imported>Alfa-ketosav: rRNS is the abbreviation in German (ribosomale Ribonukleinsäure) but not in English (ribosomal ribonucleic acid)

2025-05-08T10:37:37Z

rRNS is the abbreviation in German (ribosomale Ribonukleinsäure) but not in English (ribosomal ribonucleic acid)

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{{Short description|Part of the eukaryotic ribosomal RNA}}
{{missing information|Rfam SSU_rRNA_eukarya|date=December 2020}}

'''18S ribosomal RNA''' (abbreviated '''18S rRNA''') is a part of the [[ribosomal RNA]] in [[eukaryote]]s. It is a component of the [[Eukaryotic small ribosomal subunit (40S)]] and the [[cytosol]]ic homologue of both the [[12S ribosomal RNA|12S rRNA]] in [[mitochondria]] and the [[16S ribosomal RNA|16S rRNA]] in [[plastid]]s and [[prokaryote]]s. Similar to the prokaryotic 16S rRNA, the genes of the 18S ribosomal RNA have been widely used for [[Phylogenetics|phylogenetic studies]] and biodiversity screening of eukaryotes.<ref name="Meyer 2010">{{cite journal |vauthors=Meyer A, Todt C, Mikkelsen NT, Lieb B|title=Fast evolving 18S rRNA sequences from Solenogastres (Mollusca) resist standard PCR amplification and give new insights into mollusk substitution rate heterogeneity|journal=BMC Evolutionary Biology|year=2010|volume=10 |issue=1 |page=70 |article-number=70 |doi=10.1186/1471-2148-10-70|doi-access=free |pmid=20214780 |pmc=2841657 |bibcode=2010BMCEE..10...70M }}</ref>

==Research history==
Along with the [[28S ribosomal RNA|28S]] and [[5.8S ribosomal RNA|5.8S rRNA]] in eukaryotes, the '''18S rRNA''' was early identified as integral structural element of [[ribosome]]s which were first characterized by their sedimentation properties and named according to measured [[Svedberg|Svedberg units]].<ref name=Graw2015Springer>{{cite book |last=Graw |first=Jochen |date=2015|edition=6th |title=Genetik |trans-title=Genetics |language=German |location=Berlin, Heidelberg |publisher= Springer-Verlag Berlin Heidelberg|isbn=978-3-662-44816-8|doi=10.1007/978-3-662-44817-5}}</ref>

Given its ubiquitous presence in eukaryotic life, the evolution of the 18S rRNA was soon proposed as marker for [[phylogenetics|phylogenetic studies]] to resolve the evolution of [[eukaryote]]s.<ref name="Field1988">{{cite journal |vauthors=Field KG, Olsen GJ, Lane DJ, Giovannoni SJ, Ghiselin MT, Raff EC, Pace NR, Raff RA|title=Molecular phylogeny of the animal kingdom |journal=[[Science (journal)|Science]]|year=1988|volume=239|issue=4841 |pages=748–753 |doi=10.1126/science.3277277|pmid=3277277 |bibcode=1988Sci...239..748F }}</ref>

==Structure and function==
The '''18S ribosomal RNA''' is the [[SSU rRNA|structural RNA of the small subunit]] in the [[eukaryote|eukaryotic]] cytoplasmic [[ribosome]].

[[File:Eucaryot rdna.png|thumb|right|General organization of the eukaryotic nuclear [[ribosomal DNA|rDNA]] tandem repeats consisting of [[External transcribed spacer|ETS]], 18S rRNA, [[Internal transcribed spacer|ITS-1]], [[5.8S ribosomal RNA|5.8S rRNA]], [[Internal transcribed spacer|ITS-2]] and [[28S ribosomal RNA|28S rRNA]].]]
The genomic sequence of the 18S rRNA is organized in a group with the [[28S ribosomal RNA|28S]] and [[5.8S ribosomal RNA|5.8S rRNA]], separated and flanked by the [[Internal transcribed spacer|ITS-1, ITS-2]] and [[External transcribed spacer|ETS]] spacer regions.<ref name=Hillis1991>{{cite journal |vauthors=Hillis DM, Dixon MT |title=Ribosomal DNA: Molecular Evolution and Phylogenetic Inference |journal=[[The Quarterly Review of Biology]] |year=1991|volume=66 |issue=4 |pages=411–53 |doi=10.1086/417338 |pmid=1784710}}</ref>
These regions of [[ribosomal DNA|ribosomal DNA (rDNA)]] are present with several hundred copies in the active genome, clustered in [[Nucleolus organizer region|nucleolus organizer regions (NORs)]].<ref name=Graw2015Springer/>
In [[ribosome biogenesis]], these genes are transcribed together by the [[RNA polymerase I]] and are processed in the [[nucleolus]] structure of the [[Cell nucleus|nucleus]].

