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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/01/18 05:05:28」(JST)

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The nucleus of a human cell showing the location of euchromatin

Euchromatin is a lightly packed form of chromatin (DNA, RNA and protein) that is enriched in genes, and is often (but not always) under active transcription. Euchromatin comprises the most active portion of the genome within the cell nucleus. 92% of the human genome is euchromatic.^[1] The remainder is called heterochromatin.

Structure

The structure of euchromatin is reminiscent of an unfolded set of beads along a string, wherein those beads represent nucleosomes. Nucleosomes consist of eight proteins known as histones, with approximately 147 base pairs of DNA wound around them; in euchromatin, this wrapping is loose so that the raw DNA may be accessed. Each core histone possesses a `tail' structure, which can vary in several ways; it is thought that these variations act as "master control switches," which determine the overall arrangement of the chromatin. In particular, it is believed that the presence of methylated lysine 4 on the histone tails acts as a general marker for euchromatin.

Appearance

In general, euchromatin appears as light-colored bands when stained in G banding^{[citation needed]} and observed under an optical microscope, in contrast to heterochromatin, which stains darkly. This lighter staining is due to the less compact structure of euchromatin. The basic structure of euchromatin is an elongated, open, 10 nm microfibril, as noted by electron microscopy. In prokaryotes, euchromatin is the only form of chromatin present; this indicates that the heterochromatin structure evolved later along with the nucleus, possibly as a mechanism to handle increasing genome size.

Function

Euchromatin participates in the active transcription of DNA to mRNA products. The unfolded structure allows gene regulatory proteins and RNA polymerase complexes to bind to the DNA sequence, which can then initiate the transcription process. Not all euchromatin is necessarily transcribed, but in general that which is not is transformed into heterochromatin to protect the genes while they are not in use. There is therefore a direct link to how actively productive a cell is and the amount of euchromatin that can be found in its nucleus. It is thought that the cell uses transformation from euchromatin into heterochromatin as a method of controlling gene expression and replication, since such processes behave differently on densely compacted chromatin, known as the `accessibility hypothesis'. One example of constitutive euchromatin that is 'always turned on' is housekeeping genes, which code for the proteins needed for basic functions of cell survival.

References

^ "Finishing the euchromatic sequence of the human genome". Nature 431 (7011): 931–45. 21 October 2004. doi:10.1038/nature03001. PMID 15496913.

External links

Research news in Euchromatin
Zheng C, Hayes J (2003). "Structures and interactions of the core histone tail domains.". Biopolymers 68 (4): 539–46. doi:10.1002/bip.10303. PMID 12666178.
Muegge K (2003). "Modifications of histone cores and tails in V(D)J recombination". Genome Biol 4 (4): 211. doi:10.1186/gb-2003-4-4-211. PMC 154571. PMID 12702201. Article
Histology image: 20102loa – Histology Learning System at Boston University

English Journal

Epigenetic regulation of the Plasmodium falciparum genome.

Duffy MF, Selvarajah SA, Josling GA, Petter M.AbstractRecent research has highlighted some unique aspects of chromatin biology in the malaria parasite Plasmodium falciparum. During its erythrocytic lifecycle P. falciparum maintains its genome primarily as unstructured euchromatin. Indeed there is no clear role for chromatin-mediated silencing of the majority of the developmentally expressed genes in P. falciparum. However discontinuous stretches of heterochromatin are critical for variegated expression of contingency genes that mediate key pathogenic processes in malaria. These range from invasion of erythrocytes and antigenic variation to solute transport and growth adaptation in response to environmental changes. Despite lack of structure within euchromatin the nucleus maintains functional compartments that regulate expression of many genes at the nuclear periphery, particularly genes with clonally variant expression. The typical components of the chromatin regulatory machinery are present in P. falciparum; however, some of these appear to have evolved novel species-specific functions, e.g. the dynamic regulation of histone variants at virulence gene promoters. The parasite also appears to have repeatedly acquired chromatin regulatory proteins through lateral transfer from endosymbionts and from the host. P. falciparum chromatin regulators have been successfully targeted with multiple drugs in laboratory studies; hopefully their functional divergence from human counterparts will allow the development of parasite-specific inhibitors.
Briefings in functional genomics.Brief Funct Genomics.2013 Dec 10. [Epub ahead of print]
Recent research has highlighted some unique aspects of chromatin biology in the malaria parasite Plasmodium falciparum. During its erythrocytic lifecycle P. falciparum maintains its genome primarily as unstructured euchromatin. Indeed there is no clear role for chromatin-mediated silencing of the ma
PMID 24326119

Preferential localization of γH2AX foci in euchromatin of retina rod cells after DNA damage induction.

