Editing Protein splicing (section)

===Types of inteins===
Inteins can be classified on many criteria.
* Based on how they splice themselves out, they can be classified into ''cis-splicing'' (which means that they splice themselves out) or ''trans-splicing'' (which means they need outside help). Most studied inteins are cis-splicing. Split inteins (see below) usually involves two halves helping each other out, so they are ''trans-splicing''.<ref name="pmid33339089">{{cite journal |last1=Nanda |first1=A |last2=Nasker |first2=SS |last3=Mehra |first3=A |last4=Panda |first4=S |last5=Nayak |first5=S |title=Inteins in Science: Evolution to Application. |journal=Microorganisms |date=16 December 2020 |volume=8 |issue=12 |page=2004 |doi=10.3390/microorganisms8122004 |pmid=33339089 |pmc=7765530 |doi-access=free }}</ref>
* Based on whether they contain the endonuclease domain. Ones that have an endonuclease domain is called a "maxi-intein", otherwise a "mini-intein".<ref name="pmid33339089"/>
* Based on their splicing mechanism, which can be partially<ref name=pmid21317331/> inferred based on the sequence. Class 1 intein is the most-studied type and are marked by a cysteine or serine as the first residue. Class 2 intein, or "alanine intein" has alanine as the first residue and no WCT motif. Class 3 intein has alanine as the first residue and a noncontiguous "WCT" motif.<ref name="pmid33339089"/> It has also been proposed that inteins that start with a serine and contain a "WCT" motif should be also classed as class 3.<ref name=pmid21317331>{{cite journal |last1=Tori |first1=K |last2=Perler |first2=FB |title=Expanding the definition of class 3 inteins and their proposed phage origin. |journal=Journal of Bacteriology |date=April 2011 |volume=193 |issue=8 |pages=2035–41 |doi=10.1128/JB.01407-10 |pmid=21317331|pmc=3133030 |doi-access=free }}</ref>

==== Full and mini inteins ====
Inteins can contain a [[Homing endonuclease| homing endonuclease gene]] (HEG) domain in addition to the splicing domains. This domain is responsible for the spread of the intein by cleaving DNA at an intein-free [[allele]] on the [[homologous chromosome]], triggering the [[DNA repair#Double-strand breaks|DNA double-stranded break repair]] (DSBR) system, which then repairs the break, thus copying the intein-coding DNA into a previously intein-free site.<ref name="pmid33339089"/> The HEG domain is not necessary for intein splicing, and so it can be lost, forming a ''minimal'', or ''mini'', ''intein''. Several studies have demonstrated the modular nature of inteins by adding or removing HEG domains and determining the activity of the new construct.{{Citation needed| date=November 2011}}

==== Split inteins ====
Sometimes, the intein of the precursor protein comes from two genes. In this case, the intein is said to be a ''split intein''. For example, in [[cyanobacteria]], [[DnaE]], the catalytic subunit α of [[DNA polymerase III holoenzyme|DNA polymerase III]], is encoded by two separate genes, ''dnaE-n'' and ''dnaE-c''. The ''dnaE-n'' [[gene product|product]] consists of an N-extein sequence followed by a 123-AA intein sequence, whereas the ''dnaE-c'' product consists of a 36-AA intein sequence followed by a C-extein sequence.<ref name="DnaE">{{Cite journal | doi=10.1073/pnas.95.16.9226 | last1=Wu | first1=H. | last2=Hu | first2=Z. | last3=Liu | first3=X. Q. 
| title=Protein trans-splicing by a split intein encoded in a split DnaE gene of Synechocystis sp. PCC6803 
| journal=Proceedings of the National Academy of Sciences of the United States of America | volume=95 | issue=16 | pages=9226–9231 | year=1998 | pmid=9689062 | pmc=21320| bibcode=1998PNAS...95.9226W | doi-access=free }}</ref>