A giant virus genome is densely packaged by stable nucleosomes within virions.

The two doublet histones of Marseillevirus are distantly related to the four eukaryotic core histones and wrap 121 base pairs of DNA to form remarkably similar nucleosomes. By permeabilizing Marseillevirus virions and performing genome-wide nuclease digestion, chemical cleavage, and mass spectrometry assays, we find that the higher-order organization of Marseillevirus chromatin fundamentally differs from that of eukaryotes. Marseillevirus nucleosomes fully protect DNA within virions as closely abutted 121-bp DNA-wrapped cores without linker DNA or phasing along genes. Likewise, we observed that nucleosomes reconstituted onto multi-copy tandem repeats of a nucleosome-positioning sequence are tightly packed. Dense promiscuous packing of fully wrapped nucleosomes rather than "beads on a string" with genic punctuation represents a distinct mode of DNA packaging by histones. We suggest that doublet histones have evolved for viral genome protection and may resemble an early stage of histone differentiation leading to the eukaryotic octameric nucleosome.

Histone variants on the move: substrates for chromatin dynamics.

Most histones are assembled into nucleosomes behind the replication fork to package newly synthesized DNA. By contrast, histone variants, which are encoded by separate genes, are typically incorporated throughout the cell cycle. Histone variants can profoundly change chromatin properties, which in turn affect DNA replication and repair, transcription, and chromosome packaging and segregation. Recent advances in the study of histone replacement have elucidated the dynamic processes by which particular histone variants become substrates of histone chaperones, ATP-dependent chromatin remodellers and histone-modifying enzymes. Here, we review histone variant dynamics and the effects of replacing DNA synthesis-coupled histones with their replication-independent variants on the chromatin landscape.

Giant variations in giant virus genome packaging.

Giant viruses (Nucleocytoviricota) have a largely conserved lifecycle, yet how they cram their large genomes into viral capsids is mostly unknown. The major capsid protein and the packaging ATPase (pATPase) comprise a highly conserved morphogenesis module in giant viruses, yet some giant viruses dispense with an icosahedral capsid, and others encode multiple versions of pATPases, including conjoined ATPase doublets, or encode none. Some giant viruses have acquired DNA-condensing proteins to compact their genomes, including sheath-like structures encasing folded DNA or densely packed viral nucleosomes that show a resemblance to eukaryotic nucleosomes at the telomeres. Here, we review what is known and unknown about these ATPases and condensing proteins, and place these variations in the context of viral lifecycles.

The genetics and epigenetics of satellite centromeres.

Centromeres, the chromosomal loci where spindle fibers attach during cell division to segregate chromosomes, are typically found within satellite arrays in plants and animals. Satellite arrays have been difficult to analyze because they comprise megabases of tandem head-to-tail highly repeated DNA sequences. Much evidence suggests that centromeres are epigenetically defined by the location of nucleosomes containing the centromere-specific histone H3 variant cenH3, independently of the DNA sequences where they are located; however, the reason that cenH3 nucleosomes are generally found on rapidly evolving satellite arrays has remained unclear. Recently, long-read sequencing technology has clarified the structures of satellite arrays and sparked rethinking of how they evolve, and new experiments and analyses have helped bring both understanding and further speculation about the role these highly repeated sequences play in centromere identification.

Old cogs, new tricks: the evolution of gene expression in a chromatin context.

Sophisticated gene-regulatory mechanisms probably evolved in prokaryotes billions of years before the emergence of modern eukaryotes, which inherited the same basic enzymatic machineries. However, the epigenomic landscapes of eukaryotes are dominated by nucleosomes, which have acquired roles in genome packaging, mitotic condensation and silencing parasitic genomic elements. Although the molecular mechanisms by which nucleosomes are displaced and modified have been described, just how transcription factors, histone variants and modifications and chromatin regulators act on nucleosomes to regulate transcription is the subject of considerable ongoing study. We explore the extent to which these transcriptional regulatory components function in the context of the evolutionarily ancient role of chromatin as a barrier to processes acting on DNA and how chromatin proteins have diversified to carry out evolutionarily recent functions that accompanied the emergence of differentiation and development in multicellular eukaryotes.

