The unconventional structure of centromeric nucleosomes

Mck1-mediated proteolysis of CENP-A prevents mislocalization of CENP-A for chromosomal stability in Saccharomyces cerevisiae

Centromeric localization of evolutionarily conserved CENP-A (Cse4 in Saccharomyces cerevisiae) is essential for chromosomal stability. Mislocalization of overexpressed CENP-A to noncentromeric regions contributes to chromosomal instability in yeasts, flies, and humans. Overexpression and mislocalization of CENP-A observed in many cancers are associated with poor prognosis. Previous studies have shown that F-box proteins, Cdc4 and Met30 of the Skp, Cullin, F-box ubiquitin ligase cooperatively regulate proteolysis of Cse4 to prevent Cse4 mislocalization and chromosomal instability under normal physiological conditions. Mck1-mediated phosphorylation of Skp, Cullin, F-box-Cdc4 substrates such as Cdc6 and Rcn1 enhances the interaction of the substrates with Cdc4. Here, we report that Mck1 interacts with Cse4, and Mck1-mediated proteolysis of Cse4 prevents Cse4 mislocalization for chromosomal stability. Our results showed that mck1Δ strain overexpressing CSE4 (GAL-CSE4) exhibits lethality, defects in ubiquitin-mediated proteolysis of Cse4, mislocalization of Cse4, and reduced Cse4-Cdc4 interaction. Strain expressing GAL-cse4-3A with mutations in three potential Mck1 phosphorylation consensus sites (S10, S16, and T166) also exhibits growth defects, increased stability with mislocalization of Cse4-3A, chromosomal instability, and reduced interaction with Cdc4. Constitutive expression of histone H3 (Δ16H3) suppresses the chromosomal instability phenotype of GAL-cse4-3A strain, suggesting that the chromosomal instability phenotype is linked to Cse4-3A mislocalization. We conclude that Mck1 and its three potential phosphorylation sites on Cse4 promote Cse4-Cdc4 interaction and this contributes to ubiquitin-mediated proteolysis of Cse4 preventing its mislocalization and chromosomal instability. These studies advance our understanding of pathways that regulate cellular levels of CENP-A to prevent mislocalization of CENP-A in human cancers.

Misregulation of cell cycle-dependent methylation of budding yeast CENP-A contributes to chromosomal instability

Centromere (CEN) identity is specified epigenetically by specialized nucleosomes containing evolutionarily conserved CEN-specific histone H3 variant CENP-A (Cse4 in Saccharomyces cerevisiae, CENP-A in humans), which is essential for faithful chromosome segregation. However, the epigenetic mechanisms that regulate Cse4 function have not been fully defined. In this study, we show that cell cycle-dependent methylation of Cse4-R37 regulates kinetochore function and high-fidelity chromosome segregation. We generated a custom antibody that specifically recognizes methylated Cse4-R37 and showed that methylation of Cse4 is cell cycle regulated with maximum levels of methylated Cse4-R37 and its enrichment at the CEN chromatin occur in the mitotic cells. Methyl-mimic cse4-R37F mutant exhibits synthetic lethality with kinetochore mutants, reduced levels of CEN-associated kinetochore proteins and chromosome instability (CIN), suggesting that mimicking the methylation of Cse4-R37 throughout the cell cycle is detrimental to faithful chromosome segregation. Our results showed that SPOUT methyltransferase Upa1 contributes to methylation of Cse4-R37 and overexpression of UPA1 leads to CIN phenotype. In summary, our studies have defined a role for cell cycle-regulated methylation of Cse4 in high-fidelity chromosome segregation and highlight an important role of epigenetic modifications such as methylation of kinetochore proteins in preventing CIN, an important hallmark of human cancers.

Assembly principles and stoichiometry of a complete human kinetochore module

Centromeres are epigenetically determined chromosomal loci that seed kinetochore assembly to promote chromosome segregation during cell division. CENP-A, a centromere-specific histone H3 variant, establishes the foundations for centromere epigenetic memory and kinetochore assembly. It recruits the constitutive centromere-associated network (CCAN), which in turn assembles the microtubule-binding interface. How the specific organization of centromeric chromatin relates to kinetochore assembly and to centromere identity through cell division remains conjectural. Here, we break new ground by reconstituting a functional full-length version of CENP-C, the largest human CCAN subunit and a blueprint of kinetochore assembly. We show that full-length CENP-C, a dimer, binds stably to two nucleosomes and permits further assembly of all other kinetochore subunits in vitro with relative ratios closely matching those of endogenous human kinetochores. Our results imply that human kinetochores emerge from clustering multiple copies of a fundamental module and may have important implications for transgenerational inheritance of centromeric chromatin.

Mechanical and structural properties of archaeal hypernucleosomes

Many archaea express histones, which organize the genome and play a key role in gene regulation. The structure and function of archaeal histone–DNA complexes remain however largely unclear. Recent studies show formation of hypernucleosomes consisting of DNA wrapped around an ‘endless’ histone-protein core. However, if and how such a hypernucleosome structure assembles on a long DNA substrate and which interactions provide for its stability, remains unclear. Here, we describe micromanipulation studies of complexes of the histones HMfA and HMfB with DNA. Our experiments show hypernucleosome assembly which results from cooperative binding of histones to DNA, facilitated by weak stacking interactions between neighboring histone dimers. Furthermore, rotational force spectroscopy demonstrates that the HMfB–DNA complex has a left-handed chirality, but that torque can drive it in a right-handed conformation. The structure of the hypernucleosome thus depends on stacking interactions, torque, and force. In vivo, such modulation of the archaeal hypernucleosome structure may play an important role in transcription regulation in response to environmental changes.

Phase-plate cryo-EM structure of the Widom 601 CENP-A nucleosome core particle reveals differential flexibility of the DNA ends

The histone H3 variant CENP-A marks centromeres epigenetically and is essential for mitotic fidelity. Previous crystallographic studies of the CENP-A nucleosome core particle (NCP) reconstituted with a human α-satellite DNA derivative revealed both DNA ends to be highly flexible, a feature important for CENP-A mitotic functions. However, recent cryo-EM studies of CENP-A NCP complexes comprising primarily Widom 601 DNA reported well-ordered DNA ends. Here, we report the cryo-EM structure of the CENP-A 601 NCP determined by Volta phase-plate imaging. The data reveal that one (‘left’) 601 DNA end is well ordered whereas the other (‘right’) end is flexible and partly detached from the histone core, suggesting sequence-dependent dynamics of the DNA termini. Indeed, a molecular dynamics simulation of the CENP-A 601 NCP confirmed the distinct dynamics of the two DNA extremities. Reprocessing the image data using two-fold symmetry yielded a cryo-EM map in which both DNA ends appeared well ordered, indicating that such an artefact may inadvertently arise if NCP asymmetry is lost during image processing. These findings enhance our understanding of the dynamic features that discriminate CENP-A from H3 nucleosomes by revealing that DNA end flexibility can be fine-tuned in a sequence-dependent manner.

