Ribosomes, tetracyclines

It has been known for some time that tetracyclines are accumulated by bacteria and prevent bacterial protein synthesis (Fig. 4). Furthermore, inhibition of protein synthesis is responsible for the bacteriostatic effect (85). Inhibition of protein synthesis results primarily from dismption of codon-anticodon interaction between tRNA and mRNA so that binding of aminoacyl-tRNA to the ribosomal acceptor (A) site is prevented (85). The precise mechanism is not understood. However, inhibition is likely to result from interaction of the tetracyclines with the 30S ribosomal subunit because these antibiotics are known to bind strongly to a single site on the 30S subunit (85). 

Fig. 4. Comparison of the three types of tetracycline resistance where T represents the tetracycline molecule O, a tetracycline transporter and aaa/, the ribosome A shows the effect of tetracycline exposure on a sensitive cell B, the efflux of resistance where a cytoplasmic membrane protein ( D) pumps tetracycline out of the cell as fast as the tetracycline transporter takes it up C, the ribosomal protection type of resistance where the ribosome is modified by ( ) to block productive binding and D, the tetracycline modification type of resistance where t is an inactive form of tetracycline. Reproduced with...

Tetracycline and its derivative doxycycline are antibiotics widely used in the treatment of bacterial infections. They also exert an antimalarial activity. Tetracyclines inhibit the binding of aminoacyl-tRNA to the ribosome during protein synthesis. 

Resistance to tetracyclines is often caused by the acquisition of genes (e.g. tetO and tetM) coding for so-called ribosome protection proteins. These proteins bind to the ribosome and protect them from tetracycline action. 

Ribosomal Protein Synthesis Inhibitors. Figure 3 The chemical structure of tetracycline and possible interactions with 16S rRNA in the primary binding site. Arrows with numbers indicate distances (in A) between functional groups. There are no interactions obseived between the upper portion of the molecule and 16S rRNA consistent with data that these positions can be modified without affecting inhibitory action (from Brodersen et al. [4] with copynght permission). 

Brodersen DE, Clemons WM, Carter AP et al (2000) The structural basis for the action of the antibiotics tetracycline, pactamycin and hygromycin B on the 30S ribosomal subunit. Cell 103 1143-1154... 

Bacterial ribosome function Aminoglycosides Tetracyclines Chloramphenicol Macrolides, azalides Fusidic acid Mupirocin Distort SOS ribosomal subunit Block SOS ribosomal subunit Inhibits peptidyl transferase Block translocation Inhibits elongation factor Inhibits isoleucyl-tRNA synthesis No action on 40S subunit Excluded by mammalian cells No action on mammalian equivalent No action on mammalian equivalent Excluded by mammalian cells No action on mammalian equivalent... 

Plasmid- or transposon-encoded ribosomal protection factors are a second mechanism of resistance to the tetracyclines. These proteins are believed to alter the tetracycline binding site on the 308 ribosomal subunit, lowering the affinity for the drugs. 

Ribosome protection (tetM gene) inactivating tetracyclines... 

P = constitutive promoter, tetR = TETrepressor gene,TETR = TET repressor protein, tetO = TET operator sequence, = tetracycline, ere/CRE = Cre recombinase gene/protein, PLEA = late embryogenesis abundant promoter, RIP = gene for ribosome inactivating protein, shaded blocks are loxP sites in orientation shown by solid triangle. After [48]... 

Tetracyclines are a group of antibiotics derived from bacteria. Chlortet-racycline was isolated from Streptomyces aureofaciens and oxytetracycline from Streptomyces rimosus. Tetracychnes act by binding to receptors on the bacterial ribosome and inhibit bacterial protein synthesis. 

Newer and more generally usefnl macrolide antibiotics include azithromycin (Zithromax) and clarithromycin (Biaxin). These too are wide-spectrum antibiotics and both are semisynthetic derivatives of erythromycin. Like the tetracyclines, the macrolide antibiotics act as protein synthesis inhibitors and also do so by binding specifically to the bacterial ribosome, thongh at a site distinct from that of the tetracyclines. 

