliest in every chromosome and are clustered throughout the cell cycle. In this study, the genome-wide occupancy of the replication initiation protein Orc4 reveals its abundance at all centromeres in C. albicans Orc4 is associated with four different DNA sequence motifs, one of which coincides with tRNA genes (tDNA) that replicate early and cluster together in space. Hi-C combined with genome-wide replication timing analyses identify that early replicating Orc4-bound regions interact with themselves stronger than with late replicating Orc4-bound regions. We simulate a polymer model of chromosomes of C. albicans and propose that the early replicating and highly enriched Orc4-bound sites preferentially localize around the clustered kinetochores. We also observe that Orc4 is constitutively localized to centromeres, and both Orc4 and the helicase Mcm2 are essential for cell viability and CENPA stability in C. albicans Finally, we show that new molecules of CENPA are recruited to centromeres during late anaphase/telophase, which coincides with the stage at which the CENPA-specific chaperone Scm3 localizes to the kinetochore. We propose that the spatiotemporal localization of Orc4 within the nucleus, in collaboration with Mcm2 and Scm3, maintains centromeric chromatin stability and CENPA recruitment in C. albicans.

Structural Mechanisms for Replicating DNA in Eukaryotes

The faithful and timely copying of DNA by molecular machines known as replisomes depends on a disparate suite of enzymes and scaffolding factors working together in a highly orchestrated manner. Large, dynamic protein-nucleic acid assemblies that selectively morph between distinct conformations and compositional states underpin this critical cellular process. In this article, we discuss recent progress outlining the physical basis of replisome construction and progression in eukaryotes.

Ribonucleotide incorporation into DNA during DNA replication and its consequences

Ribonucleotides are the most abundant non-canonical nucleotides in the genome. Their vast presence and influence over genome biology is becoming increasingly appreciated. Here we review the recent progress made in understanding their genomic presence, incorporation characteristics and usefulness as biomarkers for polymerase enzymology. We also discuss ribonucleotide processing, the genetic consequences of unrepaired ribonucleotides in DNA and evidence supporting the significance of their transient presence in the nuclear genome.

Molecular mechanisms of eukaryotic origin initiation, replication fork progression, and chromatin maintenance

Eukaryotic DNA replication is a highly dynamic and tightly regulated process. Replication involves several dozens of replication proteins, including the initiators ORC and Cdc6, replicative CMG helicase, DNA polymerase α-primase, leading-strand DNA polymerase ε, and lagging-strand DNA polymerase δ. These proteins work together in a spatially and temporally controlled manner to synthesize new DNA from the parental DNA templates. During DNA replication, epigenetic information imprinted on DNA and histone proteins is also copied to the daughter DNA to maintain the chromatin status. DNA methyltransferase 1 is primarily responsible for copying the parental DNA methylation pattern into the nascent DNA. Epigenetic information encoded in histones is transferred via a more complex and less well-understood process termed replication-couple nucleosome assembly. Here, we summarize the most recent structural and biochemical insights into DNA replication initiation, replication fork elongation, chromatin assembly and maintenance, and related regulatory mechanisms.

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.

Evolutionary advantage of anti-parallel strand orientation of duplex DNA

DNA in all living systems shares common properties that are remarkably well suited to its function, suggesting refinement by evolution. However, DNA also shares some counter-intuitive properties which confer no obvious benefit, such as strand directionality and anti-parallel strand orientation, which together result in the complicated lagging strand replication. The evolutionary dynamics that led to these properties of DNA remain unknown but their universality suggests that they confer as yet unknown selective advantage to DNA. In this article, we identify an evolutionary advantage of anti-parallel strand orientation of duplex DNA, within a given set of plausible premises. The advantage stems from the increased rate of replication, achieved by dividing the DNA into predictable, independently and simultaneously replicating segments, as opposed to sequentially replicating the entire DNA, thereby parallelizing the replication process. We show that anti-parallel strand orientation is essential for such a replicative organization of DNA, given our premises, the most important of which is the assumption of the presence of sequence-dependent asymmetric cooperativity in DNA.

Delving into Eukaryotic Origins of Replication Using DNA Structural Features

DNA replication in eukaryotes is an intricate process, which is precisely synchronized by a set of regulatory proteins, and the replication fork emanates from discrete sites on chromatin called origins of replication (Oris). These spots are considered as the gateway to chromosomal replication and are stereotyped by sequence motifs. The cognate sequences are noticeable in a small group of entire origin regions or totally absent across different metazoans. Alternatively, the use of DNA secondary structural features can provide additional information compared to the primary sequence. In this article, we report the trends in DNA sequence-based structural properties of origin sequences in nine eukaryotic systems representing different families of life. Biologically relevant DNA secondary structural properties, namely, stability, propeller twist, flexibility, and minor groove shape were studied in the sequences flanking replication start sites. Results indicate that Oris in yeasts show lower stability, more rigidity, and narrow minor groove preferences compared to genomic sequences surrounding them. Yeast Oris also show preference for A-tracts and the promoter element TATA box in the vicinity of replication start sites. On the contrary, Drosophila melanogaster, humans, and Arabidopsis thaliana do not have such features in their Oris, and instead, they show high preponderance of G-rich sequence motifs such as putative G-quadruplexes or i-motifs and CpG islands. Our extensive study applies the DNA structural feature computation to delve into origins of replication across organisms ranging from yeasts to mammals and including a plant. Insights from this study would be significant in understanding origin architecture and help in designing new algorithms for predicting DNA trans-acting factor recognition events.