Centromeres under Pressure: Evolutionary Innovation in Conflict with Conserved Function

Centromeres are essential genetic elements that enable spindle microtubule attachment for chromosome segregation during mitosis and meiosis. While this function is preserved across species, centromeres display an array of dynamic features, including: (1) rapidly evolving DNA; (2) wide evolutionary diversity in size, shape and organization; (3) evidence of mutational processes to generate homogenized repetitive arrays that characterize centromeres in several species; (4) tolerance to changes in position, as in the case of neocentromeres; and (5) intrinsic fragility derived by sequence composition and secondary DNA structures. Centromere drive underlies rapid centromere DNA evolution due to the "selfish" pursuit to bias meiotic transmission and promote the propagation of stronger centromeres. Yet, the origins of other dynamic features of centromeres remain unclear. Here, we review our current understanding of centromere evolution and plasticity. We also detail the mutagenic processes proposed to shape the divergent genetic nature of centromeres. Changes to centromeres are not simply evolutionary relics, but ongoing shifts that on one side promote centromere flexibility, but on the other can undermine centromere integrity and function with potential pathological implications such as genome instability.

Differential chromatin accessibility landscape reveals structural and functional features of the allopolyploid wheat chromosomes

Background

Our understanding of how the complexity of the wheat genome influences the distribution of chromatin states along the homoeologous chromosomes is limited. Using a differential nuclease sensitivity assay, we investigate the chromatin states of the coding and repetitive regions of the allopolyploid wheat genome.

Results

Although open chromatin is found to be significantly enriched around genes, the majority of MNase-sensitive regions are located within transposable elements (TEs). Chromatin of the smaller D genome is more accessible than that of the larger A and B genomes. Chromatin states of different TEs vary among families and are influenced by the TEs’ chromosomal position and proximity to genes. While the chromatin accessibility of genes is influenced by proximity to TEs, and not by their position on the chromosomes, we observe a negative chromatin accessibility gradient along the telomere-centromere axis in the intergenic regions, positively correlated with the distance between genes. Both gene expression levels and homoeologous gene expression bias are correlated with chromatin accessibility in promoter regions. The differential nuclease sensitivity assay accurately predicts previously detected centromere locations. SNPs located within more accessible chromatin explain a higher proportion of genetic variance for a number of agronomic traits than SNPs located within more closed chromatin.

Conclusions

Chromatin states in the wheat genome are shaped by the interplay of repetitive and gene-encoding regions that are predictive of the functional and structural organization of chromosomes, providing a powerful framework for detecting genomic features involved in gene regulation and prioritizing genomic variation to explain phenotypes.

Dbf4-Dependent Kinase (DDK)-Mediated Proteolysis of CENP-A Prevents Mislocalization of CENP-A in Saccharomyces cerevisiae

The evolutionarily conserved centromeric histone H3 variant (Cse4 in budding yeast, CENP-A in humans) is essential for faithful chromosome segregation. Mislocalization of CENP-A to non-centromeric chromatin contributes to chromosomal instability (CIN) in yeast, fly, and human cells and CENP-A is highly expressed and mislocalized in cancers. Defining mechanisms that prevent mislocalization of CENP-A is an area of active investigation. Ubiquitin-mediated proteolysis of overexpressed Cse4 (GALCSE4) by E3 ubiquitin ligases such as Psh1 prevents mislocalization of Cse4, and psh1 Δ strains display synthetic dosage lethality (SDL) with GALCSE4 We previously performed a genome-wide screen and identified five alleles of CDC7 and DBF4 that encode the Dbf4-dependent kinase (DDK) complex, which regulates DNA replication initiation, among the top twelve hits that displayed SDL with GALCSE4 We determined that cdc7 -7 strains exhibit defects in ubiquitin-mediated proteolysis of Cse4 and show mislocalization of Cse4 Mutation of MCM5 (mcm5 -bob1) bypasses the requirement of Cdc7 for replication initiation and rescues replication defects in a cdc7 -7 strain. We determined that mcm5 -bob1 does not rescue the SDL and defects in proteolysis of GALCSE4 in a cdc7 -7 strain, suggesting a DNA replication-independent role for Cdc7 in Cse4 proteolysis. The SDL phenotype, defects in ubiquitin-mediated proteolysis, and the mislocalization pattern of Cse4 in a cdc7 -7 psh1 Δ strain were similar to that of cdc7 -7 and psh1 Δ strains, suggesting that Cdc7 regulates Cse4 in a pathway that overlaps with Psh1 Our results define a DNA replication initiation-independent role of DDK as a regulator of Psh1-mediated proteolysis of Cse4 to prevent mislocalization of Cse4.

Epigenetics and genome stability

Maintaining genome stability is essential to an organism's health and survival. Breakdown of the mechanisms protecting the genome and the resulting genome instability are an important aspect of the aging process and have been linked to diseases such as cancer. Thus, a large network of interconnected pathways is responsible for ensuring genome integrity in the face of the continuous challenges that induce DNA damage. While these pathways are diverse, epigenetic mechanisms play a central role in many of them. DNA modifications, histone variants and modifications, chromatin structure, and non-coding RNAs all carry out a variety of functions to ensure that genome stability is maintained. Epigenetic mechanisms ensure the functions of centromeres and telomeres that are essential for genome stability. Epigenetic mechanisms also protect the genome from the invasion by transposable elements and contribute to various DNA repair pathways. In this review, we highlight the integral role of epigenetic mechanisms in the maintenance of genome stability and draw attention to issues in need of further study.

Chromatin Organization in Early Land Plants Reveals an Ancestral Association between H3K27me3, Transposons, and Constitutive Heterochromatin

Genome packaging by nucleosomes is a hallmark of eukaryotes. Histones and the pathways that deposit, remove, and read histone modifications are deeply conserved. Yet, we lack information regarding chromatin landscapes in extant representatives of ancestors of the main groups of eukaryotes, and our knowledge of the evolution of chromatin-related processes is limited. We used the bryophyte Marchantia polymorpha, which diverged from vascular plants circa 400 mya, to obtain a whole chromosome genome assembly and explore the chromatin landscape and three-dimensional genome organization in an early diverging land plant lineage. Based on genomic profiles of ten chromatin marks, we conclude that the relationship between active marks and gene expression is conserved across land plants. In contrast, we observed distinctive features of transposons and other repetitive sequences in Marchantia compared with flowering plants. Silenced transposons and repeats did not accumulate around centromeres. Although a large fraction of constitutive heterochromatin was marked by H3K9 methylation as in flowering plants, a significant proportion of transposons were marked by H3K27me3, which is otherwise dedicated to the transcriptional repression of protein-coding genes in flowering plants. Chromatin compartmentalization analyses of Hi-C data revealed that repressed B compartments were densely decorated with H3K27me3 but not H3K9 or DNA methylation as reported in flowering plants. We conclude that, in early plants, H3K27me3 played an essential role in heterochromatin function, suggesting an ancestral role of this mark in transposon silencing.

