Genome-wide nucleosome and transcription factor responses to genetic perturbations reveal chromatin-mediated mechanisms of transcriptional regulation.

Epigenetic mechanisms contribute to gene regulation by altering chromatin accessibility through changes in transcription factor (TF) and nucleosome occupancy throughout the genome. Despite numerous studies focusing on changes in gene expression, the intricate chromatin-mediated regulatory code remains largely unexplored on a comprehensive scale. We address this by employing a factor-agnostic, reverse-genetics approach that uses MNase-seq to capture genome-wide TF and nucleosome occupancies in response to the individual deletion of 201 transcriptional regulators in , thereby assaying nearly one million mutant-gene interactions.

Spatiotemporal kinetics of CAF-1-dependent chromatin maturation ensures transcription fidelity during S-phase.

Proper maintenance of epigenetic information after replication is dependent on the rapid assembly and maturation of chromatin. Chromatin Assembly Complex 1 (CAF-1) is a conserved histone chaperone that deposits (H3-H4) tetramers as part of the replication-dependent chromatin assembly process. Loss of CAF-1 leads to a delay in chromatin maturation, albeit with minimal impact on steady-state chromatin structure.

Spatiotemporal kinetics of CAF-1-dependent chromatin maturation ensures transcription fidelity during S-phase.

Proper maintenance of epigenetic information after replication is dependent on the rapid assembly and maturation of chromatin. Chromatin Assembly Complex 1 (CAF-1) is a conserved histone chaperone that deposits (H3-H4) tetramers as part of the replication-dependent chromatin assembly process. Loss of CAF-1 leads to a delay in chromatin maturation, albeit with minimal impact on steady-state chromatin structure.

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 .

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.

Stochastic initiation of DNA replication across the human genome.

Wang et al. (2021) comprehensively map DNA replication initiation events across the human genome using single-molecule optical resolution mapping and find that initiation events are randomly distributed across broad initiation zones that are only utilized in a stochastic fashion across a population of cells.

RoboCOP: jointly computing chromatin occupancy profiles for numerous factors from chromatin accessibility data.

Chromatin is a tightly packaged structure of DNA and protein within the nucleus of a cell. The arrangement of different protein complexes along the DNA modulates and is modulated by gene expression. Measuring the binding locations and occupancy levels of different transcription factors (TFs) and nucleosomes is therefore crucial to understanding gene regulation.

Linking the dynamics of chromatin occupancy and transcription with predictive models.

Though the sequence of the genome within each eukaryotic cell is essentially fixed, it exists within a complex and changing chromatin state. This state is determined, in part, by the dynamic binding of proteins to the DNA. These proteins-including histones, transcription factors (TFs), and polymerases-interact with one another, the genome, and other molecules to allow the chromatin to adopt one of exceedingly many possible configurations.

Local nucleosome dynamics and eviction following a double-strand break are reversible by NHEJ-mediated repair in the absence of DNA replication.

We interrogated at nucleotide resolution the spatiotemporal order of chromatin changes that occur immediately following a site-specific double-strand break (DSB) upstream of the locus and its subsequent repair by nonhomologous end joining (NHEJ). We observed the immediate eviction of a nucleosome flanking the break and the repositioning of adjacent nucleosomes away from the break. These early chromatin events were independent of the end-processing Mre11-Rad50-Xrs2 (MRX) complex and preceded the MRX-dependent broad eviction of histones and DNA end-resectioning that extends up to ∼8 kb away from the break.

RoboCOP: Multivariate State Space Model Integrating Epigenomic Accessibility Data to Elucidate Genome-Wide Chromatin Occupancy.

Chromatin is the tightly packaged structure of DNA and protein within the nucleus of a cell. The arrangement of different protein complexes along the DNA modulates and is modulated by gene expression. Measuring the binding locations and level of occupancy of different transcription factors (TFs) and nucleosomes is therefore crucial to understanding gene regulation.

Capturing the primordial Kras mutation initiating urethane carcinogenesis.

The environmental carcinogen urethane exhibits a profound specificity for pulmonary tumors driven by an oncogenic QL/R mutation in the gene Kras. Similarly, the frequency, isoform, position, and substitution of oncogenic RAS mutations are often unique to human cancers. To elucidate the principles underlying this RAS mutation tropism of urethane, we adapted an error-corrected, high-throughput sequencing approach to detect mutations in murine Ras genes at great sensitivity.

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.

Sir2 suppresses transcription-mediated displacement of Mcm2-7 replicative helicases at the ribosomal DNA repeats.

