Ribosomes are cellular machines found within all living organisms, serving as the primary sites for protein synthesis. They translate genetic instructions into functional proteins, which perform nearly all cellular tasks. The 70S ribosome is a specific type found exclusively in bacteria and other prokaryotic cells. Its efficiency in producing proteins for bacterial growth and reproduction makes it fundamental for their survival.
The Architecture of the 70S Ribosome
The 70S ribosome is assembled from two subunits: the larger 50S subunit and the smaller 30S subunit. The “S” denotes Svedberg units, which measure a particle’s sedimentation rate in a centrifuge, indicating its size and shape. The 50S subunit contains two ribosomal RNA (rRNA) molecules, the 23S rRNA and the 5S rRNA, along with approximately 34 ribosomal proteins.
The smaller 30S subunit is composed of a single 16S rRNA molecule and around 21 ribosomal proteins. These rRNA molecules form the structural backbone of each subunit and possess catalytic activity, facilitating biochemical reactions. Associated proteins stabilize the structure and assist in various steps of the protein synthesis process.
The 70S Ribosome’s Role in Protein Production
The 70S ribosome translates the genetic code carried by messenger RNA (mRNA) into a sequence of amino acids, which fold into a protein. This process, known as translation, occurs in three stages: initiation, elongation, and termination. During initiation, the 30S subunit first binds to the mRNA molecule and a specialized initiator transfer RNA (tRNA) carrying the first amino acid. The 50S subunit then joins this complex, forming the complete 70S ribosome.
Once assembled, the ribosome moves along the mRNA, reading codons. In the elongation phase, transfer RNA (tRNA) molecules, each carrying a specific amino acid, enter the ribosome at the A-site (aminoacyl site). If the tRNA’s anticodon matches the mRNA codon, the amino acid is added to the growing polypeptide chain at the P-site. The ribosome then shifts, moving the tRNAs and mRNA, and spent tRNAs exit from the E-site. This sequential addition of amino acids continues until the ribosome encounters a stop codon on the mRNA.
Upon encountering a stop codon, release factors bind to the ribosome, triggering the release of the newly synthesized protein from the P-site. The ribosomal subunits then dissociate from the mRNA, becoming available to initiate another round of protein synthesis. This continuous cycle ensures bacteria rapidly produce the proteins required for their metabolic processes, structural integrity, and replication. The 70S ribosome acts as a highly efficient molecular factory, converting genetic information into the functional molecules that sustain bacterial life.
How 70S Ribosomes Differ from Other Ribosomes
The 70S ribosome has distinct characteristics that set it apart from ribosomes in other organisms, particularly the 80S ribosomes of eukaryotic cells. The most apparent distinction is their overall size and sedimentation coefficient: prokaryotic ribosomes are 70S, while eukaryotic ribosomes are larger at 80S. This difference in Svedberg units reflects variations in their mass and how they settle during ultracentrifugation.
The 70S ribosome consists of a 50S large subunit and a 30S small subunit. In contrast, the eukaryotic 80S ribosome is assembled from a larger 60S subunit and a 40S small subunit. Ribosomal RNA content also varies. The 70S ribosome contains 23S, 5S, and 16S rRNAs, while the 80S ribosome features 28S, 5.8S, 5S, and 18S rRNAs, with the 18S rRNA in the 40S subunit being a key distinguishing feature from the 16S rRNA in the 30S subunit.
The number and types of ribosomal proteins associated with each subunit also differ. Eukaryotic ribosomes generally have more proteins than their prokaryotic counterparts. Their cellular location also varies: 70S ribosomes are free-floating in the cytoplasm of prokaryotic cells, while 80S ribosomes in eukaryotic cells can be found free in the cytoplasm or attached to the endoplasmic reticulum.
Why the 70S Ribosome is Important for Medicine
The unique structural and functional characteristics of the 70S ribosome make it an effective target for antibiotic therapies. Its differences from the human 80S ribosome allow antibiotics to selectively inhibit bacterial protein synthesis without harming human cells. This selectivity is a cornerstone of effective antibacterial treatment, minimizing patient side effects. Many antibiotics exert their effects by binding to and interfering with various stages of the 70S ribosome’s function.
Aminoglycosides, such as streptomycin and gentamicin, bind to the 30S subunit, causing misreading of the mRNA code and premature termination. Tetracyclines also target the 30S subunit, preventing new tRNAs from attaching to the A-site, halting polypeptide chain elongation. Macrolides, such as erythromycin and azithromycin, bind to the 50S subunit, blocking the exit tunnel for the nascent protein, inhibiting protein synthesis.
Chloramphenicol also acts on the 50S subunit, inhibiting peptidyl transferase activity, which forms peptide bonds between amino acids. By disrupting these steps in bacterial protein production, these antibiotics stop bacterial growth and replication, allowing the host’s immune system to clear the infection. Continued study of the 70S ribosome remains an area of active research, providing opportunities for developing new antibacterial drugs to combat evolving bacterial resistance.
References
Structure of the Bacterial Ribosome. Available at: https://www.ncbi.nlm.nih.gov/books/NBK9898/.
Chapter 8. Protein Synthesis: The Ribosome and Translation. Available at: https://www.ncbi.nlm.nih.gov/books/NBK21118/.
The Prokaryotic and Eukaryotic Ribosomes. Available at: https://www.ncbi.nlm.nih.gov/books/NBK9905/.
Ribosome-targeting antibiotics. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6441467/.