A histone is a member of a family of basic proteins that associate with DNA to organize chromatin. In eukaryotes, core histones form the protein framework of the nucleosome, the fundamental repeating unit of chromatin. Histones are not simply packaging material: their interactions with DNA, chemical modifications, and replacement by specialized variants help regulate gene expression and other processes involving the genome. (nature.com)
Structure and major families
The five major histone families are H2A, H2B, H3, H4, and H1. H2A, H2B, H3, and H4 are core histones; H1 is a linker histone with a different structural organization and mode of binding. A conventional nucleosome core contains two copies of each core histone, forming an eight-subunit complex, or octamer. Its central H3–H4 tetramer associates with two H2A–H2B dimers. (nature.com)
Core histones share a histone fold, a structural motif containing three α-helices connected by loops. This fold mediates protein–protein interactions and contributes to DNA binding. Flexible regions, particularly the amino-terminal tails, extend beyond the folded core and provide sites for interactions and chemical modification. Basic amino acids, especially lysine and arginine, contribute positively charged surfaces that interact with the negatively charged DNA backbone. (nature.com)
DNA packaging and accessibility
A conventional nucleosome core particle contains approximately 146–147 DNA base pairs wrapped around the histone octamer in about 1.7 left-handed turns. Adjacent particles are connected by linker DNA. H1 binds the nucleosome and linker DNA, stabilizing DNA near its entry and exit points and influencing chromatin compaction. H1 is therefore associated with the nucleosome but is not one of the eight core subunits. (nature.com)
Histone binding creates a regulated barrier to DNA access rather than an irreversible seal. DNA can transiently unwrap from the nucleosome surface, allowing transcription factors to encounter otherwise obscured binding sites. Experiments with reconstituted nucleosomes show that H1 can reduce this accessibility, while particular histone modifications can counteract its effects. (nature.com)
Nucleosomes also support interactions between neighboring chromatin segments. Nevertheless, their presence does not require chromatin to form a uniform, regularly coiled fiber. Electron-tomography studies of human cells have observed irregular chromatin chains packed at different densities, rather than the obligatory sequence of progressively thicker fibers depicted in some classical models. (pubmed.ncbi.nlm.nih.gov)
Histone modifications
Histones undergo post-translational modifications on their tails and within their folded domains. These modifications can change chromatin’s physical properties, alter protein binding, or influence other histone modifications. Important types include acetylation, methylation, phosphorylation, and ubiquitination. (nature.com)
Acetylation modifies lysine residues and neutralizes the positive charge of their side chains. Its effects depend on the residue and chromatin context. A well-characterized example is H4 lysine-16 acetylation: in reconstituted nucleosomal arrays, this modification inhibits compact fiber formation and interactions between arrays. Histone acetylation is also connected to transcriptional regulation; the transcriptional coactivator CBP, for example, has intrinsic histone acetyltransferase activity. (pubmed.ncbi.nlm.nih.gov)
Methylation does not have a single activating or repressing meaning. Its association with gene activity depends on the modified residue and degree of methylation. Genome-wide mapping has identified distinct patterns at promoters, enhancers, and transcribed regions. H3 lysine-4 methylation is commonly associated with active regulatory regions, whereas H3 lysine-9 and lysine-27 trimethylation are associated with repressed chromatin. Different methylation states at the same residue can have different associations. (pubmed.ncbi.nlm.nih.gov)
Phosphorylation participates in DNA-damage signaling. Phosphorylation of the H2A.X variant produces γH2AX, which accumulates in chromatin surrounding DNA double-strand breaks and helps establish a platform for the damage response and DNA repair. (pubmed.ncbi.nlm.nih.gov)
Ubiquitination attaches the small protein ubiquitin to a histone. In chromatin, this need not indicate protein degradation. Experiments in yeast demonstrated that H2B ubiquitination is required for methylation at particular H3 residues, illustrating communication between modifications on different histones. (nature.com)
Modification names use a compact notation. In H3K27me3, “H3” identifies the histone, “K27” identifies lysine at position 27, and “me3” denotes trimethylation. Such labels describe a chemical state; their biological significance must be established in the relevant cellular context. (pubmed.ncbi.nlm.nih.gov)
Histone variants