Roles for DNA polymerase δ in initiating and terminating leading strand DNA replication

Most current evidence indicates that DNA polymerases ε and δ, respectively, perform the bulk of leading and lagging strand replication of the eukaryotic nuclear genome. Given that ribonucleotide and mismatch incorporation rates by these replicases influence somatic and germline patterns of variation, it is important to understand the details and exceptions to this overall division of labor. Using an improved method to map where these replicases incorporate ribonucleotides during replication, here we present evidence that DNA polymerase δ universally participates in initiating leading strand synthesis and that nascent leading strand synthesis switches from Pol ε to Pol δ during replication termination. Ribonucleotide maps from both the budding and fission yeast reveal conservation of these processes. These observations of replisome dynamics provide important insight into the mechanisms of eukaryotic replication and genome maintenance.

Genome-wide analysis of the spatiotemporal regulation of firing and dormant replication origins in human cells

In metazoan cells, only a limited number of mini chromosome maintenance (MCM) complexes are fired during S phase, while the majority remain dormant. Several methods have been used to map replication origins, but such methods cannot identify dormant origins. Herein, we determined MCM7-binding sites in human cells using ChIP-Seq, classified them into firing and dormant origins using origin data and analysed their association with various chromatin signatures. Firing origins, but not dormant origins, were well correlated with open chromatin regions and were enriched upstream of transcription start sites (TSSs) of transcribed genes. Aggregation plots of MCM7 signals revealed minimal difference in the efficacy of MCM loading between firing and dormant origins. We also analysed common fragile sites (CFSs) and found a low density of origins at these sites. Nevertheless, firing origins were enriched upstream of the TSSs. Based on the results, we propose a model in which excessive MCMs are actively loaded in a genome-wide manner, irrespective of chromatin status, but only a fraction are passively fired in chromatin areas with an accessible open structure, such as regions upstream of TSSs of transcribed genes. This plasticity in the specification of replication origins may minimize collisions between replication and transcription.

Autonomously Replicating Linear Plasmids That Facilitate the Analysis of Replication Origin Function in Candida albicans

Circular plasmids are important tools for molecular manipulation in model fungi such as baker’s yeast, yet, in Candida albicans, an important yeast pathogen of humans, prior studies were not able to generate circular plasmids that were autonomous (duplicated without inserting themselves into the chromosome). Here, we found that linearizing circular plasmids with sequences from telomeres, the chromosome ends, allows the plasmids to duplicate and segregate in C. albicans. We used this system to identify chromosomal sequences that facilitate the initiation of plasmid replication (origins) and to show that an ∼100-bp fragment of a C. albicans origin and an origin sequence from a distantly related yeast can both function as origins in C. albicans. Thus, the requirements for plasmid geometry, but not necessarily for origin sequences, differ between C. albicans and baker’s yeast.

The ability to generate autonomously replicating plasmids has been elusive in Candida albicans, a prevalent human fungal commensal and pathogen. Instead, plasmids generally integrate into the genome. Here, we assessed plasmid and transformant properties, including plasmid geometry, transformant colony size, four selectable markers, and potential origins of replication, for their ability to drive autonomous plasmid maintenance. Importantly, linear plasmids with terminal telomere repeats yielded many more autonomous transformants than circular plasmids with the identical sequences. Furthermore, we could distinguish (by colony size) transient, autonomously replicating, and chromosomally integrated transformants (tiny, medium, and large, respectively). Candida albicansURA3 and a heterologous marker, ARG4, yielded many transient transformants indicative of weak origin activity; the replication of the plasmid carrying the heterologous LEU2 marker was highly dependent upon the addition of a bona fide origin sequence. Several bona fide chromosomal origins, with an origin fragment of ∼100 bp as well as a heterologous origin, panARS, from Kluyveromyces lactis, drove autonomous replication, yielding moderate transformation efficiency and plasmid stability. Thus, C. albicans maintains linear plasmids that yield high transformation efficiency and are maintained autonomously in an origin-dependent manner.

IMPORTANCE Circular plasmids are important tools for molecular manipulation in model fungi such as baker’s yeast, yet, in Candida albicans, an important yeast pathogen of humans, prior studies were not able to generate circular plasmids that were autonomous (duplicated without inserting themselves into the chromosome). Here, we found that linearizing circular plasmids with sequences from telomeres, the chromosome ends, allows the plasmids to duplicate and segregate in C. albicans. We used this system to identify chromosomal sequences that facilitate the initiation of plasmid replication (origins) and to show that an ∼100-bp fragment of a C. albicans origin and an origin sequence from a distantly related yeast can both function as origins in C. albicans. Thus, the requirements for plasmid geometry, but not necessarily for origin sequences, differ between C. albicans and baker’s yeast.

Dynamics of DNA replication in a eukaryotic cell

Each genomic locus in a eukaryotic cell has a distinct average time of replication during S phase that depends on the spatial and temporal pattern of replication initiation events. Replication timing can affect genomic integrity because late replication is associated with an increased mutation rate. For most eukaryotes, the features of the genome that specify the location and timing of initiation events are unknown. To investigate these features for the fission yeast, Schizosaccharomyces pombe, we developed an integrative model to analyze large single-molecule and global genomic datasets. The model provides an accurate description of the complex dynamics of S. pombe DNA replication at high resolution. We present evidence that there are many more potential initiation sites in the S. pombe genome than previously identified and that the distribution of these sites is primarily determined by two factors: the sequence preferences of the origin recognition complex (ORC), and the interference of transcription with the assembly or stability of prereplication complexes (pre-RCs). We suggest that in addition to directly interfering with initiation, transcription has driven the evolution of the binding properties of ORC in S. pombe and other eukaryotic species to target pre-RC assembly to regions of the genome that are less likely to be transcribed.

