30S ribosomal subunits

The prokaryotic small ribosomal subunit, or 30S, is the smaller subunit of the 70S ribosome found in prokaryotes. It is a complex of the 16S ribosomal RNA (rRNA) and 19 proteins. This complex is implicated in the binding of Transfer RNA onto the messenger RNA (mRNA). The small subunit is responsible for the binding and the reading of the mRNA during translation. The whole subunit, both the rRNA and the 22 proteins, complexes with the larger 50S subunit to form the 70S prokaryotic ribosome in the cell. This 70S ribosome is then used to translate mRNA into proteins.

The 30S subunit is an integral part of mRNA translation. It binds three Prokaryotic initiation factors: IF-1, IF-2, and IF-3.

A portion of the 30S subunit (the 16S rRNA) guides the initiating (5')-AUG-(3') (start codon) codon of the mRNA into position by a recognizing a complimentary binding site, about 8 base pairs upstream, known as the Shine-Dalgarno (SD) sequence on the mRNA. This ensures the ribosome starts translation at the correct location. The tightness of the bonding between the SD sequence on the mRNA and the 16S rRNA determines how efficiently translation proceeds. Once the 16S rRNA recognizes the mRNA start codon, a special tRNA, f-Met-tRNA binds and protein translation begins. The binding site of the f-Met-tRNA on the 30S ribosomal subunit is referred to as 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 tranlsation.

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