The SAM–SAH riboswitch is a conserved RNA structure in certain
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
that binds
''S''-adenosylmethionine (SAM) and
''S''-adenosylhomocysteine (SAH) and is therefore presumed to be a
riboswitch
In molecular biology, a riboswitch is a regulatory segment of a messenger RNA molecule that binds a small molecule, resulting in a change in production of the proteins encoded by the mRNA. Thus, an mRNA that contains a riboswitch is directly in ...
.
SAM–SAH riboswitches do not share any apparent structural resemblance to known riboswitches that bind SAM or SAH. The binding affinities for both compounds are similar, but binding for SAH is at least somewhat stronger. SAM–SAH riboswitches are exclusively found in
Rhodobacterales
Rhodobacterales are an order of the Alphaproteobacteria.
Gene transfer agents are viruslike elements produced by Rhodobacterales which transfer DNA and may be an important factor in their evolution.
Etymology
From Greek ''rhodon'', the rose
...
, an
order
Order, ORDER or Orders may refer to:
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
* Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...
of
alphaproteobacteria
Alphaproteobacteria is a class of bacteria in the phylum Pseudomonadota (formerly Proteobacteria). The Magnetococcales and Mariprofundales are considered basal or sister to the Alphaproteobacteria. The Alphaproteobacteria are highly diverse and ...
. They are always found in the apparent
5' untranslated region
The 5′ untranslated region (also known as 5′ UTR, leader sequence, transcript leader, or leader RNA) is the region of a messenger RNA (mRNA) that is directly upstream from the initiation codon. This region is important for the regulation of t ...
s of ''metK'' genes, which encode the enzyme (
Methionine adenosyltransferase) that synthesizes SAM.
Given this gene association, it was proposed that SAM–SAH riboswitches more likely function as SAM-sensing RNAs. SAM–SAH riboswitches are relatively small among known riboswitches, which might relate to their inability to discriminate against SAH. However, the ability to reject SAH as a ligand might not be important under physiological conditions, because the cellular concentration of SAM is higher.
A region of the conserved structure of SAM–SAH riboswitches includes a predicted
Shine-Dalgarno sequence (
ribosome-binding site) of the downstream ''metK'' genes. These nucleotides are required for optimal binding to the ligand and might form a pseudoknot with the terminal loop within the main
stem-loop
Stem-loop intramolecular base pairing is a pattern that can occur in single-stranded RNA. The structure is also known as a hairpin or hairpin loop. It occurs when two regions of the same strand, usually complementary in nucleotide sequence wh ...
structure. Occlusion of the Shine-Dalgarno sequence might be the mechanism by which SAM–SAH riboswitches regulate expression of the downstream genes.
The 3-D structure of a SAM-SAH riboswitch has been determined by two groups working independently.
See also
*
SAM-I riboswitch
*
SAM-II riboswitch
The SAM-II riboswitch is a RNA element found predominantly in Alphaproteobacteria that binds S-adenosyl methionine (SAM). Its structure and sequence appear to be unrelated to the SAM riboswitch found in Gram-positive bacteria. This SAM riboswit ...
*
SAM-III riboswitch
*
SAM-IV riboswitch
SAM-IV riboswitches are a kind of riboswitch that specifically binds S-adenosylmethionine (SAM), a cofactor used in many methylation reactions. Originally identified by bioinformatics, SAM-IV riboswitches are largely confined to the Actinomyceta ...
*
SAM-V riboswitch
SAM-V riboswitch is the fifth known riboswitch to bind S-adenosyl methionine (SAM). It was first discovered in the marine bacterium ''Candidatus Pelagibacter ubique'' and can also be found in marine metagenomes. SAM-V features a similar consensus ...
*
SAM-VI riboswitch SAM-VI is a member of the riboswitch family. It is predominantly found in ''Bifidobacterium'' and exhibits some similarities to the SAM-III ( Smk box) riboswitch class, but lacks most of the highly conserved nucleotides of SAM-III class. SAM-VI ap ...
References
External links
*
{{DEFAULTSORT:SAM-SAH riboswitch
Cis-regulatory RNA elements
Riboswitch