Ribosomes are intricate cellular machines found in all living organisms, playing a fundamental role in sustaining life. These complex structures are responsible for protein synthesis, also known as translation, where genetic information encoded in messenger RNA (mRNA) is converted into functional proteins. Every cell relies on ribosomes to manufacture the proteins necessary for its structure, function, and reproduction. Without these molecular factories, cells would be unable to produce the enzymes, structural components, and signaling molecules required for their survival and activity.
The Building Blocks of Bacterial Ribosomes
Bacterial ribosomes, known as 70S ribosomes, are composed of two main subunits: a smaller 30S subunit and a larger 50S subunit. The “S” refers to Svedberg units, a measure of sedimentation rate during centrifugation, indicating size and density. The 30S subunit contains one ribosomal RNA (rRNA) molecule, 16S rRNA, and approximately 21 ribosomal proteins.
The larger 50S subunit consists of two rRNA molecules, 5S rRNA and 23S rRNA, and around 31 ribosomal proteins. Ribosomal RNA makes up about two-thirds of the ribosome’s mass, with proteins accounting for the remaining one-third. While rRNA provides the basic framework, ribosomal proteins fill structural gaps and enhance protein synthesis. This 70S structure is distinct from eukaryotic 80S ribosomes, a key characteristic that sets them apart.
Assembling Proteins: How Ribosomes Work
Protein synthesis in bacteria, or translation, is a coordinated process involving the interaction of the 30S and 50S ribosomal subunits. These subunits exist separately in the bacterial cytoplasm but come together to form a functional 70S ribosome when they encounter a messenger RNA (mRNA) molecule. The mRNA carries genetic instructions copied from DNA, which the ribosome “reads” to assemble the correct sequence of amino acids.
Translation unfolds in three main phases: initiation, elongation, and termination. During initiation, the 30S subunit binds to the mRNA at a start codon, usually AUG, and an initiator transfer RNA (tRNA) carrying the first amino acid (formylmethionine in bacteria) binds to this complex. The 50S subunit then joins, forming the complete 70S ribosome and creating an initiation complex.
Elongation involves the sequential addition of amino acids to the growing protein chain. The ribosome moves along the mRNA, reading codons in groups of three nucleotides. Each codon specifies an amino acid, delivered by a corresponding transfer RNA (tRNA) molecule. The 50S subunit catalyzes the formation of peptide bonds between incoming amino acids, linking them into a polypeptide chain. This process continues until the ribosome encounters a stop codon on the mRNA, signaling the end of protein synthesis. Termination then occurs as release factors bind, causing the polypeptide to be released and the 70S ribosome to dissociate back into its 30S and 50S subunits, ready for a new round of synthesis.
Why Bacterial Ribosomes Matter for Medicine
Bacterial ribosomes are important in medicine because their unique structural features make them targets for antibiotic drugs. The principle of “selective toxicity” is central to antibiotic action, meaning these drugs can specifically inhibit bacterial processes without significantly harming human cells. This selectivity arises from the distinct differences between bacterial 70S ribosomes and human 80S ribosomes, particularly in their subunit composition and rRNA sequences.
Antibiotics targeting bacterial ribosomes interfere with various stages of protein synthesis, preventing bacteria from producing proteins needed to survive and multiply. For instance, tetracyclines bind to the 30S ribosomal subunit, blocking transfer RNA (tRNA) attachment and halting protein elongation. Aminoglycosides also bind to the 30S subunit, causing misreading of the mRNA and leading to faulty, non-functional proteins.
Macrolides and lincosamides target the 50S ribosomal subunit. These drugs inhibit ribosome translocation along the mRNA, stopping protein chain elongation. Chloramphenicol, another 50S subunit-targeting antibiotic, inhibits peptidyl transferase activity, the enzyme responsible for forming peptide bonds between amino acids. Understanding the structure of bacterial ribosomes and how these antibiotics interact with them is valuable in combating antibiotic resistance, aiding in the development of new drugs or modifications to overcome bacterial resistance.