Identification and characterization of ARS-like sequences as putative origin(s) of replication in human malaria parasite Plasmodium falciparum

DNA replication is a fundamental process in genome maintenance, and initiates from several genomic sites (origins) in eukaryotes. In Saccharomyces cerevisiae, conserved sequences known as autonomously replicating sequences (ARSs) provide a landing pad for the origin recognition complex (ORC), leading to replication initiation. Although origins from higher eukaryotes share some common sequence features, the definitive genomic organization of these sites remains elusive. The human malaria parasite Plasmodium falciparum undergoes multiple rounds of DNA replication; therefore, control of initiation events is crucial to ensure proper replication. However, the sites of DNA replication initiation and the mechanism by which replication is initiated are poorly understood. Here, we have identified and characterized putative origins in P. falciparum by bioinformatics analyses and experimental approaches. An autocorrelation measure method was initially used to search for regions with marked fluctuation (dips) in the chromosome, which we hypothesized might contain potential origins. Indeed, S. cerevisiae ARS consensus sequences were found in dip regions. Several of these P. falciparum sequences were validated with chromatin immunoprecipitation-quantitative PCR, nascent strand abundance and a plasmid stability assay. Subsequently, the same sequences were used in yeast to confirm their potential as origins in vivo. Our results identify the presence of functional ARSs in P. falciparum and provide meaningful insights into replication origins in these deadly parasites. These data could be useful in designing transgenic vectors with improved stability for transfection in P. falciparum.

Review of stochastic hybrid systems with applications in biological systems modeling and analysis

Stochastic hybrid systems (SHS) have attracted a lot of research interests in recent years. In this paper, we review some of the recent applications of SHS to biological systems modeling and analysis. Due to the nature of molecular interactions, many biological processes can be conveniently described as a mixture of continuous and discrete phenomena employing SHS models. With the advancement of SHS theory, it is expected that insights can be obtained about biological processes such as drug effects on gene regulation. Furthermore, combining with advanced experimental methods, in silico simulations using SHS modeling techniques can be carried out for massive and rapid verification or falsification of biological hypotheses. The hope is to substitute costly and time-consuming in vitro or in vivo experiments or provide guidance for those experiments and generate better hypotheses.

Single-molecule analysis reveals that DNA replication dynamics vary across the course of schizogony in the malaria parasite Plasmodium falciparum

The mechanics of DNA replication and cell cycling are well-characterized in model organisms, but less is known about these basic aspects of cell biology in early-diverging Apicomplexan parasites, which do not divide by canonical binary fission but undergo unconventional cycles. Schizogony in the malaria parasite, Plasmodium, generates ~16–24 new nuclei via independent, asynchronous rounds of genome replication prior to cytokinesis and little is known about the control of DNA replication that facilitates this. We have characterised replication dynamics in P. falciparum throughout schizogony, using DNA fibre labelling and combing to visualise replication forks at a single-molecule level. We show that origins are very closely spaced in Plasmodium compared to most model systems, and that replication dynamics vary across the course of schizogony, from faster synthesis rates and more widely-spaced origins through to slower synthesis rates and closer-spaced origins. This is the opposite of the pattern usually seen across S-phase in human cells, when a single genome is replicated. Replication forks also appear to stall at an unusually high rate throughout schizogony. Our work explores Plasmodium DNA replication in unprecedented detail and opens up tremendous scope for analysing cell cycle dynamics and developing interventions targetting this unique aspect of malaria biology.

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.

Molecular Mechanism for Chromatin Regulation During MCM Loading in Mammalian Cells

DNA replication is a fundamental process required for the accurate and timely duplication of chromosomes. During late mitosis to G1 phase, the MCM2-7 complex is loaded onto chromatin in a manner dependent on ORC, CDC6, and Cdt1, and chromatin becomes licensed for replication. Although every eukaryotic organism shares common features in replication control, there are also some differences among species. For example, in higher eukaryotic cells including human cells, no strict sequence specificity has been observed for replication origins, unlike budding yeast or bacterial replication origins. Therefore, elements other than beyond DNA sequences are important for regulating replication. For example, the stability and precise positioning of nucleosomes affects replication control. However, little is known about how nucleosome structure is regulated when replication licensing occurs. During the last decade, histone acetylation enzyme HBO1, chromatin remodeler SNF2H, and histone chaperone GRWD1 have been identified as chromatin-handling factors involved in the promotion of replication licensing. In this review, we discuss how the rearrangement of nucleosome formation by these factors affects replication licensing.

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.

Chromosomal context and replication properties of ARS plasmids in Schizosaccharomyces pombe

Short, specific DNA sequences called as Autonomously Replicating Sequence (ARS) elements function as plasmid as well as chromosomal replication origins in yeasts. As compared to ARSs, different chromosomal origins vary greatly in their efficiency and timing of replication probably due to their wider chromosomal context. The two Schizosaccharomyces pombe ARS elements, ars727 and ars2004, represent two extremities in their chromosomal origin activity - ars727 is inactive and late replicating, while ars2004 is a highly active, early-firing origin. To determine the effect of chromosomal context on the activity of these ARS elements, we have cloned them with their extended chromosomal context as well as in the context of each other in both orientations and analysed their replication efficiency by ARS and plasmid stability assays. We found that these ARS elements retain their origin activity in their extended/altered context. However, deletion of a 133-bp region of the previously reported ars727- associated late replication enforcing element (LRE) caused advancement in replication timing of the resulting plasmid. These results confirm the role of LRE in directing plasmid replication timing and suggest that the plasmid origin efficiency of ars2004 or ars727 remains unaltered by the extended chromosomal context.

