RESUMO
Centromeres are scaffolds for the assembly of kinetochores that ensure chromosome segregation during cell division. How vertebrate centromeres obtain a three-dimensional structure to accomplish their primary function is unclear. Using super-resolution imaging, capture-C, and polymer modeling, we show that vertebrate centromeres are partitioned by condensins into two subdomains during mitosis. The bipartite structure is found in human, mouse, and chicken cells and is therefore a fundamental feature of vertebrate centromeres. Super-resolution imaging and electron tomography reveal that bipartite centromeres assemble bipartite kinetochores, with each subdomain binding a distinct microtubule bundle. Cohesin links the centromere subdomains, limiting their separation in response to spindle forces and avoiding merotelic kinetochore-spindle attachments. Lagging chromosomes during cancer cell divisions frequently have merotelic attachments in which the centromere subdomains are separated and bioriented. Our work reveals a fundamental aspect of vertebrate centromere biology with implications for understanding the mechanisms that guarantee faithful chromosome segregation.
Assuntos
Centrômero , Coesinas , Cinetocoros , Mitose , Animais , Humanos , Camundongos , Proteínas de Ciclo Celular/metabolismo , Centrômero/metabolismo , Galinhas , Proteínas Cromossômicas não Histona/metabolismo , Proteínas Cromossômicas não Histona/química , Segregação de Cromossomos , Cinetocoros/metabolismo , Microtúbulos/metabolismo , Fuso Acromático/metabolismoRESUMO
Enhancers are distal DNA elements believed to loop and contact promoters to control gene expression. Recently, we found diffraction-sized transcriptional condensates at genes controlled by clusters of enhancers (super-enhancers). However, a direct function of endogenous condensates in controlling gene expression remains elusive. Here, we develop live-cell super-resolution and multi-color 3D-imaging approaches to investigate putative roles of endogenous condensates in the regulation of super-enhancer controlled gene Sox2. In contrast to enhancer distance, we find instead that the condensate's positional dynamics are a better predictor of gene expression. A basal gene bursting occurs when the condensate is far (>1 µm), but burst size and frequency are enhanced when the condensate moves in proximity (<1 µm). Perturbations of cohesin and local DNA elements do not prevent basal bursting but affect the condensate and its burst enhancement. We propose a three-way kissing model whereby the condensate interacts transiently with gene locus and regulatory DNA elements to control gene bursting.
Assuntos
Regulação da Expressão Gênica , Fatores de Transcrição SOXB1 , Super Intensificadores , Transcrição Gênica , DNA/genética , Elementos Facilitadores Genéticos , Fatores de Transcrição SOXB1/genética , Animais , Camundongos , Células-Tronco Embrionárias/metabolismo , Microscopia/métodosRESUMO
Concomitant with DNA replication, the chromosomal cohesin complex establishes cohesion between newly replicated sister chromatids. Cohesion establishment requires acetylation of conserved cohesin lysine residues by Eco1 acetyltransferase. Here, we explore how cohesin acetylation is linked to DNA replication. Biochemical reconstitution of replication-coupled cohesin acetylation reveals that transient DNA structures, which form during DNA replication, control the acetylation reaction. As polymerases complete lagging strand replication, strand displacement synthesis produces DNA flaps that are trimmed to result in nicked double-stranded DNA. Both flaps and nicks stimulate cohesin acetylation, while subsequent nick ligation to complete Okazaki fragment maturation terminates the acetylation reaction. A flapped or nicked DNA substrate constitutes a transient molecular clue that directs cohesin acetylation to a window behind the replication fork, next to where cohesin likely entraps both sister chromatids. Our results provide an explanation for how DNA replication is linked to sister chromatid cohesion establishment.