Repetitive DNA sequences within eukaryotic heterochromatin are poorly transcribed and replicate late in S-phase. In Saccharomyces cerevisiae, the histone deacetylase Sir2 is required for both transcriptional silencing and late replication at the repetitive ribosomal DNA arrays (rDNA). Despite the widespread association between transcription and replication timing, it remains unclear how transcription might impinge on replication, or vice versa.

Correction to: DNA copy number evolution in Drosophila cell lines.

Following publication of the original article [1], the authors reported the following errors.

Chromatin conformation and transcriptional activity are permissive regulators of DNA replication initiation in .

Chromatin structure has emerged as a key contributor to spatial and temporal control over the initiation of DNA replication. However, despite genome-wide correlations between early replication of gene-rich, accessible euchromatin and late replication of gene-poor, inaccessible heterochromatin, a causal relationship between chromatin structure and replication initiation remains elusive. Here, we combined histone gene engineering and whole-genome sequencing in to determine how perturbing chromatin structure affects replication initiation.

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.

Temporal association of ORCA/LRWD1 to late-firing origins during G1 dictates heterochromatin replication and organization.

DNA replication requires the recruitment of a pre-replication complex facilitated by Origin Recognition Complex (ORC) onto the chromatin during G1 phase of the cell cycle. The ORC-associated protein (ORCA/LRWD1) stabilizes ORC on chromatin. Here, we evaluated the genome-wide distribution of ORCA using ChIP-seq during specific time points of G1.

DNA replication origins-where do we begin?

For more than three decades, investigators have sought to identify the precise locations where DNA replication initiates in mammalian genomes. The development of molecular and biochemical approaches to identify start sites of DNA replication (origins) based on the presence of defining and characteristic replication intermediates at specific loci led to the identification of only a handful of mammalian replication origins. The limited number of identified origins prevented a comprehensive and exhaustive search for conserved genomic features that were capable of specifying origins of DNA replication.

Noncoding Transcription Is a Driving Force for Nucleosome Instability in spt16 Mutant Cells.

FACT (facilitates chromatin transcription) consists of two essential subunits, Spt16 and Pob3, and functions as a histone chaperone. Mutation of spt16 results in a global loss of nucleosomes as well as aberrant transcription. Here, we show that the majority of nucleosome changes upon Spt16 depletion are alterations in nucleosome fuzziness and position shift.

Methylation of histone H4 lysine 20 by PR-Set7 ensures the integrity of late replicating sequence domains in Drosophila.

The methylation state of lysine 20 on histone H4 (H4K20) has been linked to chromatin compaction, transcription, DNA repair and DNA replication. Monomethylation of H4K20 (H4K20me1) is mediated by the cell cycle-regulated histone methyltransferase PR-Set7. PR-Set7 depletion in mammalian cells results in defective S phase progression and the accumulation of DNA damage, which has been partially attributed to defects in origin selection and activation.

Mcm2-7 Is an Active Player in the DNA Replication Checkpoint Signaling Cascade via Proposed Modulation of Its DNA Gate.

The DNA replication checkpoint (DRC) monitors and responds to stalled replication forks to prevent genomic instability. How core replication factors integrate into this phosphorylation cascade is incompletely understood. Here, through analysis of a unique mcm allele targeting a specific ATPase active site (mcm2DENQ), we show that the Mcm2-7 replicative helicase has a novel DRC function as part of the signal transduction cascade.

SnapShot: Origins of DNA replication.

The fundamental unit of DNA replication, the replicon, is governed by a cis-acting replicator sequence and a trans-activating initiator factor. Despite the increased size and complexity of eukaryotic genomes, eukaryotic DNA replication continues to be guided by the fundamental principles and concepts established in the replicon model.

Genome-wide chromatin footprinting reveals changes in replication origin architecture induced by pre-RC assembly.

Start sites of DNA replication are marked by the origin recognition complex (ORC), which coordinates Mcm2-7 helicase loading to form the prereplicative complex (pre-RC). Although pre-RC assembly is well characterized in vitro, the process is poorly understood within the local chromatin environment surrounding replication origins. To reveal how the chromatin architecture modulates origin selection and activation, we "footprinted" nucleosomes, transcription factors, and replication proteins at multiple points during the Saccharomyces cerevisiae cell cycle.

Dynamic loading and redistribution of the Mcm2-7 helicase complex through the cell cycle.

Eukaryotic replication origins are defined by the ORC-dependent loading of the Mcm2-7 helicase complex onto chromatin in G1. Paradoxically, there is a vast excess of Mcm2-7 relative to ORC assembled onto chromatin in G1. These excess Mcm2-7 complexes exhibit little co-localization with ORC or replication foci and can function as dormant origins.