DNA sequence templates adjacent nucleosome and ORC sites at gene amplification origins in Drosophila

Eukaryotic origins of DNA replication are bound by the origin recognition complex (ORC), which scaffolds assembly of a pre-replicative complex (pre-RC) that is then activated to initiate replication. Both pre-RC assembly and activation are strongly influenced by developmental changes to the epigenome, but molecular mechanisms remain incompletely defined. We have been examining the activation of origins responsible for developmental gene amplification in Drosophila. At a specific time in oogenesis, somatic follicle cells transition from genomic replication to a locus-specific replication from six amplicon origins. Previous evidence indicated that these amplicon origins are activated by nucleosome acetylation, but how this affects origin chromatin is unknown. Here, we examine nucleosome position in follicle cells using micrococcal nuclease digestion with Ilumina sequencing. The results indicate that ORC binding sites and other essential origin sequences are nucleosome-depleted regions (NDRs). Nucleosome position at the amplicons was highly similar among developmental stages during which ORC is or is not bound, indicating that being an NDR is not sufficient to specify ORC binding. Importantly, the data suggest that nucleosomes and ORC have opposite preferences for DNA sequence and structure. We propose that nucleosome hyperacetylation promotes pre-RC assembly onto adjacent DNA sequences that are disfavored by nucleosomes but favored by ORC.

The spatial and temporal organization of origin firing during the S-phase of fission yeast

Eukaryotes duplicate their genomes using multiple replication origins, but the organization of origin firing along chromosomes and during S-phase is not well understood. Using fission yeast, we report the first genome-wide analysis of the spatial and temporal organization of replication origin firing, analyzed using single DNA molecules that can approach the full length of chromosomes. At S-phase onset, origins fire randomly and sparsely throughout the chromosomes. Later in S-phase, clusters of fired origins appear embedded in the sparser regions, which form the basis of nuclear replication foci. The formation of clusters requires proper histone methylation and acetylation, and their locations are not inherited between cell cycles. The rate of origin firing increases gradually, peaking just before mid S-phase. Toward the end of S-phase, nearly all the available origins within the unreplicated regions are fired, contributing to the timely completion of genome replication. We propose that the majority of origins do not fire as a part of a deterministic program. Instead, origin firing, both individually and as clusters, should be viewed as being mostly stochastic.

Possible role of H1 histone in replication timing

AT-rich repetitive DNA sequences become late replicating during cell differentiation. Replication timing is not correlated with LINE density in human cells (Ryba et al. 2010). However, short and properly spaced runs of oligo dA or dT present in nuclear matrix attachment regions (MARs) of the genome are good candidates for elements of AT-rich repetitive late replicating DNA. MAR attachment to the nuclear matrix is negatively regulated by chromatin binding of H1 histone, but this is counteracted by H1 phosphorylation, high mobility group proteins or, indirectly, core histone acetylation. Fewer MAR attachments correlates positively with longer average DNA loop size, longer replicons and an increase of late replicating DNA.

Replicon: a software to accurately predict DNA replication timing in metazoan cells

Eukaryotic DNA replication follows a strict temporal program where genomic loci are replicated at precise times during the S phase of the cell cycle. Yet, the mechanism in control of the timing program in metazoan cells is poorly understood. In a recent publication, the authors proposed an intuitive stochastic model of DNA replication and showed that it predicts replication timing with an accuracy approaching the level of experimental biological repeats. Here, we discuss an extended software implementation of the mechanistic model: Replicon. This package allows interested researchers to predict the global replication timing program in human cells from chromatin data.

Homopolymer tract organization in the human malarial parasite Plasmodium falciparum and related Apicomplexan parasites

BACKGROUND

Homopolymeric tracts, particularly poly dA.dT, are enriched within the intergenic sequences of eukaryotic genomes where they appear to act as intrinsic regulators of nucleosome positioning. A previous study of the incomplete genome of the human malarial parasite Plasmodium falciparum reports a higher than expected enrichment of poly dA.dT tracts, far above that anticipated even in this highly AT rich genome. Here we report an analysis of the relative frequency, length and spatial arrangement of homopolymer tracts for the complete P. falciparum genome, extending this analysis to twelve additional genomes of Apicomplexan parasites important to human and animal health. In addition, using nucleosome-positioning data available for P. falciparum, we explore the correlation of poly dA.dT tracts with nucleosome-positioning data over key expression landmarks within intergenic regions.

RESULTS

We describe three apparent lineage-specific patterns of homopolymeric tract organization within the intergenic regions of these Apicomplexan parasites. Moreover, a striking pattern of enrichment of overly long poly dA.dT tracts in the intergenic regions of Plasmodium spp. uniquely extends into protein coding sequences. There is a conserved spatial arrangement of poly dA.dT immediately flanking open reading frames and over predicted core promoter sites. These key landmarks are all relatively depleted in nucleosomes in P. falciparum, as would be expected for poly dA.dT acting as nucleosome exclusion sequences.

CONCLUSIONS

Previous comparative studies of homopolymer tract organization emphasize evolutionary diversity; this is the first report of such an analysis within a single phylum. Our data provide insights into the evolution of homopolymeric tracts and the selective pressures at play in their maintenance and expansion.

Origin replication complex binding, nucleosome depletion patterns, and a primary sequence motif can predict origins of replication in a genome with epigenetic centromeres

Origins of DNA replication are key genetic elements, yet their identification remains elusive in most organisms. In previous work, we found that centromeres contain origins of replication (ORIs) that are determined epigenetically in the pathogenic yeast Candida albicans. In this study, we used origin recognition complex (ORC) binding and nucleosome occupancy patterns in Saccharomyces cerevisiae and Kluyveromyces lactis to train a machine learning algorithm to predict the position of active arm (noncentromeric) origins in the C. albicans genome. The model identified bona fide active origins as determined by the presence of replication intermediates on nondenaturing two-dimensional (2D) gels. Importantly, these origins function at their native chromosomal loci and also as autonomously replicating sequences (ARSs) on a linear plasmid. A “mini-ARS screen” identified at least one and often two ARS regions of ≥100 bp within each bona fide origin. Furthermore, a 15-bp AC-rich consensus motif was associated with the predicted origins and conferred autonomous replicating activity to the mini-ARSs. Thus, while centromeres and the origins associated with them are epigenetic, arm origins are dependent upon critical DNA features, such as a binding site for ORC and a propensity for nucleosome exclusion.

