Molecular Biology Notes: Histones, Nucleosomes, and Chromatin

Learning Objectives

  • Understand the concepts of histones, nucleosomes, and 30-nm fiber

  • Explain the role of histones in gene transcription

  • Describe how histone acetylation and chromatin remodeling impact gene expression

  • Comprehend the concept of the histone code

Eukaryotic DNA Structure

  • Eukaryotic DNA is not naked but complexed with histone proteins to form chromatin.

  • Initially considered unimportant, histones interact dynamically with DNA and transcription factors, influencing gene expression.

Histone Types

  • There are 5 main types of histones in eukaryotic cells:

    • H1

    • H2A

    • H2B

    • H3

    • H4

Properties of Histones

  • Histones are highly abundant proteins, with a mass in nuclei nearly equal to that of DNA.

  • They possess a pronounced positive charge at neutral pH, essential for binding to negatively charged DNA.

  • Histones are well-conserved across species, with Histone H4 showing only two amino acid differences between cows and peas.

Variability in Histones

  • Histone genes are highly multi-copy (e.g., 10–20 copies in mice; ~100 in Drosophila).

  • Posttranslational modifications lead to tremendous variability in histones, including:

    • Lysine N-acetylation

    • Serine O-phosphorylation

    • Threonine O-phosphorylation

    • Lysine N-phosphorylation

    • Histidine N-phosphorylation

    • Lysine methylation

Introduction to Nucleosomes

  • Nucleosomes are the first level of DNA folding, essential for condensing 2 meters of DNA into a 10 μm nucleus.

  • X-ray diffraction shows a strong repeat structure in nucleosomes at 100 Å intervals.

Nucleosome Structure

  • Each nucleosome consists of a central core formed by pairs of H2A, H2B, H3, and H4 histones, around which 147 base pairs (bp) of DNA are tightly wrapped.

  • This wrapping condenses the DNA length significantly, aiding in efficient packaging and organization.

Formation of the 30-nm Fiber

  • Nucleosomes further condense into a 30-nm fiber through interactions between histones, especially H1.

  • Higher ionic strength helps in forming and stabilizing this fiber configuration.

  • The 30-nm fiber exhibits a zig-zag structure formed by stacked nucleosomes and linker DNA.

Higher Order Chromatin Folding

  • 30-nm fibers account for most chromatin in a typical interphase nucleus, leading to additional folding, notably in mitotic chromosomes.

  • Further folding might involve the formation of radial loops, a favored model in chromatin architecture.

Histones and Transcription

  • Core histones can repress transcription when assembled on DNA, leading to significant transcriptional repression.

  • Histone H1 adds an additional layer of repression, which can be counteracted by specific transcription factors like Sp1 and GAL4.

Mechanisms of Nucleosome Positioning

  • Activators can reposition nucleosomes to uncover promoters, enhancing transcription potential.

  • They may also prevent initial nucleosome binding to promoters, facilitating active transcription initiation.

Histone Acetylation

  • Histone acetylation affects the gene activity positively: acetylated histones act as weaker repressors compared to unacetylated forms.

  • Acetylation is mediated by histone acetyltransferases (HATs) in both the cytoplasm and nucleus.

  • Specific lysine residues (e.g., K9 and K14 of H3, K5, K8, K16 of H4) are commonly acetylated in active chromatin.

Histone Deacetylation

  • Histone deacetylation is a repressing mechanism, where deacetylated core histones bind more tightly to DNA, hindering transcription.

  • Histone deacetylases (HDACs) play a crucial role in this repressive regulation.

The Histone Code Hypothesis

  • The combination of various histone modifications provides a code affecting transcriptional efficiency of nearby genes.

  • This code is epigenetic, influencing gene activity without altering the DNA sequence.

Chromatin Remodeling

  • Involves movement of nucleosomes and loosening of DNA-histone interactions to facilitate transcription factor binding.

  • ATP-driven complexes like SWI/SNF play vital roles in chromatin remodeling, making DNA regions more accessible for transcription.

  • Different combinations of histone modifications can send distinct signals to cells regarding transcription regulation.

Heterochromatin vs. Euchromatin

  • Euchromatin: Extended and open, where most active genes are located.

  • Heterochromatin: Condensed DNA, often silencing genes significantly, especially near centromeres and telomeres.

Summary of Key Points

  • Histones are vital for packaging DNA and regulating transcription.

  • Posttranslational modifications on histones, particularly acetylation and their code, critically influence gene expression.

  • Chromatin remodeling is often essential for enabling transcription in eukaryotic cells.