Triple-Helical DNA in Drosophila Heterochromatin

Polynucleotide chains obeying Watson-Crick pairing are apt to form non-canonical complexes such as triple-helical nucleic acids. From early characterization in vitro, their occurrence in vivo has been strengthened by increasing evidence, although most remain circumstantial particularly for triplex DNA. Here, different approaches were employed to specify triple-stranded DNA sequences in the Drosophila melanogaster chromosomes. Antibodies to triplex nucleic acids, previously characterized, bind to centromeric regions of mitotic chromosomes and also to the polytene section 59E of mutant strains carrying the brown dominant allele, indicating that AAGAG tandem satellite repeats are triplex-forming sequences. The satellite probe hybridized to AAGAG-containing regions omitting chromosomal DNA denaturation, as expected, for the intra-molecular triplex DNA formation model in which single-stranded DNA coexists with triplexes. In addition, Thiazole Orange, previously described as capable of reproducing results obtained by antibodies to triple-helical DNA, binds to AAGAG repeats in situ thus validating both detection methods. Unusual phenotype and nuclear structure exhibited by Drosophila correlate with the non-canonical conformation of tandem satellite arrays. From the approaches that lead to the identification of triple-helical DNA in chromosomes, facilities particularly provided by Thiazole Orange use may broaden the investigation on the occurrence of triplex DNA in eukaryotic genomes.

The kinetochore module Okp1CENP-Q/Ame1CENP-U is a reader for N-terminal modifications on the centromeric histone Cse4CENP-A

Kinetochores are supramolecular assemblies that link centromeres to microtubules for sister chromatid segregation in mitosis. For this, the inner kinetochore CCAN/Ctf19 complex binds to centromeric chromatin containing the histone variant CENP-A, but whether the interaction of kinetochore components to centromeric nucleosomes is regulated by posttranslational modifications is unknown. Here, we investigated how methylation of arginine 37 (R37Me) and acetylation of lysine 49 (K49Ac) on the CENP-A homolog Cse4 from Saccharomyces cerevisiae regulate molecular interactions at the inner kinetochore. Importantly, we found that the Cse4 N-terminus binds with high affinity to the Ctf19 complex subassembly Okp1/Ame1 (CENP-Q/CENP-U in higher eukaryotes), and that this interaction is inhibited by R37Me and K49Ac modification on Cse4. In vivo defects in cse4-R37A were suppressed by mutations in OKP1 and AME1, and biochemical analysis of a mutant version of Okp1 showed increased affinity for Cse4. Altogether, our results demonstrate that the Okp1/Ame1 heterodimer is a reader module for posttranslational modifications on Cse4, thereby targeting the yeast CCAN complex to centromeric chromatin.

Fission yeast telosomes: non-canonical histone-containing chromatin structures dependent on shelterin and RNA

Despite the prime importance of telomeres in chromosome stability, significant mysteries surround the architecture of telomeric chromatin. Through micrococcal nuclease mapping, we show that fission yeast chromosome ends are assembled into distinct protected structures ('telosomes') encompassing the telomeric DNA repeats and over half a kilobase of subtelomeric DNA. Telosome formation depends on the conserved telomeric proteins Taz1 and Rap1, and surprisingly, RNA. Although yeast telomeres have long been thought to be free of histones, we show that this is not the case; telomere repeats contain histones. While telomeric histone H3 bears the heterochromatic lys9-methyl mark, we show that this mark is dispensable for telosome formation. Therefore, telomeric chromatin is organized at an architectural level, in which telomere-binding proteins and RNAs impose a unique nucleosome arrangement, and a second level, in which histone modifications are superimposed upon the higher order architecture.

Phylogenetic analysis of the core histone doublet and DNA topo II genes of Marseilleviridae: evidence of proto-eukaryotic provenance

Background

While the genomes of eukaryotes and Archaea both encode the histone-fold domain, only eukaryotes encode the core histone paralogs H2A, H2B, H3, and H4. With DNA, these core histones assemble into the nucleosomal octamer underlying eukaryotic chromatin. Importantly, core histones for H2A and H3 are maintained as neofunctionalized paralogs adapted for general bulk chromatin (canonical H2 and H3) or specialized chromatin (H2A.Z enriched at gene promoters and cenH3s enriched at centromeres). In this context, the identification of core histone-like “doublets” in the cytoplasmic replication factories of the Marseilleviridae (MV) is a novel finding with possible relevance to understanding the origin of eukaryotic chromatin. Here, we analyze and compare the core histone doublet genes from all known MV genomes as well as other MV genes relevant to the origin of the eukaryotic replisome.

Results

Using different phylogenetic approaches, we show that MV histone domains encode obligate H2B-H2A and H4-H3 dimers of possible proto-eukaryotic origin. MV core histone moieties form sister clades to each of the four eukaryotic clades of canonical and variant core histones. This suggests that MV core histone moieties diverged prior to eukaryotic neofunctionalizations associated with paired linear chromosomes and variant histone octamer assembly. We also show that MV genomes encode a proto-eukaryotic DNA topoisomerase II enzyme that forms a sister clade to eukaryotes. This is a relevant finding given that DNA topo II influences histone deposition and chromatin compaction and is the second most abundant nuclear protein after histones.

Conclusions

The combined domain architecture and phylogenomic analyses presented here suggest that a primitive origin for MV histone genes is a more parsimonious explanation than horizontal gene transfers + gene fusions + sufficient divergence to eliminate relatedness to eukaryotic neofunctionalizations within the H2A and H3 clades without loss of relatedness to each of the four core histone clades. We thus suggest MV histone doublet genes and their DNA topo II gene possibly were acquired from an organism with a chromatinized replisome that diverged prior to the origin of eukaryotic core histone variants for H2/H2A.Z and H3/cenH3. These results also imply that core histones were utilized ancestrally in viral DNA compaction and/or protection from host endonucleases.

Electronic supplementary material

The online version of this article (10.1186/s13072-017-0162-0) contains supplementary material, which is available to authorized users.

Structure of centromere chromatin: from nucleosome to chromosomal architecture

The centromere is essential for the segregation of chromosomes, as it serves as attachment site for microtubules to mediate chromosome segregation during mitosis and meiosis. In most organisms, the centromere is restricted to one chromosomal region that appears as primary constriction on the condensed chromosome and is partitioned into two chromatin domains: The centromere core is characterized by the centromere-specific histone H3 variant CENP-A (also called cenH3) and is required for specifying the centromere and for building the kinetochore complex during mitosis. This core region is generally flanked by pericentric heterochromatin, characterized by nucleosomes containing H3 methylated on lysine 9 (H3K9me) that are bound by heterochromatin proteins. During mitosis, these two domains together form a three-dimensional structure that exposes CENP-A-containing chromatin to the surface for interaction with the kinetochore and microtubules. At the same time, this structure supports the tension generated during the segregation of sister chromatids to opposite poles. In this review, we discuss recent insight into the characteristics of the centromere, from the specialized chromatin structures at the centromere core and the pericentromere to the three-dimensional organization of these regions that make up the functional centromere.

