Editing Epigenetics (section)

===Prions===
{{further|Fungal prions}}
[[Prion]]s are [[Infection|infectious]] forms of [[protein]]s. In general, proteins fold into discrete units that perform distinct cellular functions, but some proteins are also capable of forming an infectious conformational state known as a prion. Although often viewed in the context of [[Transmissible spongiform encephalopathy|infectious disease]], prions are more loosely defined by their ability to catalytically convert other native state versions of the same protein to an infectious conformational state. It is in this latter sense that they can be viewed as epigenetic agents capable of inducing a phenotypic change without a modification of the genome.<ref>{{cite journal | title=Epigenetic inheritance and prions|vauthors=Yool A, Edmunds WJ | journal=Journal of Evolutionary Biology | year=1998 | pages=241–42 | volume=11 | doi=10.1007/s000360050085 | issue=2}}</ref>

[[Fungal prion]]s are considered by some to be epigenetic because the infectious phenotype caused by the prion can be inherited without modification of the genome. [[PSI (prion)|PSI+]] and URE3, discovered in [[Saccharomyces cerevisiae|yeast]] in 1965 and 1971, are the two best studied of this type of prion.<ref>{{cite journal|title=[PSI], a cytoplasmic suppressor of super-suppression in yeast | vauthors = Cox BS| journal=Heredity | volume=20 | pages=505–21 | year=1965 | doi=10.1038/hdy.1965.65 | issue=4| doi-access=free }}</ref><ref name="pmid5573734">{{cite journal | vauthors = Lacroute F | title = Non-Mendelian mutation allowing ureidosuccinic acid uptake in yeast | journal = Journal of Bacteriology | volume = 106 | issue = 2 | pages = 519–22 | date = May 1971 | pmid = 5573734 | pmc = 285125 | doi = 10.1128/JB.106.2.519-522.1971}}</ref> Prions can have a phenotypic effect through the sequestration of protein in aggregates, thereby reducing that protein's activity. In PSI+ cells, the loss of the Sup35 protein (which is involved in termination of translation) causes ribosomes to have a higher rate of read-through of stop [[codon]]s, an effect that results in suppression of [[nonsense mutation]]s in other genes.<ref name="pmid225301">{{cite journal | vauthors = Liebman SW, Sherman F | title = Extrachromosomal psi+ determinant suppresses nonsense mutations in yeast | journal = Journal of Bacteriology | volume = 139 | issue = 3 | pages = 1068–71 | date = September 1979 | pmid = 225301 | pmc = 218059 | doi = 10.1128/JB.139.3.1068-1071.1979}}</ref> The ability of Sup35 to form prions may be a conserved trait. It could confer an adaptive advantage by giving cells the ability to [[Evolutionary capacitance|switch into a PSI+ state]] and express dormant genetic features normally terminated by stop codon mutations.<ref name="pmid11028992">{{cite journal | vauthors = True HL, Lindquist SL | title = A yeast prion provides a mechanism for genetic variation and phenotypic diversity | journal = Nature | volume = 407 | issue = 6803 | pages = 477–83 | date = September 2000 | pmid = 11028992 | doi = 10.1038/35035005 | bibcode = 2000Natur.407..477T | s2cid = 4411231 }}</ref><ref name="pmid15931169">{{cite journal | vauthors = Shorter J, Lindquist S | title = Prions as adaptive conduits of memory and inheritance | journal = Nature Reviews. Genetics | volume = 6 | issue = 6 | pages = 435–50 | date = June 2005 | pmid = 15931169 | doi = 10.1038/nrg1616 | s2cid = 5575951 }}</ref><ref>{{cite journal | vauthors = Giacomelli MG, Hancock AS, Masel J | title = The conversion of 3' UTRs into coding regions | journal = Molecular Biology and Evolution | volume = 24 | issue = 2 | pages = 457–64 | date = February 2007 | pmid = 17099057 | pmc = 1808353 | doi = 10.1093/molbev/msl172 | author3-link = Joanna Masel }}</ref><ref>{{cite journal | vauthors = Lancaster AK, Bardill JP, True HL, Masel J | title = The spontaneous appearance rate of the yeast prion [PSI+] and its implications for the evolution of the evolvability properties of the [PSI+] system | journal = Genetics | volume = 184 | issue = 2 | pages = 393–400 | date = February 2010 | pmid = 19917766 | pmc = 2828720 | doi = 10.1534/genetics.109.110213 }}</ref>

Prion-based epigenetics has also been observed in ''[[Saccharomyces cerevisiae]]''.<ref>{{cite journal | vauthors = Garcia DM, Campbell EA, Jakobson CM, Tsuchiya M, Shaw EA, DiNardo AL, Kaeberlein M, Jarosz DF | title = A prion accelerates proliferation at the expense of lifespan | journal = eLife | volume = 10 | pages = e60917 | date = September 2021 | pmid = 34545808 | pmc = 8455135 | doi = 10.7554/eLife.60917 | doi-access = free }}</ref>