The Yin and Yang of Histone Marks in Transcription.

Nucleosomes wrap DNA and impede access for the machinery of transcription. The core histones that constitute nucleosomes are subject to a diversity of posttranslational modifications, or marks, that impact the transcription of genes. Their functions have sometimes been difficult to infer because the enzymes that write and read them are complex, multifunctional proteins. Here, we examine the evidence for the functions of marks and argue that the major marks perform a fairly small number of roles in either promoting transcription or preventing it. Acetylations and phosphorylations on the histone core disrupt histone-DNA contacts and/or destabilize nucleosomes to promote transcription. Ubiquitylations stimulate methylations that provide a scaffold for either the formation of silencing complexes or resistance to those complexes, and carry a memory of the transcriptional state. Tail phosphorylations deconstruct silencing complexes in particular contexts. We speculate that these fairly simple roles form the basis of transcriptional regulation by histone marks.

Histone variants at a glance.

Eukaryotic nucleosomes organize chromatin by wrapping 147 bp of DNA around a histone core particle comprising two molecules each of histone H2A, H2B, H3 and H4. The DNA entering and exiting the particle may be bound by the linker histone H1. Whereas deposition of bulk histones is confined to S-phase, paralogs of the common histones, known as histone variants, are available to carry out functions throughout the cell cycle and accumulate in post-mitotic cells. Histone variants confer different structural properties on nucleosomes by wrapping more or less DNA or by altering nucleosome stability. They carry out specialized functions in DNA repair, chromosome segregation and regulation of transcription initiation, or perform tissue-specific roles. In this Cell Science at a Glance article and the accompanying poster, we briefly examine new insights into histone origins and discuss variants from each of the histone families, focusing on how structural differences may alter their functions.

Histone variants--ancient wrap artists of the epigenome.

Histones wrap DNA to form nucleosome particles that compact eukaryotic genomes. Variant histones have evolved crucial roles in chromosome segregation, transcriptional regulation, DNA repair, sperm packaging and other processes. 'Universal' histone variants emerged early in eukaryotic evolution and were later displaced for bulk packaging roles by the canonical histones (H2A, H2B, H3 and H4), the synthesis of which is coupled to DNA replication. Further specializations of histone variants have evolved in some lineages to perform additional tasks. Differences among histone variants in their stability, DNA wrapping, specialized domains that regulate access to DNA, and post-translational modifications, underlie the diverse functions that histones have acquired in evolution.

Transcribing Centromeres: Noncoding RNAs and Kinetochore Assembly.

Chromosome segregation depends on the attachment of spindle microtubules to sites on chromosomal DNA known as centromeres, through kinetochore protein complexes. Although RNA was found in kinetochores in the 1970s, only with recent investigations has evidence emerged that loading of the centromere-specific nucleosomes that form the foundation of the kinetochore may be coupled to centromeric transcription. Centromeric transcripts are bound by several kinetochore proteins that require them for stabilization or localization. At least some centromeres have promoter activity, and many have non-B form DNA that may facilitate their transcription. Whereas other noncoding RNAs regulate gene expression or silence transposons, cotranscriptional assembly of kinetochores is a novel function for noncoding RNAs.

What makes a centromere?

Centromeres are the eukaryotic chromosomal sites at which the kinetochore forms and attaches to spindle microtubules to orchestrate chromosomal segregation in mitosis and meiosis. Although centromeres are essential for cell division, their sequences are not conserved and evolve rapidly. Centromeres vary dramatically in size and organization. Here we categorize their diversity and explore the evolutionary forces shaping them. Nearly all centromeres favor AT-rich DNA that is gene-free and transcribed at a very low level. Repair of frequent centromere-proximal breaks probably contributes to their rapid sequence evolution. Point centromeres are only ~125 bp and are specified by common protein-binding motifs, whereas short regional centromeres are 1-5 kb, typically have unique sequences, and may have pericentromeric repeats adapted to facilitate centromere clustering. Transposon-rich centromeres are often ~100-300 kb and are favored by RNAi machinery that silences transposons, by suppression of meiotic crossovers at centromeres, and by the ability of some transposons to target centromeres. Megabase-length satellite centromeres arise in plants and animals with asymmetric female meiosis that creates centromere competition, and favors satellite monomers one or two nucleosomes in length that position and stabilize centromeric nucleosomes. Holocentromeres encompass the length of a chromosome and may differ dramatically between mitosis and meiosis. We propose a model in which low level transcription of centromeres facilitates the formation of non-B DNA that specifies centromeres and promotes loading of centromeric nucleosomes.