Epigenetic modifications in multiple myeloma: recent advances on the role of DNA and histone methylation

INTRODUCTION

Multiple Myeloma (MM) is a clonal late B-cell disorder accounting for about 13% of hematological cancers and 1% of all neoplastic diseases. Recent studies on the molecular pathogenesis and biology of MM have highlighted a complex epigenomic landscape contributing to MM onset, prognosis and high individual variability. Areas covered: We describe here the current knowledge on epigenetic events characterizing MM initiation and progression, focusing on the role of DNA and histone methylation and on the most promising epi-therapeutic approaches targeting the methylation pathway. Expert opinion: Data published so far indicate that alterations of the epigenetic framework, which include aberrant global or gene/non-coding RNA specific methylation profiles, feature prominently in the pathobiology of MM. Indeed, the aberrant expression of components of the epigenetic machinery as well as the reversibility of the epigenetic marks make this pathway druggable, providing the basis for the design of epigenetic therapies against this still fatal malignancy.

Chromatin assembly: Journey to the CENter of the chromosome

All eukaryotic genomes are packaged into basic units of DNA wrapped around histone proteins called nucleosomes. The ability of histones to specify a variety of epigenetic states at defined chromatin domains is essential for cell survival. The most distinctive type of chromatin is found at centromeres, which are marked by the centromere-specific histone H3 variant CENP-A. Many of the factors that regulate CENP-A chromatin have been identified; however, our understanding of the mechanisms of centromeric nucleosome assembly, maintenance, and reorganization remains limited. This review discusses recent insights into these processes and draws parallels between centromeric and noncentromeric chromatin assembly mechanisms.

CENH3 morphogenesis reveals dynamic centromere associations during synaptonemal complex formation and the progression through male meiosis in hexaploid wheat

During meiosis, centromeres in some species undergo a series of associations, but the processes and progression to homologous pairing is still a matter of debate. Here, we aimed to correlate meiotic centromere dynamics and early telomere behaviour to the progression of synaptonemal complex (SC) construction in hexaploid wheat (2n = 42) by triple immunolabelling of CENH3 protein marking functional centromeres, and SC proteins ASY1 (unpaired lateral elements) and ZYP1 (central elements in synapsed chromosomes). We show that single or multiple centromere associations formed in meiotic interphase undergo a progressive polarization (clustering) at the nuclear periphery in early leptotene, leading to formation of the telomere bouquet. Critically, immunolabelling shows the dynamics of these presynaptic centromere associations and a structural reorganization of the centromeric chromatin coinciding with key events of synapsis initiation from the subtelomeric regions. As short stretches of subtelomeric synapsis emerged at early zygotene, centromere clusters lost their strong polarization, gradually resolving as individual centromeres indicated by more than 21 CENH3 foci associated with unpaired lateral elements. Only following this centromere depolarization were homologous chromosome arms connected, as observed by the alignment and fusion of interstitial ZYP1 loci elongating at zygotene so synapsis at centromeres is a continuation of the interstitial synapsis. Our results thus reveal that centromere associations are a component of the timing and progression of chromosome synapsis, and the gradual release of the individual centromeres from the clusters correlates with the elongation of interstitial synapsis between the corresponding homologues.

DNA Topology and Global Architecture of Point Centromeres

DNA is wrapped in a left-handed fashion around histone octasomes containing the centromeric histone H3 variant CENP-A. However, DNA topology studies have suggested that DNA is wrapped in a right-handed manner around the CENP-A nucleosome that occupies the yeast point centromere. Here, we determine the DNA linking number difference (ΔLk) stabilized by the yeast centromere and the contribution of the centromere determining elements (CDEI, CDEII, and CDEIII). We show that the intrinsic architecture of the yeast centromere stabilizes +0.6 units of ΔLk. This topology depends on the integrity of CDEII and CDEIII, but it is independent of cbf1 binding to CDEI and of the variable length of CDEII. These findings suggest that the interaction of the CBF3 complex with CDEIII and a distal CDEII segment configures a right-handed DNA loop that excludes CDEI. This loop is then occupied by a CENP-A histone complex, which does not have to be inherently right-handed.

Structures of archaeal DNA segregation machinery reveal bacterial and eukaryotic linkages

Although recent studies have provided a wealth of information about archaeal biology, nothing is known about the molecular basis of DNA segregation in these organisms. Here, we unveil the machinery and assembly mechanism of the archaeal Sulfolobus pNOB8 partition system. This system uses three proteins: ParA; an atypical ParB adaptor; and a centromere-binding component, AspA. AspA utilizes a spreading mechanism to create a DNA superhelix onto which ParB assembles. This supercomplex links to the ParA motor, which contains a bacteria-like Walker motif. The C domain of ParB harbors structural similarity to CenpA, which dictates eukaryotic segregation. Thus, this archaeal system combines bacteria-like and eukarya-like components, which suggests the possible conservation of DNA segregation principles across the three domains of life.

Inner Kinetochore Protein Interactions with Regional Centromeres of Fission Yeast

Centromeres of the fission yeast Schizosaccharomyces pombe lack the highly repetitive sequences that make most other "regional" centromeres refractory to analysis. To map fission yeast centromeres, we applied H4S47C-anchored cleavage mapping and native and cross-linked chromatin immunoprecipitation with paired-end sequencing. H3 nucleosomes are nearly absent from the central domain, which is occupied by centromere-specific H3 (cenH3 or CENP-A) nucleosomes with two H4s per particle that are mostly unpositioned and are more widely spaced than nucleosomes elsewhere. Inner kinetochore proteins CENP-A, CENP-C, CENP-T, CENP-I, and Scm3 are highly enriched throughout the central domain except at tRNA genes, with no evidence for preferred kinetochore assembly sites. These proteins are weakly enriched and less stably incorporated in H3-rich heterochromatin. CENP-A nucleosomes protect less DNA from nuclease digestion than H3 nucleosomes, while CENP-T protects a range of fragment sizes. Our results suggest that CENP-T particles occupy linkers between CENP-A nucleosomes and that classical regional centromeres differ from other centromeres by the absence of CENP-A nucleosome positioning.

Transcription dynamically patterns the meiotic chromosome-axis interface

Meiotic chromosomes are highly compacted yet remain transcriptionally active. To understand how chromosome folding accommodates transcription, we investigated the assembly of the axial element, the proteinaceous structure that compacts meiotic chromosomes and promotes recombination and fertility. We found that the axial element proteins of budding yeast are flexibly anchored to chromatin by the ring-like cohesin complex. The ubiquitous presence of cohesin at sites of convergent transcription provides well-dispersed points for axis attachment and thus chromosome compaction. Axis protein enrichment at these sites directly correlates with the propensity for recombination initiation nearby. A separate modulating mechanism that requires the conserved axial-element component Hop1 biases axis protein binding towards small chromosomes. Importantly, axis anchoring by cohesin is adjustable and readily displaced in the direction of transcription by the transcriptional machinery. We propose that such robust but flexible tethering allows the axial element to promote recombination while easily adapting to changes in chromosome activity.

