histones

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In biology, histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. They are the chief protein components of chromatin, acting as spools around which DNA winds, and play a role in gene regulation. Without histones, the unwound DNA in chromosomes would be very long (a length to width ratio of more than 10 million to one in human DNA). For example, each human cell has about 1.8 meters of DNA, but wound on the histones it has about 90 micrometers (0.09 mm) of chromatin, which, when duplicated and condensed during mitosis, result in about 120 micrometers of chromosomes.

Five major classes of histones exist: H1/H5, H2A, H2B, H3, and H4. Histones H2A, H2B, H3 and H4 are known as the core histones, while histones H1 and H5 are known as the linker histones.

Two of each of the core histones assemble to form one octameric nucleosome core particle, and 147 base pairs of DNA wrap around this core particle 1.65 times in a left-handed super-helical turn. The linker histone H1 binds the nucleosome and the entry and exit sites of the DNA, thus locking the DNA into place and allowing the formation of higher order structure. The most basic such formation is the 10 nm fiber or beads on a string conformation. This involves the wrapping of DNA around nucleosomes with approximately 50 base pairs of DNA separating each pair of nucleosomes (also referred to as linker DNA). The assembled histones and DNA is called chromatin. Higher-order structures include the 30 nm fiber (forming an irregular zigzag) and 100 nm fiber, these being the structures found in normal cells. During mitosis and meiosis, the condensed chromosomes are assembled through interactions between nucleosomes and other regulatory proteins.

The nucleosome core is formed of two H2A-H2B dimers and a H3-H4 tetramer, forming two nearly symmetrical halves by tertiary structure (C2 symmetry; one macromolecule is the mirror image of the other). The H2A-H2B dimers and H3-H4 tetramer also show pseudodyad symmetry. The 4 'core' histones (H2A, H2B, H3 and H4) are relatively similar in structure and are highly conserved through evolution, all featuring a 'helix turn helix turn helix' motif (which allows the easy dimerisation). They also share the feature of long 'tails' on one end of the amino acid structure - this being the location of post-translational modification.

Histones are subject to post translational modification by enzymes primarily on their N-terminal tails, but also in their globular domains. Such modifications include methylation, citrullination, acetylation, phosphorylation, SUMOylation, ubiquitination, and ADP-ribosylation. This affects their function of gene regulation.
In general, genes that are active have less bound histone, while inactive genes are highly associated with histones during interphase. It also appears that the structure of histones has been evolutionarily conserved, as any deleterious mutations would be severely maladaptive.

Histones are found in the nuclei of eukaryotic cells, and in certain Archaea, namely Euryarchaea, but not in bacteria. The unicellular algae known as dinoflagellates are the only eukaryotes that completely lack histones.