Chromatin-based transcriptional punctuation.

The long polycistronic transcription units of trypanosomes do not appear to be demarcated by the usual DNA motifs that punctuate transcription in familiar eukaryotes. In this issue of Genes & Development, Siegel and colleagues (pp. 1063-1076) describe a system for the demarcation of trypanosome transcription units based on the deposition and turnover of histone variants rather than on the binding of transcription factors. Replication-independent incorporation of histone variants and destabilization of nucleosomes is an emerging theme at promoters of more familiar eukaryotes, and it now appears that this system is an evolutionarily conserved mode of transcriptional punctuation.

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.

Cell-type-specific nuclei purification from whole animals for genome-wide expression and chromatin profiling.

An understanding of developmental processes requires knowledge of transcriptional and epigenetic landscapes at the level of tissues and ultimately individual cells. However, obtaining tissue- or cell-type-specific expression and chromatin profiles for animals has been challenging. Here we describe a method for purifying nuclei from specific cell types of animal models that allows simultaneous determination of both expression and chromatin profiles. The method is based on in vivo biotin-labeling of the nuclear envelope and subsequent affinity purification of nuclei. We describe the use of the method to isolate nuclei from muscle of adult Caenorhabditis elegans and from mesoderm of Drosophila melanogaster embryos. As a case study, we determined expression and nucleosome occupancy profiles for affinity-purified nuclei from C. elegans muscle. We identified hundreds of genes that are specifically expressed in muscle tissues and found that these genes are depleted of nucleosomes at promoters and gene bodies in muscle relative to other tissues. This method should be universally applicable to all model systems that allow transgenesis and will make it possible to determine epigenetic and expression profiles of different tissues and cell types.

Expansion of human centromeric arrays in cells undergoing break-induced replication.

Human centromeres are located within α-satellite arrays and evolve rapidly, which can lead to individual variation in array length. Proposed mechanisms for such alterations in length are unequal crossover between sister chromatids, gene conversion, and break-induced replication. However, the underlying molecular mechanisms responsible for the massive, complex, and homogeneous organization of centromeric arrays have not been experimentally validated. Here, we use droplet digital PCR assays to demonstrate that centromeric arrays can expand and contract within ∼20 somatic cell divisions of an alternative lengthening of telomere (ALT)-positive cell line. We find that the frequency of array variation among single-cell-derived subclones ranges from a minimum of ∼7% to a maximum of ∼100%. Further clonal evolution revealed that centromere expansion is favored over contraction. We find that the homologous recombination protein RAD52 and the helicase PIF1 are required for extensive array change, suggesting that centromere sequence evolution can occur via break-induced replication.

Sequencing of a rice centromere uncovers active genes.

Centromeres are the last frontiers of complex eukaryotic genomes, consisting of highly repetitive sequences that resist mapping, cloning and sequencing. The centromere of rice Chromosome 8 (Cen8) has an unusually low abundance of highly repetitive satellite DNA, which allowed us to determine its sequence. A region of approximately 750 kb in Cen8 binds rice CENH3, the centromere-specific H3 histone. CENH3 binding is contained within a larger region that has abundant dimethylation of histone H3 at Lys9 (H3-Lys9), consistent with Cen8 being embedded in heterochromatin. Fourteen predicted and at least four active genes are interspersed in Cen8, along with CENH3 binding sites. The retrotransposons located in and outside of the CENH3 binding domain have similar ages and structural dynamics. These results suggest that Cen8 may represent an intermediate stage in the evolution of centromeres from genic regions, as in human neocentromeres, to fully mature centromeres that accumulate megabases of homogeneous satellite arrays.

Expansion of human centromeric arrays in cells undergoing break-induced replication.