DOI:http://dx.doi.org/10.7554/eLife.07424.001

Chromosomes are long molecules of DNA that represent the genetic material of an organism. In most animal cells, chromosomes are found in pairs; with one inherited from the mother and the other from the father. Sex cells—egg cells and sperm—contain half the normal number of chromosomes, so that when they fuse, the resulting single-celled embryo inherits the full set.

When sex cells are being produced, a ring made from a protein called cohesin encircles each pair of chromosomes and holds them together until they are ready to be separated. The paired chromosomes also swap sections of DNA via a process called recombination. Structures, referred to as axial elements, compact the chromosomes in each pair and bring them in close contact so that recombination can take place. In the sexually reproducing baker's yeast, axial elements contain three main proteins: cohesin, Hop1, and Red1, but it remains unclear how the entire structure is anchored to the underlying chromosomes. Furthermore, the genes encoded within the DNA of the compacted chromosomes remain active, but it is also not clear how this is possible. This is because the compacted structure would be expected to prevent the molecular machinery that expresses genes from accessing the DNA.

Sun, Huang et al. have now studied this process in budding yeast cells by using a method called ChIP-seq to determine where cohesin and the Hop1 and Red1 proteins are found along the chromosomes. The experiments showed that cohesin, Hop1, and Red1 are enriched in regions between two genes that run in the opposite directions to each other. Sun, Huang et al. also observed that cohesin recruits Red1, which in turn, recruits Hop1, and that all three proteins physically interact with one another. These findings imply that it is cohesin that anchors the axial elements to the underlying chromosomes.

Further experiments showed that cohesin slides along chromosomes towards areas where genes are active. This suggests that cohesin provides a robust, but flexible, link between the axial elements and the chromosomes. This flexibility would enable recombination and gene expression to continue in compacted chromosomes. A loss of flexibility may be one of the reasons why mutations in cohesin components of the axial element cause infertility in men and condition called premature ovarian failure in women.

DOI:http://dx.doi.org/10.7554/eLife.07424.002

The Structural Determinants behind the Epigenetic Role of Histone Variants

Histone variants are an important part of the histone contribution to chromatin epigenetics. In this review, we describe how the known structural differences of these variants from their canonical histone counterparts impart a chromatin signature ultimately responsible for their epigenetic contribution. In terms of the core histones, H2A histone variants are major players while H3 variant CenH3, with a controversial role in the nucleosome conformation, remains the genuine epigenetic histone variant. Linker histone variants (histone H1 family) haven’t often been studied for their role in epigenetics. However, the micro-heterogeneity of the somatic canonical forms of linker histones appears to play an important role in maintaining the cell-differentiated states, while the cell cycle independent linker histone variants are involved in development. A picture starts to emerge in which histone H2A variants, in addition to their individual specific contributions to the nucleosome structure and dynamics, globally impair the accessibility of linker histones to defined chromatin locations and may have important consequences for determining different states of chromatin metabolism.

Chromatin remodelling during male gametophyte development

The plant life cycle alternates between a diploid sporophytic phase and haploid gametophytic phase, with the latter giving rise to the gametes. Male gametophyte development encompasses two mitotic divisions that results in a simple three-celled structure knows as the pollen grain, in which two sperm cells are encased within a larger vegetative cell. Both cell types exhibit a very different type of chromatin organization - highly condensed in sperm cell nuclei and highly diffuse in the vegetative cell. Distinct classes of histone variants have dynamic and differential expression in the two cell lineages of the male gametophyte. Here we review how the dynamics of histone variants are linked to reprogramming of chromatin activities in the male gametophyte, compaction of the sperm cell genome and zygotic transitions post-fertilization.

Discovering centromere proteins: from cold white hands to the A, B, C of CENPs

The kinetochore is a complex molecular machine that directs chromosome segregation during mitosis. It is one of the most elaborate subcellular protein structures in eukaryotes, comprising more than 100 different proteins. Inner kinetochore proteins associate with specialized centromeric chromatin containing the histone H3 variant centromere protein A (CENP-A) in place of H3. Outer kinetochore proteins bind to microtubules and signal to delay anaphase onset when microtubules are absent. Since the first kinetochore proteins were discovered and cloned 30 years ago using autoimmune sera from patients with scleroderma-spectrum disease, much has been learnt about the composition, functions and regulation of this remarkable structure.

Histone variants and epigenetics

Histones package and compact DNA by assembling into nucleosome core particles. Most histones are synthesized at S phase for rapid deposition behind replication forks. In addition, the replacement of histones deposited during S phase by variants that can be deposited independently of replication provide the most fundamental level of chromatin differentiation. Alternative mechanisms for depositing different variants can potentially establish and maintain epigenetic states. Variants have also evolved crucial roles in chromosome segregation, transcriptional regulation, DNA repair, and other processes. Investigations into the evolution, structure, and metabolism of histone variants provide a foundation for understanding the participation of chromatin in important cellular processes and in epigenetic memory.

The AAA-ATPase molecular chaperone Cdc48/p97 disassembles sumoylated centromeres, decondenses heterochromatin, and activates ribosomal RNA genes

Centromeres mediate chromosome segregation and are defined by the centromere-specific histone H3 variant (CenH3)/centromere protein A (CENP-A). Removal of CenH3 from centromeres is a general property of terminally differentiated cells, and the persistence of CenH3 increases the risk of diseases such as cancer. However, active mechanisms of centromere disassembly are unknown. Nondividing Arabidopsis pollen vegetative cells, which transport engulfed sperm by extended tip growth, undergo loss of CenH3; centromeric heterochromatin decondensation; and bulk activation of silent rRNA genes, accompanied by their translocation into the nucleolus. Here, we show that these processes are blocked by mutations in the evolutionarily conserved AAA-ATPase molecular chaperone, CDC48A, homologous to yeast Cdc48 and human p97 proteins, both of which are implicated in ubiquitin/small ubiquitin-like modifier (SUMO)-targeted protein degradation. We demonstrate that CDC48A physically associates with its heterodimeric cofactor UFD1-NPL4, known to bind ubiquitin and SUMO, as well as with SUMO1-modified CenH3 and mutations in NPL4 phenocopy cdc48a mutations. In WT vegetative cell nuclei, genetically unlinked ribosomal DNA (rDNA) loci are uniquely clustered together within the nucleolus and all major rRNA gene variants, including those rDNA variants silenced in leaves, are transcribed. In cdc48a mutant vegetative cell nuclei, however, these rDNA loci frequently colocalized with condensed centromeric heterochromatin at the external periphery of the nucleolus. Our results indicate that the CDC48A(NPL4) complex actively removes sumoylated CenH3 from centromeres and disrupts centromeric heterochromatin to release bulk rRNA genes into the nucleolus for ribosome production, which fuels single nucleus-driven pollen tube growth and is essential for plant reproduction.

