Sechenov School
Home › Pharmacology › Bacterial Protein Synthesis: Antibiotic Targets

Bacterial Protein Synthesis: Antibiotic Targets

For medical students2 min readUpdated 2026-10-10

The bacterial ribosome is a classic target for antimicrobial therapy. Understanding the precise mechanisms of translation and protein assembly allows for a clear understanding of the binding sites of various antibiotic classes at the level of the small and large ribosomal subunits.

Prokaryotic RibosomeFormed by the assembly of 30S and 50S subunits into a functional 70S structure.
Starting Amino AcidBacterial protein chain synthesis is always initiated with formylmethionine (fMet).
Ribosomal TranslocationInformation is read from the messenger RNA strictly in the 5'- to 3'-direction.
30S BlockadeThe small subunit is susceptible to tetracyclines, aminoglycosides, and glycylcyclines.

Pharmacological Targets: 30S and 50S

The bacterial translation apparatus is fundamentally different from the human one, making it an excellent target for selective drug toxicity. Protein synthesis inhibitors are divided into two major groups based on the ribosomal subunit they bind to.

Agents targeting the 30S (small subunit):

Agents targeting the 50S (large subunit):

Initiation of Translation

The synthesis process begins with the assembly of the working complex. First, messenger RNA (mRNA) binds to the small 30S subunit.

A key role at this stage is played by the starting amino acid. In prokaryotic organisms, this function is performed by formylmethionine (fMet). Transfer RNA carrying fMet locates the start codon (AUG) on the mRNA and attaches to it.

After this, the large 50S subunit joins the complex, completing the formation of the functional 70S ribosome.

Within the assembled ribosome, the fMet-tRNA molecule is located in the P-site (peptidyl site). The neighboring A-site (aminoacyl site) remains completely unoccupied at this moment, ready to accept the next transfer RNA to continue synthesis.

Elongation: Peptide Chain Extension

During the elongation phase, the ribosome moves sequentially along the mRNA in the 5'- to 3'-direction, attaching new amino acids. This cycle consists of several strict steps:

  1. Binding. A new aminoacyl-tRNA enters the empty A-site. It carries a specific amino acid and binds to the mRNA via complementary base pairing.
  2. Peptidyltransferase Reaction. The enzyme peptidyltransferase links two amino acids. A peptide bond forms between the formylmethionine located in the P-site and the newly arrived amino acid in the A-site.
  3. Transpeptidation. As a result of this linking, the tRNA from the A-site takes on the fMet. This tRNA now holds a dipeptide (a chain of two amino acids).
  4. Translocation. The ribosome steps forward by exactly three nucleotides toward the 3'-end. The enzyme translocase assists in this process by shifting the mRNA.

During this shift, tRNA rearrangement occurs: the molecule that relinquished fMet and became "empty" leaves the P-site. The tRNA carrying the dipeptide, previously located in the A-site, moves into the vacated P-site.

Cycle outcome: The A-site is free again. The system is ready to accept the next aminoacyl-tRNA, and the entire elongation cycle repeats.

Mnemonic

Antibiotic classes inhibiting the 30S subunit can be easily remembered by the rule "Two A's, Two T/G's": Aminoglycosides, Aminocyclitols, Tetracyclines, Glycylcyclines. The rest of the list targets the 50S subunit.

Frequently asked questions

What is the exact mechanism of action of tetracyclines on the 30S subunit?

Tetracyclines reversibly block the binding of incoming aminoacyl-tRNA to the A-site (acceptor site) on the 30S subunit of the bacterial ribosome. As a result, the transfer RNA carrying a specific amino acid cannot attach to the ribosome. This halts the elongation of the peptide chain by blocking the initiation phase of elongation. Inhibition is reversible, and protein synthesis resumes after the antibiotic is removed.

What is the mechanism of action of macrolides on the 50S subunit?

Macrolides bind to a specific ligand within the ribosomal exit tunnel of the 50S subunit, inhibiting translocation. The ribosome loses the ability to move along the mRNA by a single triplet (codon). Consequently, this blocks the movement of the formed peptide from the A-site to the P-site and disrupts the exit of synthesized peptides from the ribosome. This prevents the A-site from freeing up for the next tRNA and blocks the release of the "empty" tRNA from the P-site, halting further protein synthesis.

Which protein factors participate in the elongation stage of translation in bacteria?

Specific elongation protein factors participate in the elongation stage of translation in prokaryotes (bacteria). They facilitate the addition of amino acids to the growing chain as the ribosome moves from the 5'- to the 3'-end of the mRNA. These include:

  • EF-Tu — elongation factor.
  • EF-Ts — elongation factor.
  • EF-G — translocation factor.
Which antibiotics specifically inhibit the peptidyltransferase reaction?

The peptidyltransferase reaction is specifically inhibited by chloramphenicol. The drug binds to the catalytic center of the 50S ribosomal subunit and blocks the enzyme peptidyltransferase. Consequently, the transpeptidation reaction is inhibited: peptide bond formation is blocked, making it impossible to transfer the growing peptide chain from the tRNA in the P-site to the amino acid in the A-site.

Which subunit binds to mRNA at the very beginning of initiation?

Initially, messenger RNA binds to the small (30S) ribosomal subunit, and only after the attachment of fMet-tRNA is the complex closed by the large 50S subunit.

In what direction does the ribosome read mRNA?

Ribosomal movement (and the action of the translocase enzyme) always proceeds from the 5'-end to the 3'-end of the messenger RNA.

Where is the transfer RNA with the starting amino acid located after 70S ribosome assembly?

The fMet-tRNA molecule is located in the peptidyl P-site, leaving the neighboring aminoacyl A-site free for the next stage.

Go deeper

More topics in Pharmacology

Lipid-Lowering AgentsClassification of DiureticsRespiratory AnalepticsCoronary Artery Disease: Coronary Circulatory InsufficiencyPhysiological Mechanisms of Blood Pressure RegulationPhysiology of Myocardial ContractionAntiherpetic Drugs: Pharmacology and Mechanism of ActionAntimalarial DrugsVitaminsGeneral AnestheticsAnalepticsCardiotonic Agents: Pharmacology and Mechanism of ActionPharmacology →