The ribosome synthesizes proteins in all living cells. This apparatus reads genetic instructions encoded in messenger RNA (mRNA) and links amino acids together to form polypeptide chains. Ribosomes are categorized by their sedimentation rate, measured in Svedberg units (S). The 70S ribosome is the type found in bacteria, executing the protein-building process for these single-celled organisms.
Anatomy of the 70S Ribosome
The 70S ribosome is composed of two distinct subunits that assemble only when actively synthesizing a protein. These are the large 50S subunit and the small 30S subunit, which separate when the ribosome is not performing its function. The Svedberg units (S) are not strictly additive, which is why 50S plus 30S equals 70S.
The small 30S subunit contains a 16S ribosomal RNA (rRNA) molecule and about 20 associated proteins. Its primary role is decoding the genetic message carried by the mRNA. The larger 50S subunit contains the 23S and 5S rRNA molecules, along with about 31 proteins. Ribosomal RNA, rather than the proteins, forms the core functional and structural components of the assembly.
The Central Role in Bacterial Protein Synthesis
The primary function of the 70S ribosome is translation, the process that converts the nucleotide sequence of an mRNA molecule into a specific sequence of amino acids, ultimately forming a protein. This process is divided into three consecutive stages: initiation, elongation, and termination. Translation begins with initiation, where the 30S subunit binds to the mRNA, recognizes a specific start sequence, and recruits a special initiator transfer RNA (tRNA) that binds directly to the peptidyl (P) site.
Once the large 50S subunit joins the complex, the ribosome enters the elongation phase, where the amino acid chain is built. The 70S ribosome features three binding pockets for tRNAs: the aminoacyl (A) site, the P site, and the exit (E) site. A new amino acid-carrying tRNA enters the A site, matching its anticodon to the codon exposed on the mRNA, which is the first checkpoint for accuracy.
The 50S subunit then catalyzes the formation of a peptide bond between the amino acid in the A site and the growing polypeptide chain held in the P site, a reaction mediated by the 23S rRNA, which acts as a ribozyme. Following bond formation, the entire ribosome complex shifts, or translocates, by one codon along the mRNA, moving the growing chain from the A site to the P site, and pushing the now-empty tRNA from the P site into the E site for release. This cycle repeats rapidly until the ribosome encounters a stop codon on the mRNA, which signals the termination phase. Proteins called release factors recognize the stop codon in the A site, prompting the release of the completed polypeptide chain and the dissociation of the 70S ribosome back into its 50S and 30S subunits.
Unique Locations of the 70S Ribosome
The 70S ribosome is the standard protein-synthesis machinery found throughout all prokaryotic organisms, which include both bacteria and archaea. Within these cells, the ribosomes operate freely in the cytoplasm, often translating mRNA as soon as it is transcribed from the DNA. The 70S ribosome is also found inside two specific organelles within eukaryotic cells: the mitochondria and the chloroplasts.
Mitochondria and chloroplasts maintain their own separate genetic material and protein-making apparatus, which includes the 70S ribosome. This finding is foundational evidence supporting the Endosymbiotic Theory, an explanation for the evolutionary origin of these organelles. The theory suggests that mitochondria and chloroplasts originated as free-living bacteria that were engulfed by an ancestral eukaryotic cell, with their 70S ribosomes serving as a molecular vestige of their bacterial ancestry.
Why Antibiotics Target the 70S Ribosome
The distinct structural differences between the bacterial 70S ribosome and the eukaryotic 80S ribosome found in human cells provide a target for many clinically utilized antibiotics. The 70S and 80S ribosomes differ in size, the specific ribosomal RNA sequences, and the number and type of associated proteins. This structural divergence allows antibacterial drugs to selectively interfere with bacterial protein synthesis without significantly disrupting the host cell’s own machinery.
Specific classes of antibiotics bind to unique sites on the 70S ribosome, disrupting the translation process at different stages. For instance, tetracyclines target the 30S subunit, binding near the A site and physically preventing incoming aminoacyl-tRNAs from attaching, thereby halting the elongation phase. Macrolides, such as erythromycin, bind to the 50S subunit, often inhibiting the translocation step, which effectively stalls the ribosome’s movement along the mRNA. Aminoglycosides operate by binding to the 16S rRNA within the 30S subunit, causing the ribosome to misread the mRNA, leading to the incorporation of incorrect amino acids and the production of non-functional proteins.