Ectopic centromere nucleation by CENP--a in fission yeast

The centromere is a specific chromosomal locus that organizes the assembly of the kinetochore. It plays a fundamental role in accurate chromosome segregation. In most eukaryotic organisms, each chromosome contains a single centromere the position and function of which are epigenetically specified. Occasionally, centromeres form at ectopic loci, which can be detrimental to the cell. However, the mechanisms that protect the cell against ectopic centromeres (neocentromeres) remain poorly understood. Centromere protein-A (CENP-A), a centromere-specific histone 3 (H3) variant, is found in all centromeres and is indispensable for centromere function. Here we report that the overexpression of CENP-A(Cnp1) in fission yeast results in the assembly of CENP-A(Cnp1) at noncentromeric chromatin during mitosis and meiosis. The noncentromeric CENP-A preferentially assembles near heterochromatin and is capable of recruiting kinetochore components. Consistent with this, cells overexpressing CENP-A(Cnp1) exhibit severe chromosome missegregation and spindle microtubule disorganization. In addition, pulse induction of CENP-A(Cnp1) overexpression reveals that ectopic CENP-A chromatin can persist for multiple generations. Intriguingly, ectopic assembly of CENP-A(cnp1) is suppressed by overexpression of histone H3 or H4. Finally, we demonstrate that deletion of the N-terminal domain of CENP-A(cnp1) results in an increase in the number of ectopic CENP-A sites and provide evidence that the N-terminal domain of CENP-A prevents CENP-A assembly at ectopic loci via the ubiquitin-dependent proteolysis. These studies expand our current understanding of how noncentromeric chromatin is protected from mistakenly assembling CENP-A.

The quantitative architecture of centromeric chromatin

The centromere, responsible for chromosome segregation during mitosis, is epigenetically defined by CENP-A containing chromatin. The amount of centromeric CENP-A has direct implications for both the architecture and epigenetic inheritance of centromeres. Using complementary strategies, we determined that typical human centromeres contain ∼400 molecules of CENP-A, which is controlled by a mass-action mechanism. This number, despite representing only ∼4% of all centromeric nucleosomes, forms a ∼50-fold enrichment to the overall genome. In addition, although pre-assembled CENP-A is randomly segregated during cell division, this amount of CENP-A is sufficient to prevent stochastic loss of centromere function and identity. Finally, we produced a statistical map of CENP-A occupancy at a human neocentromere and identified nucleosome positions that feature CENP-A in a majority of cells. In summary, we present a quantitative view of the centromere that provides a mechanistic framework for both robust epigenetic inheritance of centromeres and the paucity of neocentromere formation.

DOI:http://dx.doi.org/10.7554/eLife.02137.001

The genetic information in a cell is packed into structures called chromosomes. These contain strands of DNA wrapped around proteins called histones, which helps the long DNA chains to fit inside the relatively small nucleus of the cell.

When a cell divides, it is important that both of the new cells contain all of the genetic information found in the parent cell. Therefore, the chromosomes duplicate during cell division, with the two copies held together at a single region of the chromosome called the centromere. The centromere then recruits and coordinates the molecular machinery that separates the two copies into different cells.

Centromeres are inherited in an epigenetic manner. This means that there is no specific DNA sequence that defines the location of this structure on the chromosomes. Rather, a special type of histone, called CENP-A, is involved in defining its location. Bodor et al. use multiple techniques to show that human centromeres normally contain around 400 molecules of CENP-A, and that this number is crucial for ensuring that centromeres form in the right place. Interestingly, only a minority of the CENP-A molecules are located at centromeres; yet this is more than at any other region of the chromosome. This explains why centromeres are only formed at a single position on each chromosome.

When the chromosomes separate, the CENP-A molecules at the centromere are randomly divided between the two copies. In this way memory of the centromere location is maintained. If the number of copies of CENP-A inherited by one of the chromosomes drops below a threshold value, a centromere will not form. However, Bodor et al. found that the number of CENP-A molecules in a centromere is large enough, not only to support the formation of the centromere structure, but also to keep it above the threshold value in nearly all cases. This threshold is also high enough to make it unlikely that a centromere will form in the wrong place because of a random fluctuation in the number of CENP-A molecules. Therefore, the number of CENP-A molecules is crucial for controlling both the formation and the inheritance of the centromere.

DOI:http://dx.doi.org/10.7554/eLife.02137.002

The budding yeast Centromere DNA Element II wraps a stable Cse4 hemisome in either orientation in vivo

In budding yeast, a single cenH3 (Cse4) nucleosome occupies the ∼120-bp functional centromere, however conflicting structural models for the particle have been proposed. To resolve this controversy, we have applied H4S47C-anchored cleavage mapping, which reveals the precise position of histone H4 in every nucleosome in the genome. We find that cleavage patterns at centromeres are unique within the genome and are incompatible with symmetrical structures, including octameric nucleosomes and (Cse4/H4)₂ tetrasomes. Centromere cleavage patterns are compatible with a precisely positioned core structure, one in which each of the 16 yeast centromeres is occupied by oppositely oriented Cse4/H4/H2A/H2B hemisomes in two rotational phases within the population. Centromere-specific hemisomes are also inferred from distances observed between closely-spaced H4 cleavages, as predicted from structural modeling. Our results indicate that the orientation and rotational position of the stable hemisome at each yeast centromere is not specified by the functional centromere sequence.

DOI:http://dx.doi.org/10.7554/eLife.01861.001

DNA is tightly packaged in cells for a variety of reasons—to allow it to fit inside the nucleus, to protect it from damage, and to help control the production of proteins from genes. The basic unit of packaged DNA is called a nucleosome, which consists of DNA wrapped around a structure formed by two pairs of four different proteins.

These proteins, which are called histones, have a role that extends beyond providing structural support for DNA. When cells divide, for example, pairs of ‘sister chromosomes’ are pulled apart to ensure that the two daughter cells both have the same chromosomes as the original cell. The sister chromosomes are pulled apart from a single position called a centromere, and the nucleosomes at this position contain a histone that is different from the histones found everywhere else in the cell. However, until recently it was not clear if the nucleosomes that contained these special cenH3 histones had the same structure as other nucleosomes.

Now Henikoff et al. have used a method called H4S47C-anchored cleavage mapping to study every nucleosome in the genome of the yeast S. cerevisiae. This mapping technique uses DNA sequencing to measure the precise distances between fixed points on the DNA in the nucleosome. Knowing these distances tells researchers a great deal about the number and position of the histones within each nucleosome in the genome.

Using this approach, Henikoff et al. found that nucleosomes at centromeres are different from other nucleosomes in histone number and arrangement. In particular, the nucleosome at each yeast centromere contains only one each of the four different histones in an asymmetrical orientation, in contrast to all other yeast nucleosomes, which contain two sets of four histones in a symmetrical arrangement. Furthermore, each nucleosome at a centromere can adopt one of two orientations: these orientations are mirror images of each other, and they occur with equal probability. It should also be possible to use the mapping technique developed by Henikoff et al. to study the larger and more complex centromeres found in other organisms, including humans.

