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Chromatin Modification in iPS Cells

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Chromatin modification alters gene expression; therefore, scientists can add histone-modifying enzymes, histone variants, and chromatin remodeling complexes to somatic cells to aid reprogramming into pluripotent stem (iPS) cells.
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The extent of chromatin compaction can be studied by staining chromatin using specific DNA binding dyes. Under the microscope, the dense-compacted regions that take up more dye are called heterochromatin. Heterochromatin is further classified into two forms – constitutive heterochromatin and facultative heterochromatin.
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The extent of chromatin compaction can be studied by staining chromatin using specific DNA binding dyes. Under the microscope, the dense-compacted regions take up more dye, appearing darker, while the less-compact areas take up less dye and appear lighter. Based on the compaction level, chromatins are classified into two primary forms – euchromatin and heterochromatin.
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Nucleosomes are the basic units of chromatin compaction. Each nucleosome consists of the DNA bound tightly around a histone core, which makes the DNA inaccessible to DNA binding proteins such as DNA polymerase and RNA polymerase. Hence, the fundamental problem is to ensure access to DNA when appropriate, despite the compact and protective chromatin structure.
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Epigenetics is the study of inherited changes in a cell's phenotype without changing the DNA sequences. It provides a form of memory for the differential gene expression pattern to maintain cell lineage, position-effect variegation, dosage compensation, and maintenance of chromatin structures such as telomeres and centromeres. For example, the structure and location of the centromere on chromosomes are epigenetically inherited. Its functionality is not dictated or ensured by the underlying...
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Gene transcription is regulated by the synergistic action of several proteins that form a complex at a gene regulatory site. This is observed in eukaryotes, where the regulation of gene expression is a complex process. Regulatory proteins in eukaryotes can broadly be classified into two types – regulators that bind directly to specific DNA sequences and co-regulators that associate with regulatory proteins but cannot directly bind to the DNA. These co-regulators are further divided into...
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Updated: Oct 1, 2025

A Method to Study de novo Formation of Chromatin Domains
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The ACF chromatin-remodeling complex is essential for Polycomb repression.

Elizabeth T Wiles1, Colleen C Mumford1, Kevin J McNaught1

  • 1Institute of Molecular Biology, University of Oregon, Eugene, United States.

Elife
|March 8, 2022
PubMed
Summary

The ACF chromatin-remodeling complex, involving ISWI and ACF1, is essential for repressing specific genes marked by Histone H3 lysine 27 (H3K27) methylation, revealing a key mechanism in gene silencing.

Keywords:
H3K27 methylationISWIN. crassaPRC2chromatin remodelerchromosomesfacultative heterochromatingene expressiongeneticsgenomicsnucleosome positioning

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Area of Science:

  • Epigenetics and Gene Regulation
  • Chromatin Biology
  • Molecular Genetics

Background:

  • Gene repression is crucial for multicellular organism development.
  • Histone H3 lysine 27 (H3K27) methylation marks repressed facultative heterochromatin, but its silencing mechanism is not fully understood.

Purpose of the Study:

  • To identify genes involved in transcriptional silencing of H3K27-methylated chromatin.
  • To elucidate the mechanism of Polycomb repression.

Main Methods:

  • Forward genetic screen in *Neurospora crassa*.
  • Identification of gene homologs for ISWI and ACF1.
  • Analysis of chromatin remodeling complex interactions and nucleosome positioning.

Main Results:

  • The *N. crassa* ISWI and ACF1 homologs form an ACF chromatin-remodeling complex.
  • This ACF complex is required for repressing a subset of H3K27-methylated genes.
  • The H3K27 methyltransferase SET-7 promotes ACF complex association with facultative heterochromatin.
  • Deletion of *iswi* or *acf1* leads to a downstream shift of the +1 nucleosome in upregulated H3K27-methylated genes.

Conclusions:

  • The ACF complex plays a direct role in Polycomb repression.
  • Proper nucleosome positioning mediated by the ACF complex is critical for facultative heterochromatin silencing.