Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 177

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 antibiotics occurs in a number of ways. One way is methylation of a specific adenine base in the 23S rRNA, which prevents binding of the antibiotic, by enzymes called the Erm methylases. Some mutational changes in the 23S rRNA can also confer resistance to these antibiotics. Resistance can also occur by acquisition of genes that encode efflux pumps that remove the antibiotic from the cell. New derivatives of the antibiotics must be made constantly to stay ahead of the advancing bacterial resistance.

Thiostrepton

Thiostrepton and other thiopeptide antibiotics block translation by binding to 23S rRNA in the region of the ribosome involved in the peptidyltransferase reaction and preventing the binding of EF-G. Thiostrepton is specific to Firmicutes because it does not cross the outer membrane of Gram-negative bacterial cells.

Most thiostrepton-resistant mutants are missing the L11 ribosomal protein from the 50S ribosomal subunit. This protein seems not to be required for protein synthesis but plays a role in guanosine tetraphosphate synthesis (see chapter 12). Other mutations confer resistance by changing nucleotides in the 23S rRNA close to where the antibiotic binds. Genes de rived from plasmids and transposons can confer resistance by directing specific methylation of 23S rRNA. Eukaryotes may be insensitive to the antibiotic because the analogous ribose sugars of the eukaryotic 28S rRNAs are normally extensively methylated.