DOI:http://dx.doi.org/10.7554/eLife.01861.002

The composition, functions, and regulation of the budding yeast kinetochore

The propagation of all organisms depends on the accurate and orderly segregation of chromosomes in mitosis and meiosis. Budding yeast has long served as an outstanding model organism to identify the components and underlying mechanisms that regulate chromosome segregation. This review focuses on the kinetochore, the macromolecular protein complex that assembles on centromeric chromatin and maintains persistent load-bearing attachments to the dynamic tips of spindle microtubules. The kinetochore also serves as a regulatory hub for the spindle checkpoint, ensuring that cell cycle progression is coupled to the achievement of proper microtubule-kinetochore attachments. Progress in understanding the composition and overall architecture of the kinetochore, as well as its properties in making and regulating microtubule attachments and the spindle checkpoint, is discussed.

CAL1 is the Drosophila CENP-A assembly factor

Representing a unique family of histone assembly factors, CAL1 assembles the histone H3 variant CENP-A on centromeric DNA in Drosophila.

Centromeres are specified epigenetically by the incorporation of the histone H3 variant CENP-A. In humans, amphibians, and fungi, CENP-A is deposited at centromeres by the HJURP/Scm3 family of assembly factors, but homologues of these chaperones are absent from a number of major eukaryotic lineages such as insects, fish, nematodes, and plants. In Drosophila, centromeric deposition of CENP-A requires the fly-specific protein CAL1. Here, we show that targeting CAL1 to noncentromeric DNA in Drosophila cells is sufficient to heritably recruit CENP-A, kinetochore proteins, and microtubule attachments. CAL1 selectively interacts with CENP-A and is sufficient to assemble CENP-A nucleosomes that display properties consistent with left-handed octamers. The CENP-A assembly activity of CAL1 resides within an N-terminal domain, whereas the C terminus mediates centromere recognition through an interaction with CENP-C. Collectively, this work identifies the “missing” CENP-A chaperone in flies, revealing fundamental conservation between insect and vertebrate centromere-specification mechanisms.

Holocentromeres are dispersed point centromeres localized at transcription factor hotspots

Centromeres vary greatly in size and sequence composition, ranging from 'point' centromeres with a single cenH3-containing nucleosome to 'regional' centromeres embedded in tandemly repeated sequences to holocentromeres that extend along the length of entire chromosomes. Point centromeres are defined by sequence, whereas regional and holocentromeres are epigenetically defined by the location of cenH3-containing nucleosomes. In this study, we show that Caenorhabditis elegans holocentromeres are organized as dispersed but discretely localized point centromeres, each forming a single cenH3-containing nucleosome. These centromeric sites co-localize with kinetochore components, and their occupancy is dependent on the cenH3 loading machinery. These sites coincide with non-specific binding sites for multiple transcription factors ('HOT' sites), which become occupied when cenH3 is lost. Our results show that the point centromere is the basic unit of holocentric organization in support of the classical polycentric model for holocentromeres, and provide a mechanistic basis for understanding how centromeric chromatin might be maintained. DOI: http://dx.doi.org/10.7554/eLife.02025.001.

Holocentromeres are dispersed point centromeres localized at transcription factor hotspots

Centromeres vary greatly in size and sequence composition, ranging from ‘point’ centromeres with a single cenH3-containing nucleosome to ‘regional’ centromeres embedded in tandemly repeated sequences to holocentromeres that extend along the length of entire chromosomes. Point centromeres are defined by sequence, whereas regional and holocentromeres are epigenetically defined by the location of cenH3-containing nucleosomes. In this study, we show that Caenorhabditis elegans holocentromeres are organized as dispersed but discretely localized point centromeres, each forming a single cenH3-containing nucleosome. These centromeric sites co-localize with kinetochore components, and their occupancy is dependent on the cenH3 loading machinery. These sites coincide with non-specific binding sites for multiple transcription factors (‘HOT’ sites), which become occupied when cenH3 is lost. Our results show that the point centromere is the basic unit of holocentric organization in support of the classical polycentric model for holocentromeres, and provide a mechanistic basis for understanding how centromeric chromatin might be maintained.

DOI:http://dx.doi.org/10.7554/eLife.02025.001

During cell division, the chromosomes in the original cell must be replicated and these ‘sister chromosomes’ must then be divided equally between the two new daughter cells. At first, the sister chromosomes are held together near a region called the centromere, which is important because the microtubules that pull the sister chromosomes apart attach themselves to the centromere. In many cases, the centromere is a small region near the middle of the chromosomes, which produces a classic X shape. However, in some organisms centromeres span the entire length of the chromosomes. There are at least 13 plant and animal lineages with such holocentromeres.

Inside the nucleus of cells, DNA is wrapped around molecules called histones. There are five major families of histones, and histones belonging to one of these families—the H3 histones—are replaced by cenH3 variant histones at both conventional centromeres and holocentromeres. There are many unanswered questions about holocentromeres. In particular, do holocentromeres truly extend along the full length of the chromosomes, or are they found at a large number of specific sites?

Now Steiner and Henikoff have studied the distribution of cenH3 in the genome of the worm C. elegans to investigate holocentromeres in greater detail. These experiments showed that the holocentromere in C. elegans is actually made of about 700 individual centromeric sites distributed along the length of the chromosomes. Each of these sites contains just one nucleosome that contains cenH3, and these sites are likely to be the sites that microtubules attach to during cell division. Surprisingly, the same sites can also act as so-called ‘HOT–sites’: these sites are bound by many proteins that are involved in regulating the process by which genes are expressed as proteins, which suggests a link between centromeres and these regulatory proteins.

The work of Steiner and Henikoff describes how centromeric nucleosomes are distributed across the genome, but why and how cenH3 ends up at these particular 700 sites remains an open question.

DOI:http://dx.doi.org/10.7554/eLife.02025.002

The centromeric nucleosome-like CENP-T-W-S-X complex induces positive supercoils into DNA

The centromere is a specific genomic region upon which the kinetochore is formed to attach to spindle microtubules for faithful chromosome segregation. To distinguish this chromosomal region from other genomic loci, the centromere contains a specific chromatin structure including specialized nucleosomes containing the histone H3 variant CENP–A. In addition to CENP–A nucleosomes, we have found that centromeres contain a nucleosome-like structure comprised of the histone-fold CENP–T–W–S–X complex. However, it is unclear how the CENP–T–W–S–X complex associates with centromere chromatin. Here, we demonstrate that the CENP–T–W–S–X complex binds preferentially to ∼100 bp of linker DNA rather than nucleosome-bound DNA. In addition, we find that the CENP–T–W–S–X complex primarily binds to DNA as a (CENP–T–W–S–X)₂ structure. Interestingly, in contrast to canonical nucleosomes that negatively supercoil DNA, the CENP–T–W–S–X complex induces positive DNA supercoils. We found that the DNA-binding regions in CENP–T or CENP–W, but not CENP–S or CENP–X, are required for this positive supercoiling activity and the kinetochore targeting of the CENP–T–W–S–X complex. In summary, our work reveals the structural features and properties of the CENP–T–W–S–X complex for its localization to centromeres.

