Editing Gene expression (section)

===Post-translational modifications===
{{Main|Post-translational modification}}
Post-translational modifications (PTMs) are [[covalent]] modifications to proteins. Like RNA splicing, they help to significantly diversify the proteome. These modifications are usually catalyzed by enzymes. Additionally, processes like covalent additions to amino acid side chain residues can often be reversed by other enzymes. However, some, like the [[proteolysis|proteolytic cleavage]] of the protein backbone, are irreversible.<ref name="WalshGarneau-Tsodikova2005">{{cite journal | vauthors = Walsh CT, Garneau-Tsodikova S, Gatto GJ | title = Protein posttranslational modifications: the chemistry of proteome diversifications | journal = Angewandte Chemie | volume = 44 | issue = 45 | pages = 7342–7372 | date = December 2005 | pmid = 16267872 | doi = 10.1002/anie.200501023 | s2cid = 32157563 }}</ref>

PTMs play many important roles in the cell.<ref name="KhouryBaliban2011">{{cite journal | vauthors = Khoury GA, Baliban RC, Floudas CA | title = Proteome-wide post-translational modification statistics: frequency analysis and curation of the swiss-prot database | journal = Scientific Reports | volume = 1 | pages = 90 | date = September 2011 | pmid = 22034591 | pmc = 3201773 | doi = 10.1038/srep00090 | number = 90 | author-link3 = Christodoulos Floudas | bibcode = 2011NatSR...1...90K }}</ref> For example, phosphorylation is primarily involved in activating and deactivating proteins and in signaling pathways.<ref name="MannJensen2003">{{cite journal | vauthors = Mann M, Jensen ON | title = Proteomic analysis of post-translational modifications | journal = Nature Biotechnology | volume = 21 | issue = 3 | pages = 255–261 | date = March 2003 | pmid = 12610572 | doi = 10.1038/nbt0303-255 | s2cid = 205266061 }}</ref> PTMs are involved in transcriptional regulation: an important function of acetylation and methylation is histone tail modification, which alters how accessible DNA is for transcription.<ref name="WalshGarneau-Tsodikova2005"/> They can also be seen in the immune system, where glycosylation plays a key role.<ref name="SeoLee2004">{{cite journal | vauthors = Seo J, Lee KJ | title = Post-translational modifications and their biological functions: proteomic analysis and systematic approaches | journal = Journal of Biochemistry and Molecular Biology | volume = 37 | issue = 1 | pages = 35–44 | date = January 2004 | pmid = 14761301 | doi = 10.5483/bmbrep.2004.37.1.035 | doi-access = free }}</ref> One type of PTM can initiate another type of PTM, as can be seen in how [[ubiquitination]] tags proteins for degradation through proteolysis.<ref name="WalshGarneau-Tsodikova2005"/> Proteolysis, other than being involved in breaking down proteins, is also important in activating and deactivating them, and in regulating biological processes such as DNA transcription and cell death.<ref name="RogersOverall2013">{{cite journal | vauthors = Rogers LD, Overall CM | title = Proteolytic post-translational modification of proteins: proteomic tools and methodology | journal = Molecular & Cellular Proteomics | volume = 12 | issue = 12 | pages = 3532–3542 | date = December 2013 | pmid = 23887885 | pmc = 3861706 | doi = 10.1074/mcp.M113.031310 | doi-access = free }}</ref>