DNA replication machinery is highly conserved, yet the definition of exactly what specifies a replication origin differs in different species. Here, we utilized computational genomics to predict origin locations in Candida albicans by combining locations of binding sites for the conserved origin replication complex, necessary for replication initiation, together with chromatin organization patterns. We identified predicted sequences that exhibited bona fide origin function and developed a linear plasmid assay to delimit the DNA fragments necessary for origin function. Additionally, we found that a short AC-rich motif, which is enriched in predicted origins, is required for origin function. Thus, we demonstrated a new machine learning paradigm for identification of potential origins from a genome with no prior information. Furthermore, this work suggests that C. albicans has two different types of origins: “hard-wired” arm origins that rely upon specific sequence motifs and “epigenetic” centromeric origins that are recruited to kinetochores in a sequence-independent manner.

GC-rich DNA elements enable replication origin activity in the methylotrophic yeast Pichia pastoris

The well-studied DNA replication origins of the model budding and fission yeasts are A/T-rich elements. However, unlike their yeast counterparts, both plant and metazoan origins are G/C-rich and are associated with transcription start sites. Here we show that an industrially important methylotrophic budding yeast, Pichia pastoris, simultaneously employs at least two types of replication origins--a G/C-rich type associated with transcription start sites and an A/T-rich type more reminiscent of typical budding and fission yeast origins. We used a suite of massively parallel sequencing tools to map and dissect P. pastoris origins comprehensively, to measure their replication dynamics, and to assay the global positioning of nucleosomes across the genome. Our results suggest that some functional overlap exists between promoter sequences and G/C-rich replication origins in P. pastoris and imply an evolutionary bifurcation of the modes of replication initiation.

[Regulation of DNA replication timing]

Although distinct chromatin types have been long known to replicate at different timepoints of S phase, fine replication control has only recently become considered as an epigenetic phenomenon. It is now clear that in course of differentiation significant changes in genome replication timing occur, and these changes are intimately linked with the changes in transcriptional activity and nuclear architecture. Temporally coordinate replication is organized spatially into discrete units having specific chromosomal organization and function. Even though the functional aspects of such tight control of replication timing remain to be explored, one can confidently consider the replication program as yet another fundamental feature characteristic of the given differentiation state. The present review touches upon the molecular mechanisms of spatial and temporal control of replication timing, involving individual replication origins as well as large chromatin domains.

Back to the origin: reconsidering replication, transcription, epigenetics, and cell cycle control

In bacteria, replication is a carefully orchestrated event that unfolds the same way for each bacterium and each cell division. The process of DNA replication in bacteria optimizes cell growth and coordinates high levels of simultaneous replication and transcription. In metazoans, the organization of replication is more enigmatic. The lack of a specific sequence that defines origins of replication has, until recently, severely limited our ability to define the organizing principles of DNA replication. This question is of particular importance as emerging data suggest that replication stress is an important contributor to inherited genetic damage and the genomic instability in tumors. We consider here the replication program in several different organisms including recent genome-wide analyses of replication origins in humans. We review recent studies on the role of cytosine methylation in replication origins, the role of transcriptional looping and gene gating in DNA replication, and the role of chromatin's 3-dimensional structure in DNA replication. We use these new findings to consider several questions surrounding DNA replication in metazoans: How are origins selected? What is the relationship between replication and transcription? How do checkpoints inhibit origin firing? Why are there early and late firing origins? We then discuss whether oncogenes promote cancer through a role in DNA replication and whether errors in DNA replication are important contributors to the genomic alterations and gene fusion events observed in cancer. We conclude with some important areas for future experimentation.

Trapping DNA replication origins from the human genome

Synthesis of chromosomal DNA is initiated from multiple origins of replication in higher eukaryotes; however, little is known about these origins’ structures. We isolated the origin-derived nascent DNAs from a human repair-deficient cell line by blocking the replication forks near the origins using two different origin-trapping methods (i.e., UV- or chemical crosslinker-treatment and cell synchronization in early S phase using DNA replication inhibitors). Single-stranded DNAs (of 0.5–3 kb) that accumulated after such treatments were labeled with bromodeoxyuridine (BrdU). BrdU-labeled DNA was immunopurified after fractionation by alkaline sucrose density gradient centrifugation and cloned by complementary-strand synthesis and PCR amplification. Competitive PCR revealed an increased abundance of DNA derived from known replication origins (c-myc and lamin B2 genes) in the nascent DNA fractions from the UV-treated or crosslinked cells. Nucleotide sequences of 85 and 208 kb were obtained from the two libraries (I and II) prepared from the UV-treated log-phase cells and early S phase arrested cells, respectively. The libraries differed from each other in their G+C composition and replication-related motif contents, suggesting that differences existed between the origin fragments isolated by the two different origin-trapping methods. The replication activities for seven out of 12 putative origin loci from the early-S phase cells were shown by competitive PCR. We mapped 117 (library I) and 172 (library II) putative origin loci to the human genome; approximately 60% and 50% of these loci were assigned to the G-band and intragenic regions, respectively. Analyses of the flanking sequences of the mapped loci suggested that the putative origin loci tended to associate with genes (including conserved sites) and DNase I hypersensitive sites; however, poor correlations were found between such loci and the CpG islands, transcription start sites, and K27-acetylated histone H3 peaks.

