{{Short description|Gene sequence removed from RNA transcripts by ''trans''-splicing}}
An '''outron''' is a nucleotide sequence at the 5' end of the primary transcript of a gene that is removed by a special form of RNA splicing during maturation of the final RNA product.<ref name="Conrad1993">{{Cite journal|last1=Conrad|first1=Richard|last2=Fen Liou|first2=Ruey|last3=Blumenthal|first3=Thomas|date=1993-02-25|title=Functional analysis of a ''C. elegans'' trans-splice acceptor|url= |journal=Nucleic Acids Research|language=en|volume=21|issue=4|pages=913–919|doi=10.1093/nar/21.4.913|issn=0305-1048|pmc=309224|pmid=8451190}}</ref> Whereas intron sequences are located inside the gene, outron sequences lie outside the gene.<ref name="Stover2006">{{Cite journal|last1=Stover|first1=Nicholas A.|last2=Kaye|first2=Michelle S.|last3=Cavalcanti|first3=Andre R. O.|date=2006-01-10|title=Spliced leader trans-splicing|journal=Current Biology|language=English|volume=16|issue=1|pages=R8–R9|doi=10.1016/j.cub.2005.12.019|issn=0960-9822|pmid=16401417|doi-access=free|bibcode=2006CBio...16...R8S }}</ref>
== Characteristics == The outron is an intron-like sequence possessing similar characteristics such as the G+C content<ref name="Lasda2011"/> and a splice acceptor site that is the signal for ''trans''-splicing.<ref>{{cite web | title = Oxford reference — Outron | url = https://www.oxfordreference.com/view/10.1093/oi/authority.20110803100257646 | access-date = 26 September 2019}}</ref><ref>{{cite web | title = The MISO Sequence Ontology Browser — Outron (SO:0001475) | url = http://www.sequenceontology.org/browser/current_svn/term/SO:0001475 | access-date = 26 September 2019}}</ref> Such a ''trans''-splice site is essentially defined as an acceptor (3') splice site without an upstream donor (5') splice site.
In eukaryotes such as euglenozoans, dinoflagellates, sponges, nematodes, cnidarians, ctenophores, flatworms, crustaceans, chaetognaths, rotifers, and tunicates, the length of spliced leader (SL) outrons range from 30 to 102 nucleotides (nt), with the SL exon length ranging from 16 to 51 nt, and the full SL RNA length ranging from 46 to 141 nt.<ref name="Lasda2011">{{Cite journal|last1=Lasda|first1=Erika L.|last2=Blumenthal|first2=Thomas|date=2011-05-01|title=''Trans''-splicing|journal=Wiley Interdisciplinary Reviews: RNA|language=en|volume=2|issue=3|pages=417–434|doi=10.1002/wrna.71|pmid=21957027|s2cid=209567118}}</ref>
== Processing == In standard cis-splicing, the donor splice site in upstream position is required together with an acceptor site located on downstream position on the same pre-RNA molecule. By contrast, the SL ''trans''-splicing relies on a 3' acceptor splice site on the outron, and a 5' donor splice site (GU dinucleotide) located on a separate RNA molecule, the SL RNA.<ref name="Lasda2011"/> Moreover, the outron of the premature mRNA contains a branchpoint adenosine — followed by a downstream polypyrimidine tract — which interacts with the intron-like portion of the SL RNA to form a 'Y' branched byproduct, reminiscent of the lasso structure formed during intron splicing. Nuclear machinery then resolves this 'Y' branching structure by ''trans''-splicing the SL RNA sequence to the 3′ ''trans''-splice acceptor site (AG dinucleotide) of the pre-mRNA.<ref name="Stover2006"/>
When outrons are processed, the SL exon is ''trans''-spliced to distinct, unpaired, downstream acceptor sites adjacent to each open reading frame of the polycistronic pre-mRNA, leading to distinct mature capped transcripts. <ref name="Clayton2002">{{Cite journal|last=Clayton|first=Christine E.|date=2002-04-15|title=Life without transcriptional control? From fly to man and back again|journal=The EMBO Journal|language=en|volume=21|issue=8|pages=1881–1888|doi=10.1093/emboj/21.8.1881|issn=1460-2075|pmc=125970|pmid=11953307}}</ref><ref name="Blumenthal2003">{{Cite journal|last1=Blumenthal|first1=Thomas|last2=Gleason|first2=Kathy Seggerson|date=February 2003|title=''Caenorhabditis elegans'' operons: form and function|journal=Nature Reviews Genetics|language=en|volume=4|issue=2|pages=110–118|doi=10.1038/nrg995|pmid=12560808|s2cid=9864778|issn=1471-0056}}</ref><ref name="Lei2016">{{cite journal | vauthors = Lei Q, Li C, Zuo Z, Huang C, Cheng H, Zhou R | title = Evolutionary Insights into RNA trans-Splicing in Vertebrates | journal = Genome Biology and Evolution | volume = 8 | issue = 3 | pages = 562–77 | date = March 2016 | pmid = 26966239 | pmc = 4824033 | doi = 10.1093/gbe/evw025 }}</ref>
== See also == * {{annotated link|Exon}} * {{annotated link|Messenger RNA}}
==References== {{reflist}}
Category:Spliceosome Category:RNA splicing Category:Gene expression Category:Non-coding DNA