Assuntos
Cromátides , Proteínas de Saccharomyces cerevisiae , Cromátides/metabolismo , Proteínas Nucleares/metabolismo , Saccharomyces cerevisiae/genética , Saccharomyces cerevisiae/metabolismo , Proteínas de Saccharomyces cerevisiae/genética , Proteínas de Saccharomyces cerevisiae/metabolismo , Replicação do DNA , Proteínas de Ciclo Celular/genética , Proteínas de Ciclo Celular/metabolismo , DNA , Acetiltransferases/genética , Acetiltransferases/metabolismoRESUMO
Alzheimer's disease (AD) is the most common cause of dementia worldwide, but the molecular and cellular mechanisms underlying cognitive impairment remain poorly understood. To address this, we generated a single-cell transcriptomic atlas of the aged human prefrontal cortex covering 2.3 million cells from postmortem human brain samples of 427 individuals with varying degrees of AD pathology and cognitive impairment. Our analyses identified AD-pathology-associated alterations shared between excitatory neuron subtypes, revealed a coordinated increase of the cohesin complex and DNA damage response factors in excitatory neurons and in oligodendrocytes, and uncovered genes and pathways associated with high cognitive function, dementia, and resilience to AD pathology. Furthermore, we identified selectively vulnerable somatostatin inhibitory neuron subtypes depleted in AD, discovered two distinct groups of inhibitory neurons that were more abundant in individuals with preserved high cognitive function late in life, and uncovered a link between inhibitory neurons and resilience to AD pathology.
Assuntos
Doença de Alzheimer , Encéfalo , Idoso , Humanos , Doença de Alzheimer/metabolismo , Doença de Alzheimer/patologia , Encéfalo/metabolismo , Encéfalo/patologia , Cognição , Disfunção Cognitiva/metabolismo , Neurônios/metabolismoRESUMO
Regulatory landscapes drive complex developmental gene expression, but it remains unclear how their integrity is maintained when incorporating novel genes and functions during evolution. Here, we investigated how a placental mammal-specific gene, Zfp42, emerged in an ancient vertebrate topologically associated domain (TAD) without adopting or disrupting the conserved expression of its gene, Fat1. In ESCs, physical TAD partitioning separates Zfp42 and Fat1 with distinct local enhancers that drive their independent expression. This separation is driven by chromatin activity and not CTCF/cohesin. In contrast, in embryonic limbs, inactive Zfp42 shares Fat1's intact TAD without responding to active Fat1 enhancers. However, neither Fat1 enhancer-incompatibility nor nuclear envelope-attachment account for Zfp42's unresponsiveness. Rather, Zfp42's promoter is rendered inert to enhancers by context-dependent DNA methylation. Thus, diverse mechanisms enabled the integration of independent Zfp42 regulation in the Fat1 locus. Critically, such regulatory complexity appears common in evolution as, genome wide, most TADs contain multiple independently expressed genes.
Assuntos
Cromatina , Placenta , Animais , Fator de Ligação a CCCTC/metabolismo , Montagem e Desmontagem da Cromatina , Elementos Facilitadores Genéticos , Evolução Molecular , Feminino , Genoma , Mamíferos/metabolismo , Placenta/metabolismo , Gravidez , Regiões Promotoras Genéticas , Fatores de Transcrição/genética , Fatores de Transcrição/metabolismoRESUMO
Structural maintenance of chromosomes (SMC) complexes organize genome topology in all kingdoms of life and have been proposed to perform this function by DNA loop extrusion. How this process works is unknown. Here, we have analyzed how loop extrusion is mediated by human cohesin-NIPBL complexes, which enable chromatin folding in interphase cells. We have identified DNA binding sites and large-scale conformational changes that are required for loop extrusion and have determined how these are coordinated. Our results suggest that DNA is translocated by a spontaneous 50 nm-swing of cohesin's hinge, which hands DNA over to the ATPase head of SMC3, where upon binding of ATP, DNA is clamped by NIPBL. During this process, NIPBL "jumps ship" from the hinge toward the SMC3 head and might thereby couple the spontaneous hinge swing to ATP-dependent DNA clamping. These results reveal mechanistic principles of how cohesin-NIPBL and possibly other SMC complexes mediate loop extrusion.