{| class="wikitable floatright"
|+ 18S rRNA nucleotide length of selected species
|'''Species'''
|'''Size [nt]'''
|-
|''[[Saccharomyces cerevisiae]]'' || 1,789<ref name=Rubstov1980>{{cite journal |vauthors=Rubtsov PM, Musakhanov MM, Zakharyev VM, Krayev AS, Skryabin KG, Bayev AA |title=The structure of the yeast ribosomal RNA genes. I. The complete nucleotide sequence of the 18S ribosomal RNA gene from Saccharomyces cerevisiae. |journal=[[Nucleic Acids Research]]|year=1980|volume=8 |issue=23 |pages=5779–5794 |doi=10.1093/nar/8.23.5779 |pmid=7008030 |pmc=324341 }}</ref>
|-
|''[[Xenopus laevis]]''|| 1,825<ref name=Salim1981>{{cite journal |vauthors=Salim M, Maden EH|title=Nucleotide sequence of ''Xenopus laevis'' 18S ribosomal RNA inferred from gene sequence. |journal=[[Nature (journal)|Nature]]|year=1981|volume=291 |issue=5812 |pages=205–208 |doi=10.1038/291205a0 |pmid=7015146 |bibcode=1981Natur.291..205S }}</ref>
|-
|''[[Homo sapiens]]''|| 1,869<ref name=NCBI18SHomoSapiens/>
|-
|''[[Drosophila melanogaster]]''|| 1,995<ref name=Tautz1988>{{cite journal |vauthors=Tautz D, Hancock JM, Webb DA, Tautz C, Dover GA|title=Complete sequences of the rRNA genes of Drosophila melanogaster |journal=[[Molecular Biology and Evolution]] |year=1988 |volume=5 |issue=4 |pages=366–376 |doi= 10.1093/oxfordjournals.molbev.a040500 |pmid= 3136294 }}</ref>
|}
The length of the 18S rRNA varies considerably in the eukaryotic phylogenetic tree, corresponding to a range of 16S-19S in [[Svedberg|Svedberg units]],<ref name=Graw2015Springer/> with the average length commonly given as around 2000 [[nucleotide]]s.<ref name=Graw2015Springer/> The 18S rRNA of [[human]]s has a length of 1869 nucleotides.<ref name=NCBI18SHomoSapiens>Page ''Homo sapiens RNA, 18S ribosomal N5 (RNA18SN5), ribosomal RNA'' on {{cite web|url=https://www.ncbi.nlm.nih.gov/nuccore/NR_003286|title=''Homo sapiens'' 18S ribosomal RNA|date=25 March 2023 | publisher=[[National Library of Medicine]]|access-date=2024-06-29}}</ref>

==Uses==
The universal presence of the '''18S rRNA''' in [[eukaryotes]] and generally high degree of conservation in evolution allow the construction of universal [[Primer (molecular biology)|primers]] for [[DNA amplification]] by [[polymerase chain reaction]].<ref name=Hillis1991/><ref name="Meyer 2010"/> The possible applications mirror molecular methods involving [[16S ribosomal RNA|16S rRNA]] of [[prokaryotes]].

=== Biodiversity screening ===
Primers binding in highly conserved regions of the 18S rRNA are an important marker for [[biodiversity]] screening,<ref name="Meyer 2010"/> allowing the amplification of unspecified or random targets from environmental samples as well as uncharacterized [[Biological specimen|specimens]] from collections for [[DNA sequencing]]. Subsequent [[sequence alignment]] covering the less strictly conserved segments then allows the assignment of a sample to biologic [[clade (biology)|clades]]. {{Citation needed|date=July 2024|reason=general citation, also differences in resolution to genus and species level}}

In the case of 18S rRNA, retrieval of DNA is improved by the abundance of repeating sequences of the [[ribosomal DNA|rDNA]] within eukaryotic cells,<ref name="Meyer 2010"/> promoting the sensitivity of the analysis.

=== Phylogenetics ===
Multiple properties of the genomic sequence of the 18S rRNA have established it as an important marker gene for large-scale phylogenetic analysis and the reconstruction of the [[metazoa]]n [[tree of life (biology)|tree of life]]. The integral role in formation and function of the [[ribosome]] is a key cause for its omnipresence in eukaryotic life. Meanwhile, the gene maintains a high degree of conservation under a persistent selective pressure in all living beings,<ref name="Meyer 2010"/> highlighting its potential for comparison between distantly related clades.

Early studies utilizing the 18S rRNA sequence constructed the first large-scale [[phylogenetic tree]]s of the [[metazoa]].<ref name=Field1988/> Evidence from further studies led to the creation of several important [[clade]]s, such as the [[Ecdysozoa]] and [[Lophotrochozoa]].<ref name="Meyer 2010"/>{{Citation needed|date=July 2024|reason=Original publications for both mentioned clades.}}

During the latter part of the 2000s, and with increased numbers of taxa included into molecular phylogenies, however, two problems became apparent. First, there are prevailing sequencing impediments in representatives of certain taxa, such as the mollusk classes [[Solenogastres]] and [[Tryblidia]], selected [[bivalve]] taxa, and the enigmatic crustacean class [[Remipedia]].<ref name="Meyer 2010"/> Failure to obtain 18S sequences of single taxa is considered a common phenomenon but is rarely ever reported.<ref name="Meyer 2010"/> Secondly, in contrast to initially high hopes, 18S cannot resolve nodes at all taxonomic levels and its efficacy varies considerably among clades. This has been discussed as an effect of rapid ancient radiation within short periods. Multigene analyses are currently thought to give more reliable results for tracing deep branching events in Metazoa but 18S still is extensively used in phylogenetic analyses.<ref name="Meyer 2010"/>

== References ==
This article incorporates CC-By-2.0 text from the reference.<ref name="Meyer 2010"/>

{{reflist}}

{{Ribosome subunits}}

{{DEFAULTSORT:18s Ribosomal Rna}}
[[Category:Ribosomal RNA]]

18S ribosomal RNA - Revision history

imported>Alfa-ketosav: rRNS is the abbreviation in German (ribosomale Ribonukleinsäure) but not in English (ribosomal ribonucleic acid)