Location, location, location: it's all in the timing for replication origins

The differential replication timing of eukaryotic replication origins has long been linked with epigenetic regulation of gene expression and more recently with genome stability and mutation rates; however, the mechanism has remained obscure. Recent studies have shed new light by identifying novel factors that determine origin timing in yeasts and mammalian cells and implicate the spatial organization of origins within nuclear territories in the mechanism. These new insights, along with recent findings that several initiation factors are limiting relative to licensed origins, support and shape an emerging model for replication timing control. The mechanisms that control the spatial organization of replication origins have potential impacts for genome regulation beyond replication.

Identification of two telomere-proximal fission yeast DNA replication origins constrained by nearby cis-acting sequences to replicate in late S phase

Telomeres of the fission yeast,  Schizosaccharomyces pombe, are known to replicate in late S phase, but the reasons for this late replication are not fully understood. We have identified two closely-spaced DNA replication origins, 5.5 to 8 kb upstream from the telomere itself. These are the most telomere-proximal of all the replication origins in the fission yeast genome. When located by themselves in circular plasmids, these origins fired in early S phase, but if flanking sequences closer to the telomere were included in the circular plasmid, then replication was restrained to late S phase - except in cells lacking the replication-checkpoint kinase, Cds1. We conclude that checkpoint-dependent late replication of telomere-associated sequences is dependent on nearby cis-acting sequences, not on proximity to the physical end of a linear chromosome.

Analysis of stress-induced duplex destabilization (SIDD) properties of replication origins, genes and intergenes in the fission yeast, Schizosaccharomyces pombe

Background

Replication and transcription, the two key functions of DNA, require unwinding of the DNA double helix. It has been shown that replication origins in the budding yeast, Saccharomyces cerevisiae contain an easily unwound stretch of DNA. We have used a recently developed method for determining the locations and degrees of stress-induced duplex destabilization (SIDD) for all the reported replication origins in the genome of the fission yeast, Schizosaccharomyces pombe.

Results

We have found that the origins are more susceptible to SIDD as compared to the non-origin intergenic regions (NOIRs) and genes. SIDD analysis of many known origins in other eukaryotes suggests that SIDD is a common property of replication origins. Interestingly, the previously shown deletion-dependent changes in the activities of the origins of the ura4 origin region on chromosome 3 are paralleled by changes in SIDD properties, suggesting SIDD’s role in origin activity. SIDD profiling following in silico deletions of some origins suggests that many of the closely spaced S. pombe origins could be clusters of two or three weak origins, similar to the ura4 origin region.

Conclusion

SIDD appears to be a highly conserved, functionally important property of replication origins in S. pombe and other organisms. The distinctly low SIDD scores of origins and the long range effects of genetic alterations on SIDD properties provide a unique predictive potential to the SIDD analysis. This could be used in exploring different aspects of structural and functional organization of origins including interactions between closely spaced origins.

Genome-wide analysis reveals extensive functional interaction between DNA replication initiation and transcription in the genome of Trypanosoma brucei

Identification of replication initiation sites, termed origins, is a crucial step in understanding genome transmission in any organism. Transcription of the Trypanosoma brucei genome is highly unusual, with each chromosome comprising a few discrete transcription units. To understand how DNA replication occurs in the context of such organization, we have performed genome-wide mapping of the binding sites of the replication initiator ORC1/CDC6 and have identified replication origins, revealing that both localize to the boundaries of the transcription units. A remarkably small number of active origins is seen, whose spacing is greater than in any other eukaryote. We show that replication and transcription in T. brucei have a profound functional overlap, as reducing ORC1/CDC6 levels leads to genome-wide increases in mRNA levels arising from the boundaries of the transcription units. In addition, ORC1/CDC6 loss causes derepression of silent Variant Surface Glycoprotein genes, which are critical for host immune evasion.

Genome-wide identification and characterization of replication origins by deep sequencing

Background

DNA replication initiates at distinct origins in eukaryotic genomes, but the genomic features that define these sites are not well understood.

Results

We have taken a combined experimental and bioinformatic approach to identify and characterize origins of replication in three distantly related fission yeasts: Schizosaccharomyces pombe, Schizosaccharomyces octosporus and Schizosaccharomyces japonicus. Using single-molecule deep sequencing to construct amplification-free high-resolution replication profiles, we located origins and identified sequence motifs that predict origin function. We then mapped nucleosome occupancy by deep sequencing of mononucleosomal DNA from the corresponding species, finding that origins tend to occupy nucleosome-depleted regions.

Conclusions

The sequences that specify origins are evolutionarily plastic, with low complexity nucleosome-excluding sequences functioning in S. pombe and S. octosporus, and binding sites for trans-acting nucleosome-excluding proteins functioning in S. japonicus. Furthermore, chromosome-scale variation in replication timing is conserved independently of origin location and via a mechanism distinct from known heterochromatic effects on origin function. These results are consistent with a model in which origins are simply the nucleosome-depleted regions of the genome with the highest affinity for the origin recognition complex. This approach provides a general strategy for understanding the mechanisms that define DNA replication origins in eukaryotes.