Assuntos
Proteínas de Ciclo Celular/metabolismo , Proteínas Cromossômicas não Histona/metabolismo , DNA/química , Conformação de Ácido Nucleico , Adenosina Trifosfatases/metabolismo , Trifosfato de Adenosina/metabolismo , Sítios de Ligação , Proteínas de Ciclo Celular/química , DNA/metabolismo , Transferência Ressonante de Energia de Fluorescência , Células HeLa , Humanos , Hidrólise , Cinética , Microscopia de Força Atômica , Modelos Moleculares , Proteínas Nucleares/metabolismo , Conformação Proteica , CoesinasRESUMO
Nuclei are central hubs for information processing in eukaryotic cells. The need to fit large genomes into small nuclei imposes severe restrictions on genome organization and the mechanisms that drive genome-wide regulatory processes. How a disordered polymer such as chromatin, which has vast heterogeneity in its DNA and histone modification profiles, folds into discernibly consistent patterns is a fundamental question in biology. Outstanding questions include how genomes are spatially and temporally organized to regulate cellular processes with high precision and whether genome organization is causally linked to transcription regulation. The advent of next-generation sequencing, super-resolution imaging, multiplexed fluorescent in situ hybridization, and single-molecule imaging in individual living cells has caused a resurgence in efforts to understand the spatiotemporal organization of the genome. In this review, we discuss structural and mechanistic properties of genome organization at different length scales and examine changes in higher-order chromatin organization during important developmental transitions.
Assuntos
Cromatina , Cromossomos , Cromatina/genética , DNA , Genoma , Hibridização in Situ FluorescenteRESUMO
CCCTC-binding factor (CTCF) and cohesin are key players in three-dimensional chromatin organization. The topologically associating domains (TADs) demarcated by CTCF are remarkably well conserved between species, although genome-wide CTCF binding has diverged substantially following transposon-mediated motif expansions. Therefore, the CTCF consensus motif poorly predicts TADs, and additional factors must modulate CTCF binding and subsequent TAD formation. Here, we demonstrate that the ChAHP complex (CHD4, ADNP, HP1) competes with CTCF for a common set of binding motifs. In Adnp knockout cells, novel insulated regions are formed at sites normally bound by ChAHP, whereas proximal canonical boundaries are weakened. These data reveal that CTCF-mediated loop formation is modulated by a distinct zinc-finger protein complex. Strikingly, ChAHP-bound loci are mainly situated within less diverged SINE B2 transposable elements. This implicates ChAHP in maintenance of evolutionarily conserved spatial chromatin organization by buffering novel CTCF binding sites that emerged through SINE expansions.
Assuntos
Fator de Ligação a CCCTC/metabolismo , Cromatina/metabolismo , Proteínas Cromossômicas não Histona/metabolismo , DNA Helicases/metabolismo , Células-Tronco Embrionárias/metabolismo , Proteínas de Homeodomínio/metabolismo , Proteínas do Tecido Nervoso/metabolismo , Retroelementos , Animais , Sítios de Ligação , Linhagem Celular , Homólogo 5 da Proteína Cromobox , Células-Tronco Embrionárias/citologia , Camundongos , Ligação Proteica , Domínios ProteicosRESUMO
As predicted by the notion that sister chromatid cohesion is mediated by entrapment of sister DNAs inside cohesin rings, there is perfect correlation between co-entrapment of circular minichromosomes and sister chromatid cohesion. In most cells where cohesin loads without conferring cohesion, it does so by entrapment of individual DNAs. However, cohesin with a hinge domain whose positively charged lumen is neutralized loads and moves along chromatin despite failing to entrap DNAs. Thus, cohesin engages chromatin in non-topological, as well as topological, manners. Since hinge mutations, but not Smc-kleisin fusions, abolish entrapment, DNAs may enter cohesin rings through hinge opening. Mutation of three highly conserved lysine residues inside the Smc1 moiety of Smc1/3 hinges abolishes all loading without affecting cohesin's recruitment to CEN loading sites or its ability to hydrolyze ATP. We suggest that loading and translocation are mediated by conformational changes in cohesin's hinge driven by cycles of ATP hydrolysis.