EXTERNAL DATABASES:
HHMD Human Histone Modification Database
NHGRI Histone Database
MethyCancer database of human DNA Methylation and Cancer
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ORTHOLOGY CLASS Homo sapiens L. (human) [HSA] Mus musculus L. (mouse) [MMU] Caenorhabditis elegans Maupas (nematode) [CEL] Drosophila melanogaster Meigen (fruit fly) [DME] Saccharomyces cerevisiae Meyen ex E.C. Hansen (budding yeast) [SCE] Schizo-saccharomyces pombe Lindner (fission yeast) [SPO] Escherichia coli Migula (bacterium) K-12 MG1655 [ECO] Arabidopsis thaliana (L.) Heynh. (mouse-ear cress) [ATH]
ko:K11254 (histone H4) HIST2H4A
HIST1H4J
HIST1H4K
HIST1H4L
HIST1H4F
HIST1H4A
HIST1H4B
HIST2H4B
HIST1H4I
HIST1H4G
HIST1H4C
HIST1H4D
HIST4H4
HIST1H4H
HIST1H4E
Hist1h4f
Hist1h4m
Hist1h4a
Hist1h4h
Hist1h4b
Hist1h4n
Hist1h4d
Hist1h4i
Hist1h4c
Hist4h4
Hist1h4k
Hist2h4
Hist1h4j
his-31
his-10
his-64
his-1
his-67
his-28
his-37
his-56
his-50
his-60
his-46
his-26
his-38
his-14
his-18
his-5
His4:CG33903
His4:CG33871
His4:CG31611
His4:CG33885
His4:CG33905
His4:CG33887
His4:CG33893
His4:CG33909
His4:CG33897
His4:CG33891
His4r
His4:CG33879
His4:CG33889
His4:CG33873
His4:CG33907
His4:CG33899
His4:CG33877
His4:CG33883
His4:CG33869
His4:CG33881
His4:CG33875
His4:CG33901
His4:CG33895
HHF1
HHF2
hhf1
hhf3
hhf2
HIS4
gi:15231283
gi:145323776
gi:15232318
gi:18424269
gi:18424305
gi:30692704
gi:18390815
ko:K11253 (histone H3) HIST1H3A
H3F3C
HIST1H3B
HIST1H3F
HIST1H3H
HIST1H3E
HIST1H3J
HIST1H3I
HIST2H3C
HIST3H3
HIST1H3D
H3F3A
HIST1H3C
HIST2H3A
HIST2H3D
HIST1H3G
H3F3B
Hist1h3g
H3f3b
Hist1h3e
Hist1h3a
Gm10257
Hist1h3d
Hist1h3f
Hist1h3i
Hist2h3c2-ps
Hist2h3c1
Hist1h3c
Hist2h3b
Hist1h3b
H3f3a
H3f3c
Hist1h3h
his-45
his-74
his-6
his-71
his-25
his-72
his-27
his-69
his-17
his-55
his-70
his-9
his-63
cpar-1
his-59
his-32
his-49
his-13
his-2
his-42
His3:CG33827
His3:CG33851
His3:CG33848
His3:CG33824
His3:CG33803
His3:CG33812
His3.3A
His3:CG33842
His3:CG33845
His3:CG33836
His3:CG33818
His3:CG33815
His3:CG33857
His3:CG33839
His3:CG33833
His3.3B
His3:CG33854
His3:CG33806
His3:CG33863
His3:CG31613
His3:CG33830
His3:CG33860
His3:CG33821
His3:CG33809
His3:CG33866
HHT1
HHT2
hht1
hht2
hht3
gi:15222272
HTR12
MGH3
gi:18416261
gi:145334271
gi:15222297
gi:15238430
gi:79326608
gi:15232146
gi:15238916
gi:18390992
gi:18416264
gi:15239940
gi:15238433
ko:K11252 (histone H2B) HIST1H2BI
HIST1H2BO
HIST2H2BF
HIST1H2BG
HIST1H2BD
HIST1H2BC
HIST1H2BB
HIST3H2BB
HIST1H2BK
HIST2H2BE
H2BFWT
HIST1H2BJ
HIST1H2BF
HIST1H2BN
HIST1H2BL
HIST1H2BH
HIST1H2BA
H2BFM
HIST1H2BM
HIST1H2BE
Hist1h2bn
Hist3h2ba
1700014N06Rik
Hist1h2bl
Hist1h2bg
Hist1h2bj
Hist1h2bh
Hist2h2bb
Hist2h2be
Hist1h2be
Hist1h2bm
Hist3h2bb-ps
Hist1h2bc
Hist1h2br
Hist1h2ba
Hist1h2bq
Hist1h2bf
Hist1h2bk
Hist1h2bp
Hist1h2bb
1700024P04Rik
his-66
his-54
his-34
his-4
his-41
his-44
his-8
his-22
his-62
his-20
his-15
his-48
his-29
his-39
his-52
his-58
his-11
His2B:CG33900
His2B:CG33904
His2B:CG33876
His2B:CG33882
His2B:CG33884
His2B:CG33890
His2B:CG33910
His2B:CG33888
His2B:CG33886
His2B:CG33874
His2B:CG33868
His2B:CG33894
His2B:CG33878
His2B:CG33892
His2B:CG33902
His2B:CG33880
His2B:CG33898
His2B:CG33872
His2B:CG33906
His2B:CG33896
His2B:CG17949
His2B:CG33908
His2B:CG33870
HTB1
HTB2
htb1 gi:15223150
HTB11
HTB2
HTB4
HTB9
gi:15231854
gi:15226943
gi:15241858
HTB1
gi:15224292
gi:15232689
ko:K11251 (histone H2A) HIST1H2AL
HIST1H2AB
H2AFB1
HIST1H2AC
HIST1H2AA
HIST1H2AG
H2AFX
H2AFZ
H2AFV
H2AFB2
HIST3H2A
H2AFB3
HIST2H2AA3
H2AFY
HIST2H2AC
HIST1H2AD
HIST1H2AI
HIST2H2AB
HIST2H2AA4
HIST1H2AM
HIST1H2AK
HIST1H2AH
HIST1H2AJ
H2AFY2
HIST1H2AE
H2AFJ
Gm14478
Gm14479
H2afv
Gm14483
H2afb1
HisH2A
Hist2h2aa2
Gm5132
Hist2h2aa1
Gm14475
H2afj
Hist1h2ah
Hist1h2an
H2afz
Hist1h2aa
Hist1h2ae
Gm4906
Gm5382
Hist1h2ap
H2afy
HisH2A
Hist1h2ag
Gm14484
Gm6026
Hist2h2ab
H2afx
Hist3h2a
Hist1h2ao
Hist1h2ak
Gm14482
Hist1h2ad
Gm14477
H2afy2
Hist1h2ac
1700012L04Rik
Hist1h2ab
Gm14501
Gm14476
Hist1h2ai
Hist1h2af
Hist2h2ac
his-12
his-47
his-61
his-65
his-43
his-3
his-30
his-16
his-53
his-33
his-51
htz-1
his-7
his-68
gi:17544704
his-19
his-21
his-57
his-35
His2A:CG33850
His2A:CG33862
His2A:CG33808
His2A:CG33865
His2A:CG33853
His2A:CG33820
His2Av
His2A:CG33838
His2A:CG33847
His2A:CG33841
His2A:CG33814
His2A:CG33844
His2A:CG33832
His2A:CG33835
His2A:CG33817
His2A:CG33859
His2A:CG33829
His2A:CG33856
His2A:CG31618
His2A:CG33826
His2A:CG33823
HTZ1
HTA2
HTA1
pht1
hta2
hta1
HTA12
HTA11
HTA6
gi:15221875
HTA9
HTA4
HTA13
HTA7
RAT5
HTA10
HTA8
H2AXA
HTA2
ko:K11275 (histone H1/5) HIST1H1A
HIST1H1B
HIST1H1T
HIST1H1C
HIST1H1D
HIST1H1E
H1FX
H1F0
H1f0
Hist1h1t
Hist1h1d
Hist1h1c
Hist1h1a
Hist1h1b
H1fx
Hist1h1e
hil-2
hil-6
his-24
hil-3
hil-5
hil-4
His1:CG33816
His1:CG33861
His1:CG33849
His1:CG33858
His1:CG33801
His1:CG33822
His1:CG33813
His1:CG33852
His1:CG33846
His1:CG33804
His1:CG33807
His1:CG33855
His1:CG33831
His1:CG33825
His1:CG33828
His1:CG33819
His1:CG33834
His1:CG33864
His1:CG33837
His1:CG33840
His1:CG33843
His1:CG33810
His1:CG31617
HHO1 gi:15222199
gi:15224536
HIS1-3

Last modification date: Oct. 11, 2011