Polo-like kinase 1 (Plk1): an Unexpected Player in DNA Replication

Regulation of cell cycle progression is important for the maintenance of genome integrity, and Polo-like kinases (Plks) have been identified as key regulators of this process. It is well established that Polo-like kinase 1 (Plk1) plays critical roles in mitosis but little is known about its functions at other stages of the cell cycle. Here we summarize the functions of Plk1 during DNA replication, focusing on the molecular events related to Origin Recognition Complex (ORC), the complex that is essential for the initiation of DNA replication. Within the context of Plk1 phosphorylation of Orc2, we also emphasize regulation of Orc2 in different organisms. This review is intended to provide some insight into how Plk1 coordinates DNA replication in S phase with chromosome segregation in mitosis, and orchestrates the cell cycle as a whole.

Kaposi's Sarcoma-Associated Herpesvirus Genome Replication, Partitioning, and Maintenance in Latency

Kaposi's sarcoma-associated herpesvirus (KSHV) is thought to be an oncogenic member of the γ-herpesvirus subfamily. The virus usually establishes latency upon infection as a default infection pattern. The viral genome replicates according to the host cell cycle by recruiting the host cellular replication machinery. Among the latently expressing viral factors, LANA plays pivotal roles in viral genome replication, partitioning, and maintenance. LANA binds with two LANA-binding sites (LBS1/2) within a terminal repeat (TR) sequence and is indispensable for viral genome replication in latency. The nuclear matrix region seems to be important as a replication site, since LANA as well as cellular replication factors accumulate there and recruit the viral replication origin in latency (ori-P) by its binding activity to LBS. KSHV ori-P consists of LBS followed by a 32-bp GC-rich segment (32GC). Although it has been reported that LANA recruits cellular pre-replication complexes (pre-RC) such as origin recognition complexes (ORCs) to the ori-P through its interaction with ORCs, this mechanism does not account completely for the requirement of the 32GC. On the other hand, there are few reports about the partitioning and maintenance of the viral genome. LANA interacts with many kinds of chromosomal proteins, including Brd2/RING3, core histones, such as H2A/H2B and histone H1, and so on. The detailed molecular mechanisms by which LANA enables KSHV genome partitioning and maintenance still remain obscure. By integrating the findings reported thus far on KSHV genome replication, partitioning, and maintenance in latency, we will summarize what we know now, discuss what questions remain to be answered, and determine what needs to be done next to understand the mechanisms underlying viral replication, partitioning, and maintenance strategy.

Novel features of ARS selection in budding yeast Lachancea kluyveri

BACKGROUND

The characterization of DNA replication origins in yeast has shed much light on the mechanisms of initiation of DNA replication. However, very little is known about the evolution of origins or the evolution of mechanisms through which origins are recognized by the initiation machinery. This lack of understanding is largely due to the vast evolutionary distances between model organisms in which origins have been examined.

RESULTS

In this study we have isolated and characterized autonomously replicating sequences (ARSs) in Lachancea kluyveri - a pre-whole genome duplication (WGD) budding yeast. Through a combination of experimental work and rigorous computational analysis, we show that L. kluyveri ARSs require a sequence that is similar but much longer than the ARS Consensus Sequence well defined in Saccharomyces cerevisiae. Moreover, compared with S. cerevisiae and K. lactis, the replication licensing machinery in L. kluyveri seems more tolerant to variations in the ARS sequence composition. It is able to initiate replication from almost all S. cerevisiae ARSs tested and most Kluyveromyces lactis ARSs. In contrast, only about half of the L. kluyveri ARSs function in S. cerevisiae and less than 10% function in K. lactis.

CONCLUSIONS

Our findings demonstrate a replication initiation system with novel features and underscore the functional diversity within the budding yeasts. Furthermore, we have developed new approaches for analyzing biologically functional DNA sequences with ill-defined motifs.

The major roles of DNA polymerases epsilon and delta at the eukaryotic replication fork are evolutionarily conserved

Coordinated replication of eukaryotic genomes is intrinsically asymmetric, with continuous leading strand synthesis preceding discontinuous lagging strand synthesis. Here we provide two types of evidence indicating that, in fission yeast, these two biosynthetic tasks are performed by two different replicases. First, in Schizosaccharomyces pombe strains encoding a polδ-L591M mutator allele, base substitutions in reporter genes placed in opposite orientations relative to a well-characterized replication origin are strand-specific and distributed in patterns implying that Polδ is primarily involved in lagging strand replication. Second, in strains encoding a polε-M630F allele and lacking the ability to repair rNMPs in DNA due to a defect in RNase H2, rNMPs are selectively observed in nascent leading strand DNA. The latter observation demonstrates that abundant rNMP incorporation during replication can be tolerated and that they are normally removed in an RNase H2-dependent manner. This provides strong physical evidence that Polε is the primary leading strand replicase. Collectively, these data and earlier results in budding yeast indicate that the major roles of Polδ and Polε at the eukaryotic replication fork are evolutionarily conserved.

Genome-wide depletion of replication initiation events in highly transcribed regions

This report investigates the mechanisms by which mammalian cells coordinate DNA replication with transcription and chromatin assembly. In yeast, DNA replication initiates within nucleosome-free regions, but studies in mammalian cells have not revealed a similar relationship. Here, we have used genome-wide massively parallel sequencing to map replication initiation events, thereby creating a database of all replication initiation sites within nonrepetitive DNA in two human cell lines. Mining this database revealed that genomic regions transcribed at moderate levels were generally associated with high replication initiation frequency. In genomic regions with high rates of transcription, very few replication initiation events were detected. High-resolution mapping of replication initiation sites showed that replication initiation events were absent from transcription start sites but were highly enriched in adjacent, downstream sequences. Methylation of CpG sequences strongly affected the location of replication initiation events, whereas histone modifications had minimal effects. These observations suggest that high levels of transcription interfere with formation of pre-replication protein complexes. Data presented here identify replication initiation sites throughout the genome, providing a foundation for further analyses of DNA-replication dynamics and cell-cycle progression.

