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Nucleoporins facilitate ORC loading onto chromatin

ABSTRACT: The origin recognition complex (ORC) binds throughout the genome to initiate DNA replication. In metazoans, it is still unclear how ORC is targeted to specific loci to facilitate helicase loading and replication initiation. Here, we performed immunoprecipitations coupled with mass spectrometry for ORC2 in Drosophila embryos. Surprisingly, we found that ORC2 associates with multiple subunits of the Nup107-160 subcomplex of the nuclear pore. Bioinformatic analysis revealed that, relative to all modENCODE factors, nucleoporins are among the most enriched factors at ORC2 binding sites. Critically, depletion of the nucleoporin Elys, a member of the Nup107-160 complex, results in decreased ORC2 loading onto chromatin. Depleting Elys also sensitized cells to replication fork stalling, which could reflect a defect in establishing dormant replication origins. Our work reveals a new connection between ORC, replication initiation and nucleoporins, highlighting a previously unrecognized function of nucleoporins in metazoan replication initiation.

INSTRUMENT(S): Q Exactive HF

ORGANISM(S): Drosophila Melanogaster (fruit Fly)

TISSUE(S): Embryo, Early Embryonic Cell

SUBMITTER: Logan Richards

LAB HEAD: Jared Nordman

PROVIDER: PXD033045 | Pride | 2022-10-19

REPOSITORIES: Pride

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Nucleoporins facilitate ORC loading onto chromatin

Project description:The origin recognition complex (ORC) binds throughout the genome to initiate DNA replication. In metazoans, it is still unclear how ORC is targeted to specific loci to facilitate helicase loading and replication initiation. Here, we performed immunoprecipitations coupled with mass spectrometry for ORC2 in Drosophila embryos. Surprisingly, we found that ORC2 associates with several subunits of the Nup107-160 subcomplex of the nuclear pore. Bioinformatic analysis revealed that, relative to all modENCODE factors, nucleoporins are among the most enriched factors at ORC2 binding sites. Critically, depletion of the nucleoporin Elys, a member of the Nup107-160 complex, results in decrease ORC2 loading onto chromatin. Depleting Elys also sensitized cells to replication fork stalling, which could reflect a defect in establishing dormant replication origins. Our work reveals a new connection between ORC, replication initiation and nucleoporins, highlighting a previously unrecognized function of nucleoporins in metazoan replication initiation.

2022-10-19 | GSE199896 | GEO

Yeast ORC sumoylation status fine-tunes origin activity

Project description:Sumoylation is emerging as a post-translation modification important for chromosome duplication and stability. The origin recognition complex (ORC), which directs DNA replication initiation by loading the MCM replicative helicases onto origins, is sumoylated in both yeast and human cells. However, the biological consequences of ORC sumoylation are largely unclear. Here we report the effects of hyper- and hypo-sumoylation of yeast ORC using multiple approaches. We show that ORC hyper-sumoylation preferentially reduces the activity of a subset of early origins, while Orc2 hypo-sumoylation has an opposing effect. Mechanistically, ORC hyper-sumoylation leads to reduced MCM loading in vitro and diminished MCM chromatin association in vivo. The importance of an appropriate level of ORC sumoylation is suggested by the data that either hyper- or hypo-sumoylation of ORC results in genome instability and a dependence on other genome maintenance factors for cell fitness. Thus, yeast ORC sumoylation status needs to be fine-tuned to achieve optimal origin activity control and genome stability.

