the cell and come together to form the complete 70S ribosome only when they are translating an mRNA. Note that sedimentation values are not additive; the complete ribosome is 70S in size, despite being composed of subunits that are 30S and 50S in size. This is because the two subunits fit together into a tight complex.
The two ribosomal subunits play very different roles in translation. To initiate translation, the 30S subunit first binds to the mRNA. Then, the 50S ribosome binds to the 30S subunit to make the 70S ribosome. From that point on, the 30S subunit mostly helps to select the correct aminoacyl-tRNA (aa-tRNA) for each codon while the 50S subunit does most of the work of forming the peptide bonds and translocating the tRNAs from one site on the ribosome to another (see below). The 70S ribosome moves along the mRNA, allowing tRNA anticodons to pair with the mRNA codons and translate the information from the nucleic acid chain into a polypeptide. After the polypeptide chain is completed, the ribosome separates again into the 30S and 50S subunits. The role of the subunits is discussed in more detail below.
Antibiotic Inhibitors of Translation
The translation apparatus of bacteria is a particularly tempting target for antibacterial drugs because it is somewhat different from the eukaryotic translation apparatus and is highly conserved among bacteria. Antibiotics that inhibit translation are among the most useful of all the antibiotics and are listed in the table, which also lists their targets and sources. Toxicity in some cases may result from the similarity of bacterial and mitochondrial ribosomes. Some of these antibiotics are also very useful in combating fungal diseases and in cancer chemotherapy.
Antibiotics that block translation
| Antibiotic | Source | Target |
| Puromycin | Streptomyces alboniger | Ribosomal A site |
| Kanamycin | Streptomyces kanamyceticus | 16S rRNA |
| Neomycin | Streptomyces fradiae | 16S rRNA |
| Streptomycin | Streptomyces griseus | 30S ribosome |
| Thiostrepton | Streptomyces azureus | 23S rRNA |
| Gentamicin | Micromonospora purpurea | 16S rRNA |
| Tetracycline | Streptomyces rimosus | Ribosomal A site |
| Chloramphenicol | Streptomyces venezuelae | Peptidyltransferase |
| Erythromycin | Saccharopolyspora erythraea | 23S rRNA |
| Fusidic acid | Fusidium coccineum | EF-G |
| Kirromycin | Streptomyces collinus | EF-Tu |
Inhibitors that Mimic tRNA
Puromycin mimics the 3′ end of tRNA with an amino acid attached (aa-tRNA). It enters the ribosome as does an aa-tRNA, and the peptidyltransferase attaches it to the growing polypeptide. However, it does not translocate properly from the A site to the P site, and the peptide with puromycin attached to its carboxyl terminus is released from the ribosome, terminating translation.
Studies with puromycin have contributed greatly to our understanding of translation. The model of the A and P sites in the ribosome and the concept that the 50S ribosome contains the enzyme for peptidyl bond formation came from studies with the antibiotic. Puromycin is not a very useful antibiotic for treating bacterial diseases, however, because it also inhibits translation in eukaryotes, making it toxic in humans and animals. It is, however, one of the few antibiotics that is useful in archaeal genetics, with the availability of resistance cassettes.
Inhibitors that Bind to the 23S rRNA
Chloramphenicol
Chloramphenicol inhibits translation by binding to ribosomes and preventing the binding of aa-tRNA to the A site. It might also inhibit the peptidyltransferase reaction, preventing the formation of peptide bonds. Structural studies have shown that chloramphenicol binds to specific nucleotides in the 23S rRNA, although ribosomal proteins are also part of the binding site.
Chloramphenicol is effective at low concentrations and therefore has been one of the most useful antibiotics for studying cellular functions. For example, it has been used to determine the time in the cell cycle when proteins required for cell division and for initiation of chromosomal replication are synthesized. It is also quite useful in treating bacterial diseases, since it is not very toxic for humans and animals because it is fairly specific for the translation apparatus of bacteria. It can cross the blood-brain barrier, making it useful for treating diseases of the central nervous system, such as bacterial meningitis. Chloramphenicol is bacteriostatic, which means that it stops the growth of bacteria without actually killing them. Such antibiotics should not be used in combination with antibiotics such as penicillin that depend on cell growth for their killing activity, since they neutralize the effect of these other antibiotics.
It takes multiple mutations in ribosomal proteins to make bacteria resistant to chloramphenicol, so resistant mutants are very rare. Some bacteria have enzymes that inactivate chloramphenicol. The genes for these enzymes are often carried on plasmids and transposons, interchangeable DNA elements that are discussed in chapters 4 and 8. The best-characterized chloramphenicol resistance gene is the cat gene of transposon Tn9, whose product is an enzyme that specifically acetylates (adds an acetyl group to) chloramphenicol, thereby inactivating it. The cat gene has been used extensively as a reporter gene to study gene expression in both bacteria and eukaryotes and has been introduced into many plasmid cloning vectors.
Macrolides
Erythromycin is a member of a large group of antibiotics called the macrolide antibiotics, which have large ring structures. These antibiotics may also inhibit translation by binding to the 23S rRNA and blocking the exit channel of the growing polypeptide. This causes the polypeptide to be released prematurely at either the peptidyltransferase reaction or the translocation step, causing the peptidyl-tRNA to dissociate from the ribosome.
Erythromycin and other macrolide antibiotics have been among the most useful antibiotics. They are effective mostly against Firmicutes but are also useful in treating some other bacterial diseases, including infections by Legionella and Rickettsia. Resistance to macrolide