Module 4 · Pharmacology
Antibiotics Targeting the Ribosome
More than half of all antibiotics in clinical use target the bacterial ribosome. The selectivity is achieved through the substantial structural differences between bacterial and eukaryotic ribosomes — exploited most systematically by aminoglycosides, macrolides, tetracyclines, and oxazolidinones. This module surveys the main drug classes, their binding sites, their mechanism of action, and the principal resistance mechanisms.
1. Aminoglycosides (30S, Decoding Centre)
Streptomycin (Waksman 1943, first tuberculosis drug), kanamycin, gentamicin, tobramycin, amikacin, neomycin. All bind the 16S rRNA decoding centre near A1492/A1493. Effect: induce the adenine flip-out that “accepts” a tRNA — even non-cognate ones — resulting in high miscoding rates and production of aberrant proteins, which kill bacteria through accumulated damage.
Resistance: (i) aminoglycoside-modifying enzymes (AACs, APHs, ANTs) acetylate, phosphorylate, or adenylate the drug; plasmid-encoded, widespread; (ii) efflux pumps (AcrB in Gram-negatives, MFS family); (iii) target modification by 16S rRNA methyltransferases (ArmA, RmtB in K. pneumoniae); (iv) decreased uptake. Clinical concerns: nephrotoxicity and irreversible cochlear ototoxicity — the latter due to mitochondrial 12S rRNA similarity to bacterial 16S.
2. Macrolides (50S, Exit Tunnel)
Erythromycin, clarithromycin, azithromycin. All bind the entrance of the peptide-exit tunnel in the 50S, near 23S nucleotide A2058. They do not stop peptide-bond formation outright but block tunnel transit: the nascent peptide can extend only a few residues before hitting the drug, causing ribosome stalling and drop-off. Macrolides are bacteriostatic at clinical concentrations.
Context-specific stalling: macrolides do not block all translation equally — certain amino-acid motifs in the nascent chain engage the tunnel in ways that promote or allow tunnel transit. This “specific” inhibition contradicts the old “plug-the-tunnel” model (Kannan 2012) and explains why macrolide-insensitive transcripts exist.
Resistance: (i) A2058 methylation (Erm methylases) — the “MLSB resistance” phenotype conferring resistance to macrolides, lincosamides (clindamycin), and streptogramin B; (ii) efflux; (iii) mutations at A2058, A2059 in species with few 23S copies. Telithromycin, a ketolide, overcomes some MLSB resistance by extra contact with A752 of 23S.
3. Chloramphenicol & Oxazolidinones (50S, PTC)
Chloramphenicol binds the PTC A-site, blocking amino-acyl tRNA accommodation. Bacteriostatic, broad-spectrum, used in resource-limited settings and for rickettsial and specific anaerobic infections. Toxicity (aplastic anaemia) limits use in developed countries.
Linezolid (oxazolidinone, 2000) binds the PTC A-site in a partially overlapping manner with chloramphenicol. Critical agent against Gram-positives including MRSA, VRE. Resistance is rare but emerging via 23S rRNA mutations and plasmid-encoded cfr methyltransferase. Tedizolid (2014) is a second-generation oxazolidinone with improved pharmacokinetics.
4. Tetracyclines (30S A Site)
Tetracycline, doxycycline, minocycline bind the A-site of the 30S, blocking aa-tRNA accommodation. Bacteriostatic, broad-spectrum. Tigecycline (2005, a glycylcycline) overcomes many classical tetracycline-resistance mechanisms and is used for MDR infections. Resistance: (i) TetA/TetB efflux pumps; (ii) ribosome protection proteins (TetM, TetO) that physically displace the drug; (iii) tetracycline-inactivating monooxygenases (Tet(X)).
5. Other Ribosome-Targeting Classes
- Pleuromutilins (lefamulin, retapamulin, tiamulin): bind 50S PTC at a distinct site. Lefamulin (2019) for community-acquired pneumonia.
- Streptogramins (quinupristin-dalfopristin, Synercid): synergistic pair binding 50S near the macrolide site + the peptide-exit-tunnel region.
- Fusidic acid: binds EF-G on the ribosome, blocking translocation. Used for staphylococcal skin/bone infection.
- Mupirocin: inhibits isoleucyl-tRNA synthetase, blocking tRNA charging. Topical (nasal) decolonisation of MRSA.
- Puromycin: an aa-tRNA analogue that enters the A site and forms a peptide bond with the nascent chain, releasing a puromycylated truncated peptide. Research tool (labels active-translation sites, e.g., SUnSET assay); not used clinically.
6. Why the Ribosome Remains a Successful Drug Target
Despite decades of use and extensive resistance, the ribosome continues to deliver new clinical agents. Three reasons: (i) the ribosome’s functional core is catalytic RNA rather than protein, so drugs can exploit RNA-specific binding pockets that are hard to modify by point mutation (RNA has fewer degrees of freedom than protein); (ii) bacterial ribosomes differ structurally enough from eukaryotic to permit selectivity; (iii) cryo-EM and X-ray crystallography have made structure-guided design efficient. Several oxazolidinone, pleuromutilin, and aminoglycoside analogues are in clinical development for drug-resistant Gram-negative and Mycobacterium tuberculosis infections.