Assuntos
Proteínas de Ciclo Celular/química , Cromátides/química , Proteínas Cromossômicas não Histona/química , DNA/química , Trifosfato de Adenosina/química , Animais , Sítios de Ligação , Cromatina/química , Humanos , Hidrólise , Lisina/química , Camundongos , Mutação , Proteínas Nucleares/genética , Conformação Proteica , Saccharomyces cerevisiae/genética , Proteínas de Saccharomyces cerevisiae/genética , CoesinasRESUMO
The ring-shaped structural maintenance of chromosome (SMC) complexes are multi-subunit ATPases that topologically encircle DNA. SMC rings make vital contributions to numerous chromosomal functions, including mitotic chromosome condensation, sister chromatid cohesion, DNA repair, and transcriptional regulation. They are thought to do so by establishing interactions between more than one DNA. Here, we demonstrate DNA-DNA tethering by the purified fission yeast cohesin complex. DNA-bound cohesin efficiently and topologically captures a second DNA, but only if that is single-stranded DNA (ssDNA). Like initial double-stranded DNA (dsDNA) embrace, second ssDNA capture is ATP-dependent, and it strictly requires the cohesin loader complex. Second-ssDNA capture is relatively labile but is converted into stable dsDNA-dsDNA cohesion through DNA synthesis. Our study illustrates second-DNA capture by an SMC complex and provides a molecular model for the establishment of sister chromatid cohesion.
Assuntos
Proteínas de Ciclo Celular/metabolismo , Cromátides/genética , Proteínas Cromossômicas não Histona/metabolismo , DNA/metabolismo , Proteínas de Saccharomyces cerevisiae/metabolismo , Trifosfato de Adenosina/metabolismo , Cromátides/metabolismo , Replicação do DNA , Saccharomyces cerevisiae , Schizosaccharomyces , CoesinasRESUMO
Cohesin extrusion is thought to play a central role in establishing the architecture of mammalian genomes. However, extrusion has not been visualized in vivo, and thus, its functional impact and energetics are unknown. Using ultra-deep Hi-C, we show that loop domains form by a process that requires cohesin ATPases. Once formed, however, loops and compartments are maintained for hours without energy input. Strikingly, without ATP, we observe the emergence of hundreds of CTCF-independent loops that link regulatory DNA. We also identify architectural "stripes," where a loop anchor interacts with entire domains at high frequency. Stripes often tether super-enhancers to cognate promoters, and in B cells, they facilitate Igh transcription and recombination. Stripe anchors represent major hotspots for topoisomerase-mediated lesions, which promote chromosomal translocations and cancer. In plasmacytomas, stripes can deregulate Igh-translocated oncogenes. We propose that higher organisms have coopted cohesin extrusion to enhance transcription and recombination, with implications for tumor development.
Assuntos
Trifosfato de Adenosina/metabolismo , Proteínas de Ciclo Celular/metabolismo , Proteínas Cromossômicas não Histona/metabolismo , Genoma , Animais , Linfócitos B/citologia , Linfócitos B/metabolismo , Fator de Ligação a CCCTC/genética , Fator de Ligação a CCCTC/metabolismo , Proteínas de Ciclo Celular/química , Proteínas de Ciclo Celular/genética , Linhagem Celular , Proteoglicanas de Sulfatos de Condroitina/genética , Proteoglicanas de Sulfatos de Condroitina/metabolismo , Cromatina/metabolismo , Proteínas Cromossômicas não Histona/química , Proteínas Cromossômicas não Histona/genética , Cromossomos/metabolismo , Proteínas de Ligação a DNA , Humanos , Camundongos , Mutagênese , Proteínas Nucleares/genética , Proteínas Nucleares/metabolismo , Fosfoproteínas/genética , Fosfoproteínas/metabolismo , Fatores de Transcrição/genética , Fatores de Transcrição/metabolismo , Transcrição Gênica , CoesinasRESUMO
How transcription affects genome 3D organization is not well understood. We found that during influenza A (IAV) infection, rampant transcription rapidly reorganizes host cell chromatin interactions. These changes occur at the ends of highly transcribed genes, where global inhibition of transcription termination by IAV NS1 protein causes readthrough transcription for hundreds of kilobases. In these readthrough regions, elongating RNA polymerase II disrupts chromatin interactions by inducing cohesin displacement from CTCF sites, leading to locus decompaction. Readthrough transcription into heterochromatin regions switches them from the inert (B) to the permissive (A) chromatin compartment and enables transcription factor binding. Data from non-viral transcription stimuli show that transcription similarly affects cohesin-mediated chromatin contacts within gene bodies. Conversely, inhibition of transcription elongation allows cohesin to accumulate at previously transcribed intragenic CTCF sites and to mediate chromatin looping and compaction. Our data indicate that transcription elongation by RNA polymerase II remodels genome 3D architecture.