2023-03-11 | PXD032977 | Pride

Orc6 is Required for Dynamic Recruitment of Cdt1 During Repeated Mcm2-7 Loading

Project description:The origin recognition complex (ORC) nucleates DNA replication initiation in eukaryotic cells. This six-protein complex binds replication origin DNA, recruits other initiation factors and facilitates loading of the DNA helicase. Studying the function of individual ORC subunits during pre-RC formation has been hampered by the requirement of most subunits for DNA binding. In this study, we investigate the function of the S. cerevisiae Orc6 subunit, the only subunit not required for DNA binding. In vivo, depletion of Orc6 inhibits pre-replicative complex (pre-RC) assembly and maintenance. In vitro, ORC lacking Orc6 fails to interact with Cdt1 and to load the Mcm2-7 helicase onto origin DNA. We demonstrate that two regions of Orc6 bind Cdt1 directly and that the extreme C-terminus of Orc6 (Orc6-CTD) interacts tightly with the remaining five ORC subunits. Replacing Orc6 with a fusion protein linking Cdt1 to the Orc6-CTD results in an ORC complex that loads Mcm2-7 onto DNA. Interestingly, this complex can only perform a single round of Mcm2-7 loading, suggesting that a dynamic association of Cdt1 with ORC is required for multiple rounds of pre-RC assembly. Keywords: ChIP-chip

2007-11-14 | GSE9318 | GEO

Cross-linking MS of S. cerevisiae ORC bound to DNA and Cdc6

Project description:During DNA replication initiation, the Origin Recognition Complex (ORC) and Cdc6 co-associate on DNA to load replicative helicases onto origins of replication. We apply DSSO crosslinking mass spectrometry of reconstituted ORC-DNA-Cdc6 to identify interactions between different subunits and domains in the complex.

2022-02-28 | PXD031033 | Pride

The Origin Recognition Complex interacts with a subset of metabolic genes tightly linked to origins of replication.

Project description:The origin recognition complex (ORC) marks chromosomal sites as replication origins and is essential for replication initiation. In yeast, ORC also binds to DNA elements called silencers, where its primary function is to recruit silent information regulator (SIR) proteins to establish transcriptional silencing. Indeed, silencers function poorly as chromosomal origins. Several genetic, molecular, and biochemical studies of HMR-E have led to a model proposing that when ORC becomes limiting in the cell, such as in the orc2-1 mutant, only sites that bind ORC tightly, such as HMR-E, remain fully occupied by ORC, while lower affinity sites, including most origins, lose ORC occupancy. Since HMR-E possessed a unique non-replication function, we reasoned that other tight sites might reveal novel functions for ORC on chromosomes. Therefore, we comprehensively determined ORC “affinity” genome-wide by performing an ORC ChIP-on-chip in ORC2 and orc2-1 strains. Here we describe a novel group of orc2-1-resistant ORC-interacting chromosomal sites (ORF-ORC sites) that did not function as replication origins or silencers. Instead, ORF-ORC sites were comprised of protein-coding regions of highly transcribed metabolic genes. In contrast to the ORC-silencer paradigm, transcriptional activation promoted ORC association with these genes. Remarkably, ORF-ORC genes were enriched in proximity to origins of replication, and, in several instances, were transcriptionally regulated by these origins. Taken together, these results suggest a surprising connection between ORC, replication origins and cellular metabolism.

2009-11-07 | GSE18895 | GEO

ChIP-chip from Drosophila egg chambers using ORC2 antibody

Project description:Chromatin immunoprecipitation of the Origin Recognition Complex (ORC) followed by hybridization to genome-wide tiling microarrays demonstrated that ORC does not localize to DAFC-34B after stage 10, despite a second round of replication initiation