Genes encoding efflux pumps confer resistance to tetracyclines Genes encoding proteins protecting the ribosome from the inhibiting effects of tetracycline... 

Pioletti, M. Schlunzen, F. Harms, J. Zarivach, R. Gluhmann, M. Avila, H. Bashan, A. Bartels, H. Auerbach, T. Jacobi, C. Hartsch, T. Yonath, A. Franceschi, F. Crystal structures of complexes of the small ribosomal subunit with tetracycline, edeine and IF3. EMBO J. 2001, 20, 1829-1839. 

As the name implies, this class of compounds has four linearly attached six membered rings. Tetracyclines are bacteriostatic and reversibly bind to the 30S ribosomal subunit. They interfere with the binding of aminoacyl tRNA at the A-site of the ribosome. ... 

In many ways, mitochondria resemble bacteria for example, the mitochondrial ribosomal RNA genes of all eukaryotes have been traced back to the eubacteria [10]. This can explain why some antibacterial compounds with the target of inhibiting bacterial protein synthesis also inhibit mitochondrial protein synthesis [6, 11, 12], resulting in hematotoxicity. Tetracycline, chloramphemcol and some oxazolidinone antibiotics have been shown to induce hematotoxicity by inhibiting mitochondrial protein synthesis [13]. 

Tetracycline and its derivatives Inhibit entry of the aminoacyl-tRNAs into the A site of both eukaryotic and prokaryotic ribosomes, but eukaryotic plasma membranes are impermeable to these drugs. 

Although all tetracyclines have a similar mechanism of action, they have different chemical structures and are produced by different species of Streptomyces. In addition, structural analogues of these compounds have been synthesized to improve pharmacokinetic properties and antimicrobial activity. While several biological processes in the bacterial cells are modified by the tetracyclines, their primary mode of action is inhibition of protein synthesis. Tetracyclines bind to the SOS ribosome and thereby prevent the binding of aminoacyl transfer RNA (tRNA) to the A site (acceptor site) on the 50S ri-bosomal unit. The tetracyclines affect both eukaryotic and prokaryotic cells but are selectively toxic for bacteria, because they readily penetrate microbial membranes and accumulate in the cytoplasm through an energy-dependent tetracycline transport system that is absent from mammalian cells. 

Resistance is related largely to changes in cell permeability and a resultant decreased accumulation of drug due to increased efflux from the cell by an energy-dependent mechanism. Other mechanisms, such as production of a protein that alters the interaction of tetracycline with the ribosome and enzymatic inactivation of the drug, have been reported. 

Mechanism of Action A tetracycline antibacterial that inhibits bacterial protein synthesis by binding to ribosomal receptor sites also inhibits ADH-induced water reabsorption. Therapeutic Effect Bacteriostatic also produces water diuresis. Pharmacokinetics Food and dairy products interfere with absorption. Protein binding 41 %-91%. Metabolized in liver. Excreted in urine. Removed by hemodialysis. Half-life 10-15 hr. 

Mechanism of Action A tetracycline antibacterial that inhibits bacterial protein synthesis by binding to ribosomes. Therapeutic Effect Bacteriostatic. 

Chloramphenicol (bacteriostatic interrupts protein synthesis at the ribosome) Macrolides (bacteriostatic interrupt protein synthesis at the SOS ribosome subunit) e.g., erythromycin, azithromycin, clarithromycin Lincomycins (bacteriostatic interrupt protein synthesis at the SOS subunit) Aminoglycosides (bactericidal interrupt protein synthesis at the 30S subunit) e.g., gentamicin, amikacin, kanamycin, neomycin, tobramycin Tetracyclines (bacteriostatic interrupt protein synthesis at the 30S subunit) e.g., tetracycline, doxycycline, minocycline... 

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