Assuntos
Cromatina/metabolismo , Genoma Humano , Virus da Influenza A Subtipo H5N1/metabolismo , Sítios de Ligação , Fator de Ligação a CCCTC/química , Fator de Ligação a CCCTC/metabolismo , Proteínas de Transporte/antagonistas & inibidores , Proteínas de Transporte/genética , Proteínas de Transporte/metabolismo , Proteínas de Ciclo Celular/metabolismo , Linhagem Celular , Cromatina/química , Proteínas Cromossômicas não Histona/metabolismo , Flavonoides/farmacologia , Humanos , Interferon beta/farmacologia , Macrófagos/citologia , Macrófagos/metabolismo , Macrófagos/virologia , Proteínas Nucleares/antagonistas & inibidores , Proteínas Nucleares/genética , Proteínas Nucleares/metabolismo , Piperidinas/farmacologia , Ligação Proteica , Proteínas Proto-Oncogênicas/antagonistas & inibidores , Proteínas Proto-Oncogênicas/genética , Proteínas Proto-Oncogênicas/metabolismo , Interferência de RNA , RNA Polimerase II/genética , RNA Polimerase II/metabolismo , RNA Interferente Pequeno/metabolismo , Transcrição Gênica/efeitos dos fármacos , Proteínas não Estruturais Virais/genética , Proteínas não Estruturais Virais/metabolismo , CoesinasRESUMO
The ring-shaped cohesin complex brings together distant DNA domains to maintain, express, and segregate the genome. Establishing specific chromosomal linkages depends on cohesin recruitment to defined loci. One such locus is the budding yeast centromere, which is a paradigm for targeted cohesin loading. The kinetochore, a multiprotein complex that connects centromeres to microtubules, drives the recruitment of high levels of cohesin to link sister chromatids together. We have exploited this system to determine the mechanism of specific cohesin recruitment. We show that phosphorylation of the Ctf19 kinetochore protein by a conserved kinase, DDK, provides a binding site for the Scc2/4 cohesin loading complex, thereby directing cohesin loading to centromeres. A similar mechanism targets cohesin to chromosomes in vertebrates. These findings represent a complete molecular description of targeted cohesin loading, a phenomenon with wide-ranging importance in chromosome segregation and, in multicellular organisms, transcription regulation.
Assuntos
Proteínas de Ciclo Celular/metabolismo , Proteínas Cromossômicas não Histona/metabolismo , Cinetocoros/metabolismo , Saccharomyces cerevisiae/metabolismo , Centrômero/metabolismo , Proteínas do Citoesqueleto/metabolismo , Complexos Multiproteicos/metabolismo , Fosforilação , Filogenia , Saccharomyces cerevisiae/citologia , Proteínas de Saccharomyces cerevisiae/metabolismo , Difração de Raios X , CoesinasRESUMO
The spatial organization of chromosomes influences many nuclear processes including gene expression. The cohesin complex shapes the 3D genome by looping together CTCF sites along chromosomes. We show here that chromatin loop size can be increased and that the duration with which cohesin embraces DNA determines the degree to which loops are enlarged. Cohesin's DNA release factor WAPL restricts this loop extension and also prevents looping between incorrectly oriented CTCF sites. We reveal that the SCC2/SCC4 complex promotes the extension of chromatin loops and the formation of topologically associated domains (TADs). Our data support the model that cohesin structures chromosomes through the processive enlargement of loops and that TADs reflect polyclonal collections of loops in the making. Finally, we find that whereas cohesin promotes chromosomal looping, it rather limits nuclear compartmentalization. We conclude that the balanced activity of SCC2/SCC4 and WAPL enables cohesin to correctly structure chromosomes.