The CentO satellite confers translational and rotational phasing on cenH3 nucleosomes in rice centromeres

Plant and animal centromeres comprise megabases of highly repeated satellite sequences, yet centromere function can be specified epigenetically on single-copy DNA by the presence of nucleosomes containing a centromere-specific variant of histone H3 (cenH3). We determined the positions of cenH3 nucleosomes in rice (Oryza sativa), which has centromeres composed of both the 155-bp CentO satellite repeat and single-copy non-CentO sequences. We find that cenH3 nucleosomes protect 90-100 bp of DNA from micrococcal nuclease digestion, sufficient for only a single wrap of DNA around the cenH3 nucleosome core. cenH3 nucleosomes are translationally phased with 155-bp periodicity on CentO repeats, but not on non-CentO sequences. CentO repeats have an ∼10-bp periodicity in WW dinucleotides and in micrococcal nuclease cleavage, providing evidence for rotational phasing of cenH3 nucleosomes on CentO and suggesting that satellites evolve for translational and rotational stabilization of centromeric nucleosomes.

A comparison of in vitro nucleosome positioning mapped with chicken, frog and a variety of yeast core histones

Using high-throughput sequencing, we have mapped sequence-directed nucleosome positioning in vitro on four plasmid DNAs containing DNA fragments derived from the genomes of sheep, drosophila, human and yeast. Chromatins were prepared by reconstitution using chicken, frog and yeast core histones. We also assembled yeast chromatin in which histone H3 was replaced by the centromere-specific histone variant, Cse4. The positions occupied by recombinant frog and native chicken histones were found to be very similar. In contrast, nucleosomes containing the canonical yeast octamer or, in particular, the Cse4 octamer were assembled at distinct populations of locations, a property that was more apparent on particular genomic DNA fragments. The factors that may contribute to this variation in nucleosome positioning and the implications of the behavior are discussed.

Global 'bootprinting' reveals the elastic architecture of the yeast TFIIIB-TFIIIC transcription complex in vivo

TFIIIB and TFIIIC are multi-subunit factors required for transcription by RNA polymerase III. We present a genome-wide high-resolution footprint map of TFIIIB–TFIIIC complexes in Saccharomyces cerevisiae, obtained by paired-end sequencing of micrococcal nuclease-resistant DNA. On tRNA genes, TFIIIB and TFIIIC form stable complexes with the same distinctive occupancy pattern but in mirror image, termed ‘bootprints’. Global analysis reveals that the TFIIIB–TFIIIC transcription complex exhibits remarkable structural elasticity: tRNA genes vary significantly in length but remain protected by TFIIIC. Introns, when present, are markedly less protected. The RNA polymerase III transcription terminator is flexibly accommodated within the transcription complex and, unexpectedly, plays a major structural role by delimiting its 3′-boundary. The ETC sites, where TFIIIC binds without TFIIIB, exhibit different bootprints, suggesting that TFIIIC forms complexes involving other factors. We confirm six ETC sites and report a new site (ETC10). Surprisingly, TFIIIC, but not TFIIIB, interacts with some centromeric nucleosomes, suggesting that interactions between TFIIIC and the centromere may be important in the 3D organization of the nucleus.

Reconstitution of hemisomes on budding yeast centromeric DNA

The structure of nucleosomes that contain the cenH3 histone variant has been controversial. In budding yeast, a single right-handed cenH3/H4/H2A/H2B tetramer wraps the ∼80-bp Centromere DNA Element II (CDE II) sequence of each centromere into a ‘hemisome’. However, attempts to reconstitute cenH3 particles in vitro have yielded exclusively ‘octasomes’, which are observed in vivo on chromosome arms only when Cse4 (yeast cenH3) is overproduced. Here, we show that Cse4 octamers remain intact under conditions of low salt and urea that dissociate H3 octamers. However, particles consisting of two DNA duplexes wrapped around a Cse4 octamer and separated by a gap efficiently split into hemisomes. Hemisome dimensions were confirmed using a calibrated gel-shift assay and atomic force microscopy, and their identity as tightly wrapped particles was demonstrated by gelFRET. Surprisingly, Cse4 hemisomes were stable in 4 M urea. Stable Cse4 hemisomes could be reconstituted using either full-length or tailless histones and with a 78-bp CDEII segment, which is predicted to be exceptionally stiff. We propose that CDEII DNA stiffness evolved to favor Cse4 hemisome over octasome formation. The precise correspondence between Cse4 hemisomes resident on CDEII in vivo and reconstituted on CDEII in vitro without any other factors implies that CDEII is sufficient for hemisome assembly.

Functions of the centromere and kinetochore in chromosome segregation

Centromeres play essential roles in equal chromosome segregation by directing the assembly of the microtubule binding kinetochore and serving as the cohesion site between sister chromatids. Here, we review the significant recent progress in our understanding of centromere protein assembly and how centromere proteins form the foundation of the kinetochore.

Epigenetics and aging

Over the past two decades, a growing interest on the research of the biological basis of human longevity has emerged, in order to clarify the intricacy of biological and environmental factors affecting (together with stochastic factors) the quality and the rate of human aging. These researches have outlined a complex scenario in which epigenetic marks, such as DNA methylation and numerous histone modifications, are emerging as important factors of the overall variation in life expectancy. In fact, epigenetic marks, that are responsible of the establishment of specific expression programs and of genome stability, represent a "drawbridge" across genetic, environmental and stochastic factors. In this review we provide an overview on the current knowledge and the general features of the epigenetic modifications characterizing the aging process.

CENP-A: the key player behind centromere identity, propagation, and kinetochore assembly

Chromosome segregation is the one of the great problems in biology with complexities spanning from biophysics and polymer dynamics to epigenetics. Here, we summarize the current knowledge and highlight gaps in understanding of the mechanisms controlling epigenetic regulation of chromosome segregation.

The split personality of CENP-A nucleosomes

The composition and structure of centromeric nucleosomes, which contain the histone H3 variant CENP-A, is intensely debated. Two independent studies in this issue, in yeast and human cells, now suggest that CENP-A nucleosomes adopt different structures depending on the stage of the cell cycle.

Placing the Plasmodium falciparum epigenome on the map

It is becoming increasingly evident that epigenetic mechanisms that act on and regulate chromatin structure play a key role in the development, adaptation, and survival of the malaria parasite within its human host. The study of epigenetics in Plasmodium falciparum started to flourish in recent years due to improvement of genomic technologies. Here we summarize the knowledge gained from genome-wide localization profiling of different epigenetic features, and discuss hypotheses emerging from the analysis of these 'descriptive' epigenetic maps. Furthermore, we highlight key questions to be answered, and provide a glimpse of developments required to gain true mechanistic understanding and to lift this maturing field to the next level.