2011-09-07 | GSE31890 | GEO

ChIP-chip from Drosophila egg chambers using ORC2 antibody

2011-09-06 | E-GEOD-31890 | biostudies-arrayexpress

Genome-wide EMSA (gEMSA) of ORC and yeast genomic DNA

Project description:Eukaryotic DNA replication origins are selected in G1-phase when the origin recognition complex (ORC) binds chromosomal DNA, triggering a series of molecular events that culminate in the initiation of DNA replication (a.k.a. origin firing) during S-phase. Each chromosome requires multiple origins for its duplication, and each origin fires at a characteristic time during S-phase, creating a cell-type specific genome replication pattern with relevance to differentiation, genome stability and evolution. It is unclear whether ORC-origin interactions are relevant to the regulation of origin activation time. Here we applied a novel genome-wide strategy to classify origins in the model eukaryote Saccharomyces cerevisiae based on the types of molecular interactions used for ORC-origin binding. Specifically, origins were classified as DNA-dependent when the strength of ORC-origin binding in vivo could be explained by the affinity of ORC for origin DNA in vitro, and, conversely, as ‘chromatin-dependent’ when the ORC-DNA interaction in vitro was insufficient to explain the strength of ORC-origin binding in vivo. The two classes of origins were distinct in terms of local nucleosome architecture and dependence on origin-flanking sequences in plasmid replication assays, consistent with local features of chromatin promoting ORC binding at ‘chromatin-dependent’ origins. Importantly, origins that fired in late S-phase, but not those that fired in early S-phase, showed a significant dependence on the ORC-origin DNA interaction for ORC binding in vivo. Therefore we demonstrate for the first time a connection between ORC-origin binding mechanisms and the regulation of origin activation time at a genome-wide level. Analysis of ORC genomic DNA binding across three ORC concentrations.

2013-09-30 | E-GEOD-48440 | biostudies-arrayexpress

Post-licensing specification of eukaryotic replication origins by facilitated Mcm2-7 sliding along DNA

Project description:Eukaryotic genomes are replicated from many origin sites that are licensed by the loading of inactive double hexamers of the replicative DNA helicase, Mcm2-7. How eukaryotic origin positions are specified remains elusive. Here we show that, contrary to the bacterial paradigm, eukaryotic origins are not irrevocably defined by selection of the loading site for the replicative helicase, but can shift in position after helicase loading. Using purified proteins, we show that DNA translocases, including RNA polymerase, can push budding yeast Mcm2-7 double hexamers along DNA. Displaced Mcm2-7 double hexamers support DNA replication initiation distal to the loading site in vitro. In yeast cells that are defective for transcription termination, collisions with RNA polymerase induce a shift in origin positions that correlates with the direction of transcription. These results reveal a eukaryotic origin specification mechanism that departs from the classical replicon model, helping eukaryotic cells to negotiate transcription-replication conflict. 4 samples: one replicate for WT at 37C, two replicates for rat1-1 at 37C, and one replicate for rat1-1 at 24C. All are single-end sequenced via Ion Torrent PGM methodology. rat1 = Nuclear 5' to 3' single-stranded RNA exonuclease; involved in RNA metabolism (http://www.yeastgenome.org/locus/S000005574/overview). 4 ChIP seq samples and their duplicates are submitted. rat1-1 ORC ChIP at 24C and 37C; rat1-1 MCM ChIP at 24C and 37C.

2016-01-07 | E-GEOD-69065 | biostudies-arrayexpress

Establishment and Function of Chromatin Organization at Replication Origins

Project description:The Origin Recognition Complex (ORC) is essential for the initiation of eukaryotic chromosome replication as it loads the replicative helicase, the minichromosome maintenance (MCM) complex, at replication origins. Origins display a stereotypic chromatin structure with nucleosome depletion at ORC binding sites and flanking arrays of regularly spaced nucleosomes. Although discovered long ago, it is unknown how this chromatin struture is established and if it matters for replication. By genome-scale biochemical reconstitution we screened ~300 origins and 17 purified budding yeast chromatin factors and found that ORC established nucleosome depletion over origins and flanking nucleosome arrays by orchestrating the chromatin remodelers INO80, ISW1a, ISW2 and Chd1. The functional importance of ORC’s chromatin organizing activity was directly demonstrated by ORC mutations that maintained classical MCM loader activity but lost array generation activity. These mutations impaired replication through chromatin in vitro and were lethal in vivo. Our results establish that ORC, besides its canonical role as MCM loader, has a second essential function as master regulator of the origin chromatin structure that is a crucial prerequisite for efficient chromosome replication.

2023-03-30 | GSE209681 | GEO

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