Assuntos
Proteínas de Transporte/metabolismo , Núcleo Celular/metabolismo , Cromatina/metabolismo , Proteínas Nucleares/metabolismo , Proteínas Proto-Oncogênicas/metabolismo , Acetiltransferases/metabolismo , Fator de Ligação a CCCTC , Proteínas de Ciclo Celular/metabolismo , Proteínas Cromossômicas não Histona/metabolismo , Proteínas de Ligação a DNA , Elongases de Ácidos Graxos , Edição de Genes , Humanos , Complexos Multiproteicos/metabolismo , Proteínas Repressoras/metabolismo , CoesinasRESUMO
It is now established that Bcl11b specifies T cell fate. Here, we show that in developing T cells, the Bcl11b enhancer repositioned from the lamina to the nuclear interior. Our search for factors that relocalized the Bcl11b enhancer identified a non-coding RNA named ThymoD (thymocyte differentiation factor). ThymoD-deficient mice displayed a block at the onset of T cell development and developed lymphoid malignancies. We found that ThymoD transcription promoted demethylation at CTCF bound sites and activated cohesin-dependent looping to reposition the Bcl11b enhancer from the lamina to the nuclear interior and to juxtapose the Bcl11b enhancer and promoter into a single-loop domain. These large-scale changes in nuclear architecture were associated with the deposition of activating epigenetic marks across the loop domain, plausibly facilitating phase separation. These data indicate how, during developmental progression and tumor suppression, non-coding transcription orchestrates chromatin folding and compartmentalization to direct with high precision enhancer-promoter communication.
Assuntos
Elementos Facilitadores Genéticos , Regiões Promotoras Genéticas , RNA não Traduzido/genética , Proteínas Repressoras/genética , Linfócitos T/citologia , Proteínas Supressoras de Tumor/genética , Animais , Fator de Ligação a CCCTC , Cromatina/metabolismo , Leucemia/genética , Região de Controle de Locus Gênico , Linfoma/genética , Camundongos , Lâmina Nuclear/metabolismo , Proteínas Repressoras/metabolismo , Linfócitos T/metabolismo , Timo/citologia , Timo/metabolismo , Transcrição GênicaRESUMO
The topological state of chromosomes determines their mechanical properties, dynamics, and function. Recent work indicated that interphase chromosomes are largely free of entanglements. Here, we use Hi-C, polymer simulations, and multi-contact 3C and find that, by contrast, mitotic chromosomes are self-entangled. We explore how a mitotic self-entangled state is converted into an unentangled interphase state during mitotic exit. Most mitotic entanglements are removed during anaphase/telophase, with remaining ones removed during early G1, in a topoisomerase-II-dependent process. Polymer models suggest a two-stage disentanglement pathway: first, decondensation of mitotic chromosomes with remaining condensin loops produces entropic forces that bias topoisomerase II activity toward decatenation. At the second stage, the loops are released, and the formation of new entanglements is prevented by lower topoisomerase II activity, allowing the establishment of unentangled and territorial G1 chromosomes. When mitotic entanglements are not removed in experiments and models, a normal interphase state cannot be acquired.
Assuntos
Cromossomos , DNA Topoisomerases Tipo II , DNA Topoisomerases Tipo II/genética , Cromossomos/genética , Mitose/genética , Interfase/genética , PolímerosRESUMO
Eukaryotic genomes are folded into DNA loops mediated by structural maintenance of chromosomes (SMC) complexes such as cohesin, condensin, and Smc5/6. This organization regulates different DNA-related processes along the cell cycle, such as transcription, recombination, segregation, and DNA repair. During the G2 stage, SMC-mediated DNA loops coexist with cohesin complexes involved in sister chromatid cohesion (SCC). However, the articulation between the establishment of SCC and the formation of SMC-mediated DNA loops along the chromatin remains unknown. Here, we show that SCC is indeed a barrier to cohesin-mediated DNA loop expansion along G2/M Saccharomyces cerevisiae chromosomes.
