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16S ribosomal RNA

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16S ribosomal RNA

16S ribosomal RNA (or 16S rRNA) is the component of the 30S small subunit of a prokaryotic ribosome that binds to the Shine-Dalgarno sequence. The genes coding for it are referred to as 16S rRNA gene and are used in reconstructing phylogenies, due to the slow rates of evolution of this region of the gene. Carl Woese and George E. Fox were two of the people who pioneered the use of 16S rRNA in phylogenies.

Contents

Multiple sequences of the 16S rRNA gene can exist within a single bacterium.

Functions

It has several functions:

  • Like the large (23S) ribosomal RNA, it has a structural role, acting as a scaffold defining the positions of the ribosomal proteins.
  • The 3' end contains the anti-Shine-Dalgarno sequence, which binds upstream to the AUG start codon on the mRNA. The 3'-end of 16S RNA binds to the proteins S1 and S21 known to be involved in initiation of protein synthesis; RNA-protein cross-linking by A.P. Czernilofsky et al. (FEBS Lett. Vol 58, pp 281–284, 1975).
  • Interacts with 23S, aiding in the binding of the two ribosomal subunits (50S+30S)
  • Stabilizes correct codon-anticodon pairing in the A site, via a hydrogen bond formation between the N1 atom of Adenine (see image of Purine chemical structure) residues 1492 and 1493 and the 2'OH group of the mRNA backbone

    • Universal primers

    The 16S rRNA gene is used for phylogenetic studies as it is highly conserved between different species of bacteria and archaea. Carl Woese pioneered this use of 16S rRNA. Some (hyper)thermophilic archaea (i.e. order Thermoproteales) contain 16S rRNA gene introns that are located in highly conserved regions and can impact the annealing of "universal" primers. Mitochondrial and chloroplastic rRNA are also amplified.

    The most common primer pair was devised by Weisburg et al. and is currently referred to as 27F and 1492R; however, for some applications shorter amplicons may be necessary for example for 454 sequencing with Titanium chemistry (500-ish reads are ideal) the primer pair 27F-534R covering V1 to V3. Often 8F is used rather than 27F. The two primers are almost identical, but 27F has an M instead of a C. AGAGTTTGATCMTGGCTCAG compared with 8F.

    PCR applications

    In addition to highly conserved primer binding sites, 16S rRNA gene sequences contain hypervariable regions that can provide species-specific signature sequences useful for identification of bacteria. As a result, 16S rRNA gene sequencing has become prevalent in medical microbiology as a rapid and cheap alternative to phenotypic methods of bacterial identification. Although it was originally used to identify bacteria, 16S sequencing was subsequently found to be capable of reclassifying bacteria into completely new species, or even genera. It has also been used to describe new species that have never been successfully cultured.

    16S ribosomal databases

    The 16S rRNA gene is used as the standard for classification and identification of microbes, because it is present in most microbes and shows proper changes. Type strains of 16S rRNA gene sequences for most bacteria and archaea are available on public databases such as NCBI. However, the quality of the sequences found on these databases are often not validated. Therefore, secondary databases that collect only 16S rRNA sequences are widely used. The most frequently used databases are listed below:

    EzTaxon-e

    http://eztaxon-e.ezbiocloud.net/ The EzTaxon-e database is an extension of the original EzTaxon database. It contains comprehensive 16S rRNA gene sequences of taxa with valid names as well as sequences of uncultured taxa. EzTaxon-e contains complete hierarchical taxonomic structure (from phylum rank to species rank) for the domain of bacteria and archaea.

    Ribosomal Database Project

    http://rdp.cme.msu.edu/ The Ribosomal Database Project (RDP) is a curated database that offers ribosome data along with related programs and services. The offerings include phylogenetically ordered alignments of ribosomal RNA (rRNA) sequences, derived phylogenetic trees, rRNA secondary structure diagrams and various software packages for handling, analyzing and displaying alignments and trees. The data are available via ftp and electronic mail. Certain analytic services are also provided by the electronic mail server.

    SILVA

    SILVA provides comprehensive, quality checked and regularly updated datasets of aligned small (16S/18S, SSU) and large subunit (23S/28S, LSU) ribosomal RNA (rRNA) sequences for all three domains of life as well as a suite of search, primer-design and alignment tools (Bacteria, Archaea and Eukarya). (Warning: the latest version with taxonomies, SILVA_123.1_SSURef_Nr99_tax, contains errors in the taxonomy because the species name is taken from the source and the taxonomy, up to genus, is found by similarity, e.g. like a pseudomonas lineage up to genus with a cricket species: Pseudomonas;Teleogryllus commodus.)

    Greengenes

    Greengenes is a quality controlled, comprehensive 16S reference database and taxonomy based on a de novo phylogeny that provides standard operational taxonomic unit sets. The official home page for the site is http://greengenes.secondgenome.com, and is licensed under the Creative Commons BY-SA 3.0 license.

    References

    16S ribosomal RNA Wikipedia
    (Text) CC BY-SA


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