Lafon-Hughes L, Di Tomaso MV, Liddle P, Toledo A, Reyes-Ábalos AL, Folle GA.SourceDepartamento de Genética, Instituto de Investigaciones Biológicas Clemente Estable, Montevideo, Uruguay, llafon@iibce.edu.uy.
Chromosome research : an international journal on the molecular, supramolecular and evolutionary aspects of chromosome biology.Chromosome Res.2013 Dec 10. [Epub ahead of print]
DNA damage may lead to cell transformation, senescence, or death. Histone H2AX phosphorylation, immunodetected as γH2AX foci, is an early response to DNA damage persisting even after DNA repair. In cycling mammalian cells with canonical nuclear architecture, i.e., central euchromatin and peripheral
PMID 24323064

The N-terminus and Tudor domains of Sgf29 are important for its heterochromatin boundary formation function.

Kamata K, Goswami G, Kashio S, Urano T, Nakagawa R, Uchida H, Oki M.SourceDepartment of Applied Chemistry and Biotechnology, Graduate School of Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan; Department of Biochemistry, Shimane University Faculty of Medicine, Izumo, Japan; Laboratory for Chromatin Dynamics, Center for Developmental Biology, RIKEN, Kobe 650-0047, Japan; Research and Education Program for Life Science, University of Fukui; PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan.
Journal of biochemistry.J Biochem.2013 Dec 3. [Epub ahead of print]
Eukaryotic chromosomes are organized into heterochromatin and euchromatin domains. Heterochromatin domains are transcriptionally repressed and prevented from spreading into neighboring genes by chromatin boundaries. Previously, we identified 55 boundary-related genes in Saccharomyces cerevisiae. In
PMID 24307402

Japanese Journal

近縁種ゲノム配列を用いたヒトゲノム配列未決定領域の塩基配列決定

福尾悠平,山根翔太郎,小柳香奈子,渡邉日出海
情報処理学会研究報告. BIO, バイオ情報学 2012-BIO-28(6), 1-2, 2012-03-21
ヒトゲノムプロジェクトは2004年に完了し、解読されたヒトゲノム配列は広く利用されている。しかしながら現在でもユークロマチン領域に関しても1%程度が配列未決定領域として残されている。一方でヒトゲノム上にマッピングされないヒトcDNA配列が存在しており、機能既知の遺伝子が未だ配列未決定領域内に存在することが示唆される。本研究ではヒトに近縁な霊長類のゲノム配列を利用することで、ヒトゲノム配列未決定領域 …
NAID 110008803111

Fub1p,a novel protein isolated by boundary screening,binds the proteasome complex

Hatanaka Akira,Chen Bo,Sun Jing-Qian [他],MANO Yasunobu,FUNAKOSHI Minoru,KOBAYASHI Hideki,JU Yunfeng,MIZUTANI Tetsuya,SHINMYOZU Kaori,NAKAYAMA Jun-ichi,MIYAMOTO Kaoru,UCHIDA Hiroyuki,OKI Masaya
Genes & genetic systems 86(5), 305-314, 2011-10-25
Silenced chromatin domains are restricted to specific regions. Eukaryotic chromosomes are organized into discrete domains delimited by domain boundaries. From approximately 6,000 genes in Saccharomyce …
NAID 10030209843

Boundaries of transcriptionally silent chromatin in Saccharomyces cerevisiae

Sun Jing-Qian,Hatanaka Akira,Oki Masaya
Genes & genetic systems 86(2), 73-81, 2011-04-25
In the budding yeast Saccharomyces cerevisiae, heterochromatic gene silencing has been found within HMR and HML silent mating type loci, the telomeres, and the rRNA-encoding DNA. There may be boundary …
NAID 10029516319