![010 small subunit-1FKA](https://upload.wikimedia.org/wikipedia/commons/9/90/010_small_subunit-1FKA.gif)
The prokaryotic small ribosomal subunit, or 30
S subunit, is the smaller subunit of the 70S
ribosome found in
prokaryote
A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Conne ...
s. It is a complex of the
16S ribosomal RNA
16 S ribosomal RNA (or 16 S rRNA) is the RNA component of the 30S subunit of a prokaryotic ribosome ( SSU rRNA). It binds to the Shine-Dalgarno sequence and provides most of the SSU structure.
The genes coding for it are referred to as 16S rR ...
(rRNA) and 19
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s.
This complex is implicated in the binding of
transfer RNA to
messenger RNA (mRNA). The small subunit is responsible for the binding and the reading of the mRNA during
translation
Translation is the communication of the meaning of a source-language text by means of an equivalent target-language text. The English language draws a terminological distinction (which does not exist in every language) between ''transla ...
. The small subunit, both the rRNA and its proteins, complexes with the large
50S
The 50s decade ran from January 1, 50, to December 31, 59. It was the sixth decade in the Anno Domini/Common Era, if the nine-year period from 1 AD to 9 AD is considered as a "decade".
Significant people
* Claudius, Roman Emperor (AD 41 ...
subunit to form the
70S
Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to for ...
prokaryotic
ribosome in prokaryotic cells. This 70S ribosome is then used to translate mRNA into proteins.
Function
The 30S subunit is an integral part of
mRNA translation
In molecular biology and genetics, translation is the process in which ribosomes in the cytoplasm or endoplasmic reticulum synthesize proteins after the process of transcription of DNA to RNA in the cell's nucleus. The entire process ...
. It binds three
prokaryotic initiation factors: IF-1, IF-2, and IF-3.
A portion of the 30S subunit (the
16S rRNA 16S rRNA may refer to:
* 16S ribosomal RNA
16 S ribosomal RNA (or 16 S rRNA) is the RNA component of the 30S subunit of a prokaryotic ribosome ( SSU rRNA). It binds to the Shine-Dalgarno sequence and provides most of the SSU structure.
The g ...
) guides the initiating
start codon (5′)-AUG-(3′) of
mRNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein.
mRNA is created during the ...
into position by recognizing the
Shine-Dalgarno sequence, a
complementary binding site about 8 base pairs upstream from the start codon.
This ensures the ribosome starts translation at the correct location. The tightness of the bonding between the Shine-Dalgarno sequence on the mRNA and the 16S rRNA determines how efficiently translation proceeds.
Once the 16S rRNA recognizes the mRNA start codon, a special
transfer RNA,
f-Met
''N''-Formylmethionine (fMet, HCO-Met, For-Met) is a derivative of the amino acid methionine in which a formyl group has been added to the amino group. It is specifically used for initiation of protein synthesis from bacterial and organellar DNA, ...
-tRNA, binds and protein translation begins.
The binding site of the f-Met-tRNA on the 30S ribosomal subunit is called the "D-site"
This step is required in order for
protein synthesis to occur. Then the large ribosomal subunit will bind and protein synthesis will continue. The binding of the large subunit causes a
conformational change in the 70S, which opens another site for protein translation.
In order to form the translation complex with the 50S subunit, the 30S subunit must bind IF-1, IF-2, IF-3, mRNA, and f-met-tRNA. Next, the 50S subunit binds and a
guanosine triphosphate
Guanosine-5'-triphosphate (GTP) is a purine nucleoside triphosphate. It is one of the building blocks needed for the synthesis of RNA during the transcription process. Its structure is similar to that of the guanosine nucleoside, the only d ...
is
cleaved to
guanosine diphosphate
Guanosine diphosphate, abbreviated GDP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside guanosine. GDP consists of a pyrophosphate group, a pentose sugar ribose, and the nucleobase guanine.
GDP is the product ...
and
inorganic phosphate
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phosph ...
, thus dissociating the
initiation factors Initiation factors are proteins that bind to the small subunit of the ribosome during the initiation of translation, a part of protein biosynthesis.
Initiation factors can interact with repressors to slow down or prevent translation. They have the ...
and resulting in protein translation.
This process is called "initiation" and is the slowest process of translation.
Structure
The small ribosomal subunit is made up of 16S rRNA and 19 full proteins.
There is also one
polypeptide chain
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
...
that consists of 26
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
s.
Conventionally, the rRNA is labeled with "H#" to indicate the helix number in high resolution images. Proteins are labelled "S#" to indicate the different peptides involved in rRNA stabilization. S11 and H45 are located near the Shine-Dalgarno binding site, which is also near the IF-3 binding site. Proteins S3, S4, S5, and S12, along with H18, are located near the channel where mRNA is present in the 30S subunit.
Inhibition
The 30S subunit is the target of
antibiotics such as
tetracycline
Tetracycline, sold under various brand names, is an oral antibiotic in the tetracyclines family of medications, used to treat a number of infections, including acne, cholera, brucellosis, plague, malaria, and syphilis.
Common side effects in ...
and
gentamicin.
These antibiotics specifically target the prokaryotic ribosomes, hence their usefulness in treating bacterial infections in
eukaryotes. Tetracycline interacts with H27 in the small subunit as well as binding to the
A-site
The A-site (A for aminoacyl) of a ribosome is a binding site for charged t-RNA molecules during protein synthesis. One of three such binding sites, the A-site is the first location the t-RNA binds during the protein synthesis process, the other ...
in the large subunit.
Puromycin
Puromycin is an antibiotic protein synthesis inhibitor which causes premature chain termination during translation.
Inhibition of translation
Puromycin is an aminonucleoside antibiotic, derived from the '' Streptomyces alboniger'' bacterium ...
is an inhibitor of ribosomal translation.
Pactamycin interrupts the binding in the Shine-Dalgarno binding region in the small subunit, thus disrupting activity.
Hygromycin B
Hygromycin B is an antibiotic produced by the bacterium '' Streptomyces hygroscopicus''. It is an aminoglycoside that kills bacteria, fungi and higher eukaryotic cells by inhibiting protein synthesis.
History
Hygromycin B was originally devel ...
also interacts with H44 and inhibits the translocation movement that is necessary during protein synthesis.
See also
*
Prokaryotic large ribosomal subunit (50S)
*
Ribosomal RNA
Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosom ...
*
Antibiotics
An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention o ...
References
External links
16S rRNA, BioMineWiki* http://pathmicro.med.sc.edu/mayer/antibiot.htm
*
{{Ribosome subunits
Ribosomal RNA
Protein biosynthesis