Chromatin Remodeling and Histone Code – Part 3 Notes

Histone H1 and the histone core particle

  • Core histone octamer comprises two copies of each core histone: H2A,H2B,H3,H4H2A, H2B, H3, H4 per nucleosome.
  • Histone H1 is not part of the core octamer; it is a linker histone, larger than the core histones, and binds nucleosomes at a 1:1 ratio (one H1 per nucleosome).
  • H1 binding is not universal to every nucleosome and its presence is thought to be particularly important during mitosis.
  • When H1 binds, it does not become part of the core particle; instead it binds on the outside and constrains the movement of an additional 2020 base pairs of DNA, changing the exit angle of the DNA from the nucleosome.
  • This alters chromatin compaction: H1 promotes tighter packing of nucleosomes.
  • Histone N-terminal tails (protruding from the nucleosome) also contribute to packing by allowing interactions between tails from adjacent nucleosomes, cooperatively with H1 to influence chromatin compactness.

Chromatin remodeling and how chromatin can be altered in cells

  • Chromatin is dynamic; nucleosome positions can shift in response to cellular signals, altering DNA accessibility for transcription and other processes.
  • This remodeling is driven by ATP-dependent chromatin remodeling complexes.
  • Mechanism (nucleosome sliding):
    • Remodelers bind nucleosomes, contacting the histone octamer and the DNA.
    • They use ATP hydrolysis to temporarily loosen the DNA-histone interaction.
    • This enables repositioning of the nucleosome along the DNA, moving nucleosomes closer together or further apart (sliding).
  • Example in yeast: the RSC chromatin remodeling complex is a large ATP-dependent remodeler.
    • RSC is described as a dimer of ISW1 proteins; it couples neighboring nucleosomes (colored in the diagram) and can reposition them.
    • Depending on the ISW1-type members, remodeling can be involved in transcriptional repression, proximal nucleosome shifting, or regulation of RNA polymerase traversal across a gene body.
  • Not all functions of ATP-dependent remodelers involve sliding: some remove histone octamers entirely to replace with new octamers containing histone variants (e.g., H2AZ, H2AX).
  • Histone chaperones assist in exchanging histone dimers or whole octamers that are not currently part of a nucleosome.
  • In addition to full octamer exchange, dimers can be swapped or reassembled with the help of chaperones.

Gamma H2AX and DNA damage signaling

  • A specific histone variant, H2AX, becomes phosphorylated at sites of DNA double-strand breaks (DSBs);
    • The phosphorylated form is called gamma-H2AX.
    • Gamma-H2AX serves as a marker for DNA breaks and helps recruit and organize DNA repair machinery.
  • This is an example of how chromatin remodeling and histone modifications participate in genome maintenance.

Euchromatin vs. heterochromatin: definitions, locations, and gene content

  • Euchromatin: relaxed, open chromatin associated with active transcription; more accessible regions.
  • Heterochromatin: compact, transcriptionally silent chromatin.
  • Localization and genome features of heterochromatin:
    • Concentrated at telomeres and centromeres.
    • In the nucleus, often associated with the nuclear lamina at the periphery.
    • Contains relatively few genes; the genes it contains are typically silenced.
    • Accounts for roughly 10%%10\%\% or more of the genome in a given cell type (the transcript notes say "just over 10% of the genome").
  • Dynamics:
    • Euchromatin can become heterochromatin (transcriptional silencing) and heterochromatin can spread into neighboring euchromatin.
    • Barrier DNA sequences can halt this spreading.

Barrier DNA sequences and the control of heterochromatin spreading

  • Heterochromatin spreading is a positive feedback process requiring coordinated action of reader and writer complexes:
    • Writer proteins add histone tail modifications that signal heterochromatin formation (e.g., specific methylations).
    • Reader proteins recognize these modifications and recruit the same type of writer to neighboring nucleosomes, propagating the modification to adjacent regions.
    • This propagates a spreading wave of chromatin condensation.
  • Barrier sequences block spreading via different strategies depending on the recruited barrier proteins:
    • A) Barrier protein tethering DNA to a nuclear structure (e.g., nuclear pore) creates a physical barrier to spreading.
    • B) A barrier protein that covers several adjacent nucleosomes, making them inaccessible to enzymes that propagate heterochromatin.
    • C) An enzyme recruited to the barrier sequence that erases heterochromatin-signaling histone modifications.
  • Barrier sequences thus help to preserve euchromatin and prevent inappropriate silencing of genes.

A concrete example: position effect variegation in Drosophila

  • In Drosophila, the white gene (orange in the diagram) determines red eye pigment production in wild-type flies; the gene is located in euchromatin and normally expressed (red eyes).
  • A barrier sequence exists downstream of the white gene that prevents heterochromatin spread and maintains gene expression.
  • If a chromosomal inversion places the barrier sequence away from the euchromatin area or disrupts barrier function, heterochromatin can spread into the white gene region during early development in some cells.
  • Resulting pattern:
    • Some cells silence the white gene, others do not, depending on how much heterochromatin spreads in each cell lineage.
    • This produces a variegated or mottled eye phenotype in the adult fly.
  • Key concept: heterochromatin spreading and barrier action are heritable through cell divisions, leading to clonal patterns of gene expression in tissues.

