Editing Nucleosome (section)

===Histone post-translational modifications===
[[File:Histone tails and their function in chromatin formation.svg|thumb|Histone tails and their function in chromatin formation]]

Since they were discovered in the mid-1960s, histone modifications have been predicted to affect transcription.<ref name="pmid14172992">{{cite journal | vauthors = Allfrey VG, Faulkner R, Mirsky AE | title = Acetylation and Methylation of Histones and Their Possible Role in the Regulation of RNA Synthesis | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 51 | issue = 5 | pages = 786–794 | date = May 1964 | pmid = 14172992 | pmc = 300163 | doi = 10.1073/pnas.51.5.786 | doi-access = free | bibcode = 1964PNAS...51..786A }}</ref> The fact that most of the early post-translational modifications found were concentrated within the tail extensions that protrude from the nucleosome core lead to two main theories regarding the mechanism of histone modification. The first of the theories suggested that they may affect electrostatic interactions between the histone tails and DNA to "loosen" chromatin structure. Later it was proposed that combinations of these modifications may create binding epitopes with which to recruit other proteins.<ref name="pmid10638745">{{cite journal | vauthors = Strahl BD, Allis CD | title = The language of covalent histone modifications | journal = Nature | volume = 403 | issue = 6765 | pages = 41–45 | date = January 2000 | pmid = 10638745 | doi = 10.1038/47412 | s2cid = 4418993 | bibcode = 2000Natur.403...41S }}</ref> Recently, given that more modifications have been found in the structured regions of histones, it has been put forward that these modifications may affect histone-DNA<ref name="pmid15523479">{{cite journal | vauthors = Cosgrove MS, Boeke JD, Wolberger C | title = Regulated nucleosome mobility and the histone code | journal = Nature Structural & Molecular Biology | volume = 11 | issue = 11 | pages = 1037–1043 | date = November 2004 | pmid = 15523479 | doi = 10.1038/nsmb851 | s2cid = 34704745 }}</ref> and histone-histone<ref name="pmid15808514">{{cite journal | vauthors = Ye J, Ai X, Eugeni EE, Zhang L, Carpenter LR, Jelinek MA, Freitas MA, Parthun MR | display-authors = 6 | title = Histone H4 lysine 91 acetylation a core domain modification associated with chromatin assembly | journal = Molecular Cell | volume = 18 | issue = 1 | pages = 123–130 | date = April 2005 | pmid = 15808514 | pmc = 2855496 | doi = 10.1016/j.molcel.2005.02.031 }}</ref> interactions within the nucleosome core. Modifications (such as acetylation or phosphorylation) that lower the charge of the globular histone core are predicted to "loosen" core-DNA association; the strength of the effect depends on location of the modification within the core.<ref name="pmid20816070">{{cite journal | vauthors = Fenley AT, Adams DA, Onufriev AV | title = Charge state of the globular histone core controls stability of the nucleosome | journal = Biophysical Journal | volume = 99 | issue = 5 | pages = 1577–1585 | date = September 2010 | pmid = 20816070 | pmc = 2931741 | doi = 10.1016/j.bpj.2010.06.046 | bibcode = 2010BpJ....99.1577F }}</ref>
Some modifications have been shown to be correlated with gene silencing; others seem to be correlated with gene activation. Common modifications include [[acetylation]], [[methylation]], or [[ubiquitination]] of [[lysine]]; [[methylation]] of [[arginine]]; and [[phosphorylation]] of [[serine]]. The information stored in this way is considered [[epigenetic]], since it is not encoded in the DNA but is still inherited to daughter cells. The maintenance of a repressed or activated status of a gene is often necessary for [[cellular differentiation]].<ref name="pmid12540921"/>