Histone variants are alternative histone proteins encoded by distinct genes. Unlike a modification added to an existing histone, a variant changes the protein’s amino-acid sequence. Even small sequence differences can affect its incorporation into chromatin or the properties of the resulting nucleosome. (pubmed.ncbi.nlm.nih.gov)
Several variants have extensively studied functions:
- H3.3 can be incorporated through pathways that do not require DNA replication. Its deposition provides a means of replacing histones at particular genomic locations, including transcriptionally active regions. (pubmed.ncbi.nlm.nih.gov)
- H2A.Z occurs at many functional regulatory elements. Structural studies show that replacing H2A with H2A.Z alters localized interactions within the nucleosome, while genomic studies associate it with promoters and other regulatory regions. (nature.com)
- H2A.X serves as a substrate for phosphorylation during the DNA-damage response. (pubmed.ncbi.nlm.nih.gov)
- CENP-A, a specialized H3 variant, marks centromeric chromatin and helps specify the site where the kinetochore assembles for accurate chromosome segregation. Human CENP-A nucleosomes retain an octameric organization but differ from conventional H3 nucleosomes in structural details and DNA-end flexibility. (nature.com)
Histone assembly and turnover
Histones are incorporated into chromatin through regulated assembly pathways involving histone chaperones. These proteins bind histones and help deliver them into nucleosomes. Studies of human H3 complexes distinguished a CAF-1-associated pathway for H3.1 assembly coupled to DNA synthesis from a HIRA-associated pathway for replication-independent H3.3 assembly. (pubmed.ncbi.nlm.nih.gov)
Chromatin is also remodeled during transcription. Histones may be displaced, exchanged, and redeposited as the transcription machinery passes through a region. Experiments in yeast have shown that histone chaperones influence this exchange, demonstrating that nucleosomes are dynamic structures rather than permanently fixed DNA–protein complexes. (pubmed.ncbi.nlm.nih.gov)
Histones and epigenetic information
Histones are central to epigenetics because chromatin organization can contribute to the maintenance of functional states without changing DNA sequence. CENP-A-containing chromatin provides a particularly well-established example: experimental studies have separated the maintenance of centromere identity from the recruitment of proteins needed for kinetochore function. (nature.com)
The histone-code hypothesis, proposed in 2000, suggested that combinations of histone modifications could be recognized by other proteins to produce distinct downstream events. This framework emphasizes interactions among modifications rather than treating each mark in isolation. It does not establish a universal dictionary in which every modification has one fixed outcome. Genome-wide associations between histone marks and transcriptional states must also be distinguished from evidence that a mark causes that state. (nature.com)
Evolution and history of research
Histone-based DNA organization is not restricted to eukaryotes. Many archaea possess related histones. Structural work has shown archaeal histone dimers assembling into extended DNA-wrapping structures, sharing important features with eukaryotic nucleosomes while differing from their fixed octameric organization. (pubmed.ncbi.nlm.nih.gov)
A major historical advance came in 1974, when Roger Kornberg proposed a repeating chromatin unit composed of histones and DNA; experimental work that year supported a subunit organization. In 1997, X-ray crystallography revealed the nucleosome core particle at 2.8 Å resolution, establishing an atomic description of histone assembly and DNA wrapping. The subsequent development of genome-wide chromatin mapping connected particular histone states with regulatory elements across the genome. (pubmed.ncbi.nlm.nih.gov)
References
- Crystal structure of the nucleosome core particle at 2.8 Å resolutionnature.com
- The language of covalent histone modificationsnature.com
- Linker histone H1 and H3K56 acetylation are antagonistic regulators of nucleosome dynamicsnature.com
- Linker histone defines structure and self-association behaviour of the 177 bp human chromatosomenature.com
- ChromEMT: Visualizing 3D chromatin structure and compaction in interphase and mitotic cellspubmed.ncbi.nlm.nih.gov
- Histone H4-K16 acetylation controls chromatin structure and protein interactionspubmed.ncbi.nlm.nih.gov
- The CBP co-activator is a histone acetyltransferasepubmed.ncbi.nlm.nih.gov
- High-resolution profiling of histone methylations in the human genomepubmed.ncbi.nlm.nih.gov
- DNA double-strand breaks induce H2Ax phosphorylation domains in a contact-dependent mannerpubmed.ncbi.nlm.nih.gov
- Trans-histone regulatory pathway in chromatinnature.com
- The histone variant H3.3 marks active chromatin by replication-independent nucleosome assemblypubmed.ncbi.nlm.nih.gov
- Crystal structure of a nucleosome core particle containing the variant histone H2A.Znature.com