Assuntos
Proteínas Cromossômicas não Histona , Proteínas de Saccharomyces cerevisiae , Proteínas Cromossômicas não Histona/genética , Proteínas Cromossômicas não Histona/metabolismo , Proteínas de Ciclo Celular/metabolismo , Cromátides/genética , Cromátides/metabolismo , Coesinas , Saccharomyces cerevisiae/genética , Saccharomyces cerevisiae/metabolismo , DNA/genética , DNA/metabolismo , Proteínas de Saccharomyces cerevisiae/genética , Proteínas de Saccharomyces cerevisiae/metabolismoRESUMO
Homology search is a central step of DNA double-strand break (DSB) repair by homologous recombination (HR). How it operates in cells remains elusive. We developed a Hi-C-based methodology to map single-stranded DNA (ssDNA) contacts genome-wide in S. cerevisiae, which revealed two main homology search phases. Initial search conducted by short Rad51-ssDNA nucleoprotein filaments (NPFs) is confined in cis by cohesin-mediated chromatin loop folding. Progressive growth of stiff NPFs enables exploration of distant genomic sites. Long-range resection drives this transition from local to genome-wide search by increasing the probability of assembling extensive NPFs. DSB end-tethering promotes coordinated search by opposite NPFs. Finally, an autonomous genetic element on chromosome III engages the NPF, which stimulates homology search in its vicinity. This work reveals the mechanism of the progressive expansion of homology search that is orchestrated by chromatin organizers, long-range resection, end-tethering, and specialized genetic elements and that exploits the stiff NPF structure conferred by Rad51 oligomerization.
RESUMO
Partitioning of repressive from actively transcribed chromatin in mammalian cells fosters cell-type-specific gene expression patterns. While this partitioning is reconstructed during differentiation, the chromatin occupancy of the key insulator, CCCTC-binding factor (CTCF), is unchanged at the developmentally important Hox clusters. Thus, dynamic changes in chromatin boundaries must entail other activities. Given its requirement for chromatin loop formation, we examined cohesin-based chromatin occupancy without known insulators, CTCF and Myc-associated zinc-finger protein (MAZ), and identified a family of zinc-finger proteins (ZNFs), some of which exhibit tissue-specific expression. Two such ZNFs foster chromatin boundaries at the Hox clusters that are distinct from each other and from MAZ. PATZ1 was critical to the thoracolumbar boundary in differentiating motor neurons and mouse skeleton, while ZNF263 contributed to cervicothoracic boundaries. We propose that these insulating activities act with cohesin, alone or combinatorially, with or without CTCF, to implement precise positional identity and cell fate during development.
RESUMO
The structural maintenance of chromosomes (SMC) protein complexes-cohesin, condensin, and the Smc5/6 complex (Smc5/6)-are essential for chromosome function. At the molecular level, these complexes fold DNA by loop extrusion. Accordingly, cohesin creates chromosome loops in interphase, and condensin compacts mitotic chromosomes. However, the role of Smc5/6's recently discovered DNA loop extrusion activity is unknown. Here, we uncover that Smc5/6 associates with transcription-induced positively supercoiled DNA at cohesin-dependent loop boundaries on budding yeast (Saccharomyces cerevisiae) chromosomes. Mechanistically, single-molecule imaging reveals that dimers of Smc5/6 specifically recognize the tip of positively supercoiled DNA plectonemes and efficiently initiate loop extrusion to gather the supercoiled DNA into a large plectonemic loop. Finally, Hi-C analysis shows that Smc5/6 links chromosomal regions containing transcription-induced positive supercoiling in cis. Altogether, our findings indicate that Smc5/6 controls the three-dimensional organization of chromosomes by recognizing and initiating loop extrusion on positively supercoiled DNA.