Heterochromatin

Source: Wikipedia, the free encyclopedia.

Heterochromatin is a tightly packed form of

facultative heterochromatin. Both play a role in the expression of genes. Because it is tightly packed, it was thought to be inaccessible to polymerases and therefore not transcribed; however, according to Volpe et al. (2002),[1] and many other papers since,[2] much of this DNA is in fact transcribed, but it is continuously turned over via RNA-induced transcriptional silencing (RITS). Recent studies with electron microscopy and OsO4 staining reveal that the dense packing is not due to the chromatin.[3]

Constitutive heterochromatin can affect the genes near itself (e.g.

telomeres
, in addition to acting as an attractor for other gene-expression or repression signals.

Facultative heterochromatin is the result of genes that are silenced through a mechanism such as histone deacetylation or Piwi-interacting RNA (piRNA) through RNAi. It is not repetitive and shares the compact structure of constitutive heterochromatin. However, under specific developmental or environmental signaling cues, it can lose its condensed structure and become transcriptionally active.[4]

Heterochromatin has been associated with the di- and tri -methylation of H3K9 in certain portions of the human genome.[5] H3K9me3-related methyltransferases appear to have a pivotal role in modifying heterochromatin during lineage commitment at the onset of organogenesis and in maintaining lineage fidelity.[6]

Structure

Heterochromatin vs. euchromatin

Chromatin is found in two varieties: euchromatin and heterochromatin.[7] Originally, the two forms were distinguished cytologically by how intensely they get stained – the euchromatin is less intense, while heterochromatin stains intensely, indicating tighter packing. Heterochromatin is usually localized to the periphery of the nucleus. Despite this early dichotomy, recent evidence in both animals

epigenetic
marks.

Heterochromatin mainly consists of genetically inactive satellite sequences,[10] and many genes are repressed to various extents, although some cannot be expressed in euchromatin at all.[11] Both centromeres and telomeres are heterochromatic, as is the Barr body of the second, inactivated X-chromosome in a female.

Function

General model for duplication of heterochromatin during cell division
Microscopy of heterochromatic versus euchromatic nuclei (H&E stain).

Heterochromatin has been associated with several functions, from gene regulation to the protection of chromosome integrity;[12] some of these roles can be attributed to the dense packing of DNA, which makes it less accessible to protein factors that usually bind DNA or its associated factors. For example, naked double-stranded DNA ends would usually be interpreted by the cell as damaged or viral DNA, triggering cell cycle arrest, DNA repair or destruction of the fragment, such as by endonucleases in bacteria.

Some regions of chromatin are very densely packed with fibers that display a condition comparable to that of the chromosome at

epigenetic inheritance. Variations cause heterochromatin to encroach on adjacent genes or recede from genes at the extremes of domains. Transcribable material may be repressed by being positioned (in cis) at these boundary domains. This gives rise to expression levels that vary from cell to cell,[13] which may be demonstrated by position-effect variegation.[14] Insulator sequences may act as a barrier in rare cases where constitutive heterochromatin and highly active genes are juxtaposed (e.g. the 5'HS4 insulator upstream of the chicken β-globin locus,[15] and loci in two Saccharomyces spp.[16][17]
).

Constitutive heterochromatin

Main article: Constitutive heterochromatin

All cells of a given species package the same regions of DNA in

16, and the Y-chromosome
contain large regions of constitutive heterochromatin. In most organisms, constitutive heterochromatin occurs around the chromosome centromere and near telomeres.

Facultative heterochromatin

Schematic karyogram of a human, showing an overview of the human genome using G banding, which is a method that includes Giemsa staining, wherein the lighter staining regions are generally more euchromatic, whereas darker regions generally are more heterochromatic.
Further information: Karyotype

The regions of DNA packaged in facultative heterochromatin will not be consistent between the cell types within a species, and thus a sequence in one cell that is packaged in facultative heterochromatin (and the genes within are poorly expressed) may be packaged in euchromatin in another cell (and the genes within are no longer silenced). However, the formation of facultative heterochromatin is regulated, and is often associated with morphogenesis or differentiation. An example of facultative heterochromatin is X chromosome inactivation in female mammals: one X chromosome is packaged as facultative heterochromatin and silenced, while the other X chromosome is packaged as euchromatin and expressed.

Among the molecular components that appear to regulate the spreading of heterochromatin are the

Xist. The mechanism for such spreading is still a matter of controversy.[18] The polycomb repressive complexes PRC1 and PRC2 regulate chromatin compaction and gene expression and have a fundamental role in developmental processes. PRC-mediated epigenetic aberrations are linked to genome instability and malignancy and play a role in the DNA damage response, DNA repair and in the fidelity of replication.[19]

Yeast heterochromatin

Saccharomyces cerevisiae, or budding yeast, is a model eukaryote and its heterochromatin has been defined thoroughly. Although most of its genome can be characterized as euchromatin, S. cerevisiae has regions of DNA that are transcribed very poorly. These loci are the so-called silent mating type loci (HML and HMR), the rDNA (encoding ribosomal RNA), and the sub-telomeric regions. Fission yeast (

RNAi
pathway. Double-stranded RNA is believed to result in silencing of the region through a series of steps.

In the fission yeast Schizosaccharomyces pombe, two RNAi complexes, the RITS complex and the RNA-directed RNA polymerase complex (RDRC), are part of an RNAi machinery involved in the initiation, propagation and maintenance of heterochromatin assembly. These two complexes localize in a

siRNA-dependent manner on chromosomes, at the site of heterochromatin assembly. RNA polymerase II synthesizes a transcript that serves as a platform to recruit RITS, RDRC and possibly other complexes required for heterochromatin assembly.[20][21] Both RNAi and an exosome-dependent RNA degradation process contribute to heterochromatic gene silencing. These mechanisms of Schizosaccharomyces pombe may occur in other eukaryotes.[22] A large RNA structure called RevCen has also been implicated in the production of siRNAs to mediate heterochromatin formation in some fission yeast.[23]

See also

References

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  8. ^ van Steensel B (May 2011). "Chromatin: constructing the big picture". The EMBO Journal. 30 (10): 1885–95.
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  11. ^ Lu BY, Emtage PC, Duyf BJ, Hilliker AJ, Eissenberg JC (June 2000). "Heterochromatin protein 1 is required for the normal expression of two heterochromatin genes in Drosophila". Genetics. 155 (2): 699–708.
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  12. ^ Grewal SI, Jia S (January 2007). "Heterochromatin revisited". Nature Reviews. Genetics. 8 (1): 35–46.
    S2CID 31811880
    . An up-to-date account of the current understanding of repetitive DNA, which usually doesn't contain genetic information. If evolution makes sense only in the context of the regulatory control of genes, we propose that heterochromatin, which is the main form of chromatin in higher eukaryotes, is positioned to be a deeply effective target for evolutionary change. Future investigations into assembly, maintenance and the many other functions of heterochromatin will shed light on the processes of gene and chromosome regulation.
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  16. ^ Allis CD, Grewal SI (August 2001). "Transitions in distinct histone H3 methylation patterns at the heterochromatin domain boundaries". Science. 293 (5532): 1150–5.
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  17. ^ Donze D, Kamakaka RT (February 2001). "RNA polymerase III and RNA polymerase II promoter complexes are heterochromatin barriers in Saccharomyces cerevisiae". The EMBO Journal. 20 (3): 520–31.
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External links
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