Ribosomes function as the universal protein factories within living cells, manufacturing the complex chains of amino acids that fold into functional proteins. In bacteria, this essential machinery is known as the 70S ribosome, a large and intricate ribonucleoprotein complex responsible for translating genetic instructions carried by messenger RNA (mRNA) into new proteins. This process of protein synthesis is fundamental to prokaryotic life, allowing bacteria to grow, replicate, and adapt to their environment.
Architectural Layout: Subunits and Components
The complete bacterial ribosome is designated as a 70S particle, a measurement derived from its sedimentation rate (Svedberg unit, S). The 70S structure is composed of two unequal pieces that associate to become functional: the smaller 30S subunit and the larger 50S subunit. These subunits only join together when actively synthesizing a protein.
The small 30S subunit serves as the decoding center, built around a single strand of 16S ribosomal RNA (rRNA) and approximately 21 ribosomal proteins. The interaction of the 16S rRNA with the messenger RNA ensures the correct genetic code is read during protein construction.
The larger 50S subunit is the catalytic center where the peptide bonds are formed. It contains two distinct rRNA molecules, the 23S rRNA and the smaller 5S rRNA, alongside about 31 to 34 ribosomal proteins. Remarkably, the peptidyl transferase activity—the function that links amino acids—is carried out not by a protein, but by the 23S rRNA, classifying the ribosome as a ribozyme.
The Mechanism of Translation: Building Proteins
The ribosome’s primary task, protein synthesis, occurs in a cycle of three distinct phases: initiation, elongation, and termination. The process begins with initiation, where the components required for translation are first assembled on the mRNA template. In bacteria, the small 30S subunit binds to a specific sequence on the mRNA, known as the Shine-Dalgarno sequence, positioning the ribosome correctly at the start codon.
Three initiation factors (IF-1, IF-2, and IF-3) help recruit a special initiator transfer RNA (tRNA) carrying N-formylmethionine directly into the peptidyl (P) site of the 30S subunit. Once the initiator tRNA is secured, the large 50S subunit joins the complex, forming the complete 70S initiation complex, ready to begin protein synthesis.
The next phase, elongation, is a repetitive cycle where amino acids are added to the growing polypeptide chain. Each cycle begins with a new aminoacyl-tRNA, delivered by elongation factor EF-Tu to the aminoacyl (A) site. The ribosome confirms the tRNA’s anticodon matches the mRNA’s codon at the A site, ensuring accuracy.
Once the match is verified, the 23S rRNA in the 50S subunit catalyzes the formation of a peptide bond, transferring the growing chain from the P site tRNA to the amino acid on the A site tRNA. Next, the complex shifts in a process called translocation, driven by elongation factor EF-G. This movement shifts the tRNAs: the A-site tRNA (carrying the polypeptide) moves into the P site, and the P-site tRNA (now empty) moves into the exit (E) site. The empty tRNA in the E site is then ejected, leaving the A site open for the next aminoacyl-tRNA.
The final stage is termination, which occurs when the ribosome encounters one of three specific stop codons (UAA, UAG, or UGA) in the mRNA sequence. Since no tRNA recognizes these stop codons, protein release factors (RF1 or RF2) instead bind to the A site. These release factors trigger the peptidyl transferase center to add a water molecule to the polypeptide chain, which hydrolyzes the bond connecting the finished protein to the tRNA in the P site. This action releases the completed polypeptide into the cell. Finally, a third release factor (RF3) and the ribosome recycling factor (RRF) help dismantle the complex, separating the 70S ribosome back into its 30S and 50S subunits for reuse.
Key Differences from Human Ribosomes
The bacterial 70S ribosome possesses distinct structural differences when compared to the human, or eukaryotic, 80S ribosome. This variation reflects the evolutionary separation between prokaryotic and eukaryotic life forms. The primary difference lies in the overall sedimentation rate, where the bacterial ribosome is smaller at 70S, while the eukaryotic ribosome is larger at 80S.
The subunit composition also varies significantly between the two cell types. The bacterial 70S particle is made of a 30S small subunit and a 50S large subunit. Conversely, the human 80S ribosome is constructed from a 40S small subunit and a 60S large subunit. The Svedberg units are not additive because they measure sedimentation rate, which is influenced by shape and density, not just mass.
The ribosomal RNA components are also specific to each type of ribosome. The bacterial 30S subunit contains the 16S rRNA, while the 50S subunit contains 23S and 5S rRNA molecules. In contrast, the eukaryotic 40S subunit contains the larger 18S rRNA, and the 60S subunit features 28S, 5.8S, and 5S rRNA molecules.
Exploiting the Differences: Ribosome-Targeting Antibiotics
The structural differences between the bacterial 70S and human 80S ribosomes are exploited by medicine to create selective antibiotics. These drugs bind specifically to the unique features of the bacterial ribosome, interfering with protein synthesis without harming the host cell. This selective targeting is the basis for their effectiveness.
Targeting the 30S Subunit
Various classes of antibiotics target different steps and sites on the bacterial ribosome. Aminoglycosides, such as Streptomycin, bind to the 30S small subunit, causing misreading of the mRNA code and leading to the production of faulty proteins. Similarly, Tetracyclines also target the 30S subunit, but their mechanism involves blocking the A site, which prevents the attachment of incoming aminoacyl-tRNAs.
Targeting the 50S Subunit
Other antibiotic classes focus on the 50S large subunit, often interfering with the catalytic peptidyl transferase center or the exit tunnel. Macrolides like Azithromycin, along with Lincosamides and Chloramphenicol, bind to the 23S rRNA in the 50S subunit. This binding action physically obstructs the nascent peptide exit tunnel, thereby preventing the growing protein chain from leaving the ribosome and halting the elongation phase of translation.