Theprokaryotic small ribosomal subunit, or30S subunit, is the smaller subunit of the 70Sribosome found inprokaryotes. It is a complex of the16S ribosomal RNA (rRNA) and 19proteins.^[1] This complex is implicated in the binding oftransfer RNA tomessenger RNA (mRNA).^[2] The small subunit is responsible for the binding and the reading of the mRNA duringtranslation. The small subunit, both the rRNA and its proteins, complexes with the large50S subunit to form the70S prokaryoticribosome in prokaryotic cells. This 70S ribosome is then used to translate mRNA into proteins.

The 30S subunit is an integral part ofmRNA translation. It binds threeprokaryotic initiation factors: IF-1, IF-2, and IF-3.^[3]

A portion of the 30S subunit (the16S rRNA) guides the initiatingstart codon (5′)-AUG-(3′) ofmRNA into position by recognizing theShine-Dalgarno sequence, acomplementary binding site about 8 base pairs upstream from the start codon.^[4] 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.^[4] Once the 16S rRNA recognizes the mRNA start codon, a specialtransfer RNA,f-Met-tRNA, binds and protein translation begins.^[5] The binding site of the f-Met-tRNA on the 30S ribosomal subunit is called the "D-site"^[6] This step is required in order forprotein synthesis to occur. Then the large ribosomal subunit will bind and protein synthesis will continue.^[7] The binding of the large subunit causes aconformational change in the 70S, which opens another site for protein translation.^[6]

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 aguanosine triphosphate iscleaved toguanosine diphosphate andinorganic phosphate, thus dissociating theinitiation factors and resulting in protein translation.^[8][5] This process is called "initiation" and is the slowest process of translation.^[5]

The small ribosomal subunit is made up of 16S rRNA and 19 full proteins.^[9] There is also onepolypeptide chain that consists of 26amino acids.^[10] 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.^[1]

The 30S subunit is the target ofantibiotics such astetracycline andgentamicin.^[11] These antibiotics specifically target the prokaryotic ribosomes, hence their usefulness in treating bacterial infections ineukaryotes. Tetracycline interacts with H27 in the small subunit as well as binding to theA-site in the large subunit.^[11]Puromycin is an inhibitor of ribosomal translation.^[6]Pactamycin interrupts the binding in the Shine-Dalgarno binding region in the small subunit, thus disrupting activity.Hygromycin B also interacts with H44 and inhibits the translocation movement that is necessary during protein synthesis.^[11]

• Prokaryotic large ribosomal subunit (50S)
• Ribosomal RNA
• Antibiotics

• 16S rRNA, BioMineWikiArchived 2020-09-07 at theWayback Machine
• http://pathmicro.med.sc.edu/mayer/antibiot.htm
• 16S+Ribosomal+RNA at the U.S. National Library of MedicineMedical Subject Headings (MeSH)