The histone code: covalent modifications, enzymes, and readers

  • Covalent modifications to histone N-terminal tails are central to chromatin regulation:
    • Lysine acetylation (on tails): additive mark for open chromatin; generally associated with active transcription.
    • Lysine methylation: can be mono-, di-, or tri-methylated; effects depend on position and number of methyl groups.
    • Serine phosphorylation: dynamic modification regulated by kinases and phosphatases.
    • Ubiquitination of lysines can also occur (noted as possible extension to the methylation/acetylation scheme in the slide).
  • Enzymes involved in adding/removing these modifications:
    • Histone acetyltransferases (HATs) add acetyl groups to lysines.
    • Histone deacetylases (HDACs) remove acetyl groups.
    • Histone methyltransferases (HMTs) add methyl groups to lysines.
    • Histone demethylases (HDMs) remove methyl groups.
    • Kinases add phosphate groups to serine residues; phosphatases remove them.
  • These modifications are dynamic and reversible, enabling rapid remodeling of chromatin states in response to cellular cues.
  • The histone tails contain multiple modification sites; the exact pattern and combination of modifications influence chromatin structure and the recruitment of other proteins.
  • The cumulative effect of multiple modifications constitutes a histone code, read by specialized reader protein complexes.

Readers, writers, and the histone code in action

  • Reader protein modules recognize specific histone tail modifications (often in combination with other marks).
  • Binding by reader complexes can recruit chromatin remodelers, transcriptional activators or repressors, or writer complexes that propagate modifications to adjacent nucleosomes.
  • Importantly, readers often recognize multiple histone marks simultaneously; stable binding usually requires a cluster of compatible marks, not a single modification.
  • A concrete example (on histone H3):
    • Trimethylation at lysine 4 (H3K4me3) is commonly associated with active/open chromatin and gene expression.
    • Trimethylation at lysine 9 (H3K9me3) and acetylation at lysine 9 (H3K9ac) are mutually exclusive at that site; H3K9me3 is associated with heterochromatin and silencing, while H3K9ac is associated with euchromatin and expression.
    • Another example is H3K27 trimethylation (H3K27me3), which signals gene silencing.
  • Note: Specific effects are context-dependent; marks are not read in isolation, and different combinations yield different outcomes.

Putting it together: how barrier and reader/writer dynamics control heterochromatin spreading

  • Model of spreading:
    • An open chromatin region contains a regulatory DNA sequence bound by a transcription factor or regulatory protein (shown in purple).
    • This regulator recruits a writer complex that introduces a heterochromatin-associated histone modification (e.g., a methyl mark such as H3K9me3 or H3K27me3).
    • A reader protein recognizes this modification and recruits the same or another writer that propagates the modification to the next nucleosome, continuing the spread.
    • The spreading continues as long as reader-writer feedback persists.
  • Barrier sequences block spreading by different mechanisms (see section on barrier DNA sequences).

Key takeaways and connections

  • Chromatin remodeling and histone modifications are dynamic and interconnected:
    • Nucleosome positioning can be shifted by ATP-dependent remodelers (e.g., RSC) to regulate access to DNA.
    • H1 linker histone helps compact chromatin by constraining extra DNA (≈2020 base pairs) and by promoting interactions between adjacent nucleosomes via tails.
    • The histone tail modifications—the histone code—modulate packing, protein binding, and transcriptional outcomes.
  • Heterochromatin spread is a regulated, self-propagating process that can silence genes over large regions, but barrier sequences and specific context can prevent inappropriate silencing and preserve gene expression where needed.
  • Biological significance:
    • Dynamic chromatin states underlie gene expression programs during development and in response to environmental cues.
    • Dysregulation of chromatin states and the barrier systems can lead to aberrant gene silencing or misregulation of genome organization.
  • Practical implications:
    • Understanding barrier elements and histone modifications informs how gene expression is stably inherited through cell divisions and how epigenetic states can be reprogrammed.
    • The gamma-H2AX signal is an example of how chromatin modifications participate directly in DNA repair pathways.

References to numbers, constants, and units mentioned

  • Nucleosome core: two copies of each of the core histones per nucleosome: H2A,H2B,H3,H4H2A, H2B, H3, H4.
  • Linker histone H1: binds nucleosomes at a 1:1 ratio (one H1 per nucleosome). The core octamer has two copies of each core histone: 22 copies per histone type per nucleosome.
  • H1 constrains the movement of an additional 2020 base pairs of DNA.
  • Heterochromatin typically contains relatively few genes and is associated with the nuclear lamina and telomeres/centromeres, with more compact packing.
  • Barrier sequences can stop heterochromatin spread, yielding cell-type- or lineage-specific patterns of gene expression during development (e.g., clonal heterochromatin patterns).
  • In the Drosophila eye example, position effect variegation arises when barrier protection is lost or disrupted, leading to a mottled eye phenotype due to partial silencing of the white gene across cells.

Glossary (quick reference)

  • Nucleosome: DNA wrapped around a histone octamer (core particle).
  • Histone tail: N-terminal region of histones that protrudes from the nucleosome and is heavily modified.
  • HAT: Histone acetyltransferase; adds acetyl groups to lysines.
  • HDAC: Histone deacetylase; removes acetyl groups.
  • HMT: Histone methyltransferase; adds methyl groups to lysines.
  • HDM: Histone demethylase; removes methyl groups.
  • Gamma-H2AX: phosphorylated H2AX at sites of DNA double-strand breaks; a marker for DNA damage and repair.
  • Barrier DNA sequence: DNA elements that prevent spread of heterochromatin into euchromatin by various mechanisms (physical barrier, covering nucleosomes, erasing marks).
  • Position effect variegation: variegated gene expression pattern due to position of a gene near heterochromatin resulting in mosaic silencing.
  • Reader complex: proteins that recognize specific histone modifications and recruit additional factors.
  • Writer complex: enzymes that add histone modifications; can repropagate marks to neighboring nucleosomes.

Next topics preview

  • How barrier sequences are established and what kinds of barrier proteins exist.
  • How euchromatin-to-heterochromatin transitions are regulated during development.
  • The broader organization of chromosomes and chromatin in the nucleus (to be covered in the next lecture).