Human centromeres are located within α-satellite arrays and evolve rapidly, which can lead to individual variation in array lengths. Proposed mechanisms for such alterations in lengths are unequal cross-over between sister chromatids, gene conversion, and break-induced replication. However, the underlying molecular mechanisms responsible for the massive, complex, and homogeneous organization of centromeric arrays have not been experimentally validated. Here, we use droplet digital PCR assays to demonstrate that centromeric arrays can expand and contract within ~20 somatic cell divisions of a cell line. We find that the frequency of array variation among single-cell-derived subclones ranges from a minimum of ~7% to a maximum of ~100%. Further clonal evolution revealed that centromere expansion is favored over contraction. We find that the homologous recombination protein RAD52 and the helicase PIF1 are required for extensive array change, suggesting that centromere sequence evolution can occur via break-induced replication.

Characterization of CENH3 proteins and centromere-associated DNA sequences in diploid and allotetraploid Brassica species.

CENH3 is a centromere-specific histone H3 variant and has been used as a marker to identify active centromeres and DNA sequences associated with functional centromere/kinetochore complexes. In this study, up to four distinct CENH3 (BrCENH3) cDNAs were identified in individuals of each of three diploid species of Brassica. Comparison of the BrCENH3 cDNAs implied three related gene families: BrCENH3-A in Brassica rapa (AA), BrCENH3-B in B. nigra (BB), and BrCENH3-C in B. oleracea (CC). Each family encoded a histone fold domain and N-terminal histone tails that vary in length in all three families. The BrCENH3-B cDNAs have a deletion of two exons relative to BrCENH3-A and BrCENH3-C, consistent with the more ancient divergence of the BB genome. Chromatin immunoprecipitation and immunolabeling tests with anti-BrCENH3 antibodies indicated that both centromeric tandem repeats and the centromere-specific retrotransposons of Brassica are directly associated with BrCENH3 proteins. In three allotetraploid species, we find either co-transcription of the BrCENH3 genes of the ancestral diploid species or gene suppression of the BrCENH3 from one ancestor. Although B genome centromeres are occupied by BrCENH3-B in the ancestral species B. nigra, in allotetraploids both BrCENH3-A and BrCENH3-C proteins appear to assemble at these centromeres.

Viral histones: pickpocket's prize or primordial progenitor?

The common histones H2A, H2B, H3, and H4 are the characteristic components of eukaryotic nucleosomes, which function to wrap DNA and compact the genome as well as to regulate access to DNA for transcription and replication in all eukaryotes. In the past two decades, histones have also been found to be encoded in some DNA viruses, where their functions and properties are largely unknown, though recently histones from two related viruses have been shown to form nucleosome-like structures in vitro. Viral histones can be highly similar to eukaryotic histones in primary sequence, suggesting they have been recently picked up from eukaryotic hosts, or they can be radically divergent in primary sequence and may occur as conjoined histone doublets, triplets, or quadruplets, suggesting ancient origins prior to the divergence of modern eukaryotes. Here, we review what is known of viral histones and discuss their possible origins and functions. We consider how the viral life cycle may affect their properties and histories, and reflect on the possible roles of viruses in the origin of the nucleus of modern eukaryotic cells.

A standardized nomenclature for mammalian histone genes.

Histones have a long history of research in a wide range of species, leaving a legacy of complex nomenclature in the literature. Community-led discussions at the EMBO Workshop on Histone Variants in 2011 resulted in agreement amongst experts on a revised systematic protein nomenclature for histones, which is based on a combination of phylogenetic classification and historical symbol usage. Human and mouse histone gene symbols previously followed a genome-centric system that was not applicable across all vertebrate species and did not reflect the systematic histone protein nomenclature. This prompted a collaboration between histone experts, the Human Genome Organization (HUGO) Gene Nomenclature Committee (HGNC) and Mouse Genomic Nomenclature Committee (MGNC) to revise human and mouse histone gene nomenclature aiming, where possible, to follow the new protein nomenclature whilst conforming to the guidelines for vertebrate gene naming. The updated nomenclature has also been applied to orthologous histone genes in chimpanzee, rhesus macaque, dog, cat, pig, horse and cattle, and can serve as a framework for naming other vertebrate histone genes in the future.