Defining the replication program through the chromatin landscape

DNA replication is an essential cell cycle event required for the accurate and timely duplication of the chromosomes. It is essential that the genome is replicated accurately and completely within the confines of S-phase. Failure to completely copy the genome has the potential to result in catastrophic genomic instability. Replication initiates in a coordinated manner from multiple locations, termed origins of replication, distributed across each of the chromosomes. The selection of these origins of replication is a dynamic process responding to both developmental and tissue-specific signals. In this review, we explore the role of the local chromatin environment in regulating the DNA replication program at the level of origin selection and activation. Finally, there is increasing molecular evidence that the DNA replication program itself affects the chromatin landscape, suggesting that DNA replication is critical for both genetic and epigenetic inheritance.

Genome-scale analysis of metazoan replication origins reveals their organization in specific but flexible sites defined by conserved features

In metazoans, thousands of DNA replication origins (Oris) are activated at each cell cycle. Their genomic organization and their genetic nature remain elusive. Here, we characterized Oris by nascent strand (NS) purification and a genome-wide analysis in Drosophila and mouse cells. We show that in both species most CpG islands (CGI) contain Oris, although methylation is nearly absent in Drosophila, indicating that this epigenetic mark is not crucial for defining the activated origin. Initiation of DNA synthesis starts at the borders of CGI, resulting in a striking bimodal distribution of NS, suggestive of a dual initiation event. Oris contain a unique nucleotide skew around NS peaks, characterized by G/T and C/A overrepresentation at the 5' and 3' of Ori sites, respectively. Repeated GC-rich elements were detected, which are good predictors of Oris, suggesting that common sequence features are part of metazoan Oris. In the heterochromatic chromosome 4 of Drosophila, Oris correlated with HP1 binding sites. At the chromosome level, regions rich in Oris are early replicating, whereas Ori-poor regions are late replicating. The genome-wide analysis was coupled with a DNA combing analysis to unravel the organization of Oris. The results indicate that Oris are in a large excess, but their activation does not occur at random. They are organized in groups of site-specific but flexible origins that define replicons, where a single origin is activated in each replicon. This organization provides both site specificity and Ori firing flexibility in each replicon, allowing possible adaptation to environmental cues and cell fates.

The DNA damage response pathway contributes to the stability of chromosome III derivatives lacking efficient replicators

In eukaryotic chromosomes, DNA replication initiates at multiple origins. Large inter-origin gaps arise when several adjacent origins fail to fire. Little is known about how cells cope with this situation. We created a derivative of Saccharomyces cerevisiae chromosome III lacking all efficient origins, the 5ORIΔ-ΔR fragment, as a model for chromosomes with large inter-origin gaps. We used this construct in a modified synthetic genetic array screen to identify genes whose products facilitate replication of long inter-origin gaps. Genes identified are enriched in components of the DNA damage and replication stress signaling pathways. Mrc1p is activated by replication stress and mediates transduction of the replication stress signal to downstream proteins; however, the response-defective mrc1^AQ allele did not affect 5ORIΔ-ΔR fragment maintenance, indicating that this pathway does not contribute to its stability. Deletions of genes encoding the DNA-damage-specific mediator, Rad9p, and several components shared between the two signaling pathways preferentially destabilized the 5ORIΔ-ΔR fragment, implicating the DNA damage response pathway in its maintenance. We found unexpected differences between contributions of components of the DNA damage response pathway to maintenance of ORIΔ chromosome derivatives and their contributions to DNA repair. Of the effector kinases encoded by RAD53 and CHK1, Chk1p appears to be more important in wild-type cells for reducing chromosomal instability caused by origin depletion, while Rad53p becomes important in the absence of Chk1p. In contrast, RAD53 plays a more important role than CHK1 in cell survival and replication fork stability following treatment with DNA damaging agents and hydroxyurea. Maintenance of ORIΔ chromosomes does not depend on homologous recombination. These observations suggest that a DNA-damage-independent mechanism enhances ORIΔ chromosome stability. Thus, components of the DNA damage response pathway contribute to genome stability, not simply by detecting and responding to DNA template damage, but also by facilitating replication of large inter-origin gaps.

Loss of genome integrity underlies aspects of aging and human disease. During DNA replication, two parallel signaling pathways play important roles in the maintenance of genome integrity. One pathway detects DNA damage, while the other senses replication stress. Both pathways activate responses that include arrest of cell cycle progression, giving cells time to cope with the problem. These pathways have been defined by treating cells with compounds that induce either replication stress or DNA damage, but little is known about their roles during unperturbed DNA replication. They may be important when several adjacent replication origins fail to initiate and forks from flanking origins must replicate longer regions of DNA than normal to complete replication. We have used a derivative of budding yeast chromosome III lacking all efficient replication origins to identify mutants that preferentially destabilize this chromosome fragment, which mimics a chromosome with a large inter-origin gap. We found that the DNA damage response pathway, but not the replication stress response pathway, plays an important role in maintaining this fragment. The signal recognized in this case may be replisome failure rather than forks stalled at endogenous DNA damage.

Evaluating genome-scale approaches to eukaryotic DNA replication

Mechanisms regulating where and when eukaryotic DNA replication initiates remain a mystery. Recently, genome-scale methods have been brought to bear on this problem. The identification of replication origins and their associated proteins in yeasts is a well-integrated investigative tool, but corresponding data sets from multicellular organisms are scarce. By contrast, standardized protocols for evaluating replication timing have generated informative data sets for most eukaryotic systems. Here, I summarize the genome-scale methods that are most frequently used to analyse replication in eukaryotes, the kinds of questions each method can address and the technical hurdles that must be overcome to gain a complete understanding of the nature of eukaryotic replication origins.