Ribosomes, bacterial

Ribosomes Bacterial cells contain about 15,000 ribosomes. Each is composed of a small (SOS) subunit and a large (50S) subunit. The mass of a single ribosome is 2.3 X 10 daltons. It consists of 65% RNA and 35% protein. Ribosomes are the sites of protein synthesis. The mRNA binds to ribosomes, and the mRNA nucleotide sequence specifies the protein that is synthesized. 

Ribosomes Bacterial ribosomes are smaller than eukaryotic ribosomes, but serve the same function— protein synthesis from an RNA message. 

Macrolides inhibit growth of bacteria by inhibiting protein synthesis on ribosomes. Bacterial resistance to macrolides is often accompanied by cross-resistance to lincosamide and sireptogramin B antibiotics (MLS-resistance), which can be either inducible or constitutive. 14-Membered... 

A third difference between bacterial and human cells involves their ribosomes. Bacterial ribosomes are neither the same size nor have the same composition as human ribosomes. Thus drugs that bind more to bacterial than to human ribosomes can inhibit bacterial protein synthesis and have a selective toxicity for these cells. 

Ribosome Bacterial 30S subunit Tetracyclines (protein synthesis inhibition)... 

One of the most fascinating recent developments in biology has been the discovery of numerous highly complex biopolymer assemblies (see also section C2.14.2.3) such as the ribosome or the bacterial flagellum [93, 94 and 95], the envy of nanoteclmologists seeking to miniaturize man-made mechanical devices (note that the word machinery is also sometimes used to refer to multienzyme complexes such as the proteasome [96]), and an entire... 

Mechanism of Action. THie earliest studies on the mechanism of action of lincomycin showed that lincomycin had the immediate effect on Staphjlococcus aureus of complete inhibition of protein synthesis (23). TThis inhibition results from the blocking of the peptidyltransferase site of the SOS subunit of the bacterial ribosome (24). Litde effect on DNA and RNA synthesis was observed. 

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). 

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. 

Oxazolidinones are a new class of synthetic antimicrobial agents, which have activity against many important pathogens, including methicillin-resistant Staphylococcus aureus and others. Oxazolidinones (e.g. linezolid or eperezolid) inhibit bacterial protein synthesis by inhibiting the formation of the 70S initiation complex by binding to the 50S ribosomal subunit close to the interface with the 3OS subunit. 

Good L., Nielsen P.E. Inhibition of translation and bacterial growth by peptide nucleic acid targeted to ribosomal RNA. Proc. Natl Acad. Sci. USA 1998 95 2073-2076. 

Ribosomes in bacteria and in the mitochondria of higher eukaryotic cells differ from the mammalian ribosome described in Ghapter 35. The bacterial ribosome is smaller (70S rather than SOS) and has a different, somewhat simpler complement of RNA and protein... 

The eytoplasm is a viscous fluid and contains within it systems of paramount importance. These are the nucleus, responsible for the genehc make-up of the cell, and the ribosomes, whieh are the site of protein synthesis, hi addihon are found granules of reserve material suehas polylydioxybutyric add, an energy reserve, and polyphosphate or volutin granules, the exact funchon of which has not yet been elucidated. The prokaiyohc nueleus or bacterial chromosome exists in the cytoplasm in the form of a loop and is not surrounded by a nuclear membrane. Bacteria cany other chromosomal elements episomes, which are portions of the main chromosome that have become isolated firm it, and plasmids, whieh may be called miniature chromosomes. These are small annular pieees of DNA whieh carry a limited amount of genetic information. 

Chlamydias, however, are bacteria and have been shown to possess a cell wall containing muramic acid (section 2.2.1), to contain ribosomes of the bacterial (prokaryotic) type, to reproduce themselves by binary fission and to be inhibited by antibiotics active against bacteria. 

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... 

In the absence of an AGAC the ribosomes will prodnce the artificial polypeptides, polyphenylalanine (as specified by the codon UUU) or polyproline (as specified by the codon CCC). However, when streptomycin is added, the ribosomes prodnce a mixture of polythreonine (codon ACU) and poly serine (codon UCU). The misreading of the codons does not appear to be random U is read as A or C and C is read as A or U. If such misreading occurs in whole cells the accumulation of non-functional or toxic proteins would eventually prove fatal to the cells. There is some evidence that the bacterial cell membrane is damaged when the cells attempt to excrete the faulty proteins. 

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