Protein synthesis in bacteria

MANY ANTIBIOTICS WORK BECAUSE THEY SELECTIVELY INHIBIT PROTEIN SYNTHESIS IN BACTERIA... 

The answer is b. (Hardman, p 1131.) Chloramphenicol inhibits protein synthesis in bacteria and, to a lesser extent, in eukaryotic cells. The drug binds reversibly to the. 505 ribosomal subunit and prevents attachment of aminoacybtransfer RNA (tRNA) to its binding site. The amino acid substrate is unavailable for peptidyl transferase and peptide bond formation. 

FIGURE 27-26 Termination of protein synthesis in bacteria. Termination occurs in response to a termination codon in the A site. First, a release factor, RF (RF-1 or RF-2, depending on which termination codon is present), binds to the A site. This leads to hydrolysis of the ester linkage between the nascent polypeptide and the tRNA in the P site and release of the completed polypeptide. Finally, the mRNA, de-acylated tRNA, and release factor leave the ribosome, and the ribosome dissociates into its 30S and 50S subunits. 

Protein synthesis is a central function in cellular physiology and is the primary target of many naturally occurring antibiotics and toxins. Except as noted, these antibiotics inhibit protein synthesis in bacteria. The differences between bacterial and eukaryotic protein synthesis, though in some cases subtle, are sufficient that most of the compounds discussed below are relatively harmless to eukaryotic cells. Natural selection has favored the evolution of compounds that exploit minor differences in order to affect bacterial systems selectively, such that these biochemical weapons are synthesized by some microorganisms and are extremely toxic to others. Because nearly every step in protein synthesis can be specifically inhibited by one antibiotic or another, antibiotics have become valuable tools in the study of protein biosynthesis. 

Tetracyclines inhibit protein synthesis in bacteria by blocking the A site on the ribosome, preventing the binding of aminoacyl-tRNAs. Chloramphenicol inhibits protein synthesis by bacterial (and mitochondrial... 

A special initiator tRNA, tRNAme i (I stands for initiator) is used for beginning protein synthesis. In bacteria, this initiator tRNA carries the modified amino acid N-formylmethionine (fmet). The formylation reaction transfers the formyl group from formyl-tetrahydrofolate to... 

Laursen BS, Sorensen HP, Mortensen KK, SperUng-Petersen HU. Initiation of protein synthesis in bacteria. Microbiol. Mol. Biol. Rev. 2005 69(1) 101-123. 

The methionine residue found at the amino-terminal end of E. coli proteins is usually modified. In fact, protein synthesis in bacteria starts with H-formylmethionine (fMet). A special tRNA brings formylmethionine to the ribosome to initiate protein synthesis. This initiator tRNA (abbreviated as tRNAf) differs from the one that inserts methionine in internal positions (abbreviated as tRNA ). The subscript "f indicates that methionine attached to the initiator tRNA can be formylated, whereas it cannot be formyl-ated when attached to tRNA. In approximately one-half of E. coli proteins, N-formylmethionine is removed when the nascent chain is 10 amino acids long. 

Initiation. Protein synthesis in bacteria begins by the association of one 308 subunit (not the 708 ribosome), an mRNA, a charged tRNA , three protein initiation factors, and guanosine 5 -triphosphate (GTP). These molecules make up the 308 preinitiation complex. Association occurs at an initiator AUG codon, whose selection was described above. A 508 subunit joins to the 308 subunit to form a 708 initiation complex (Figure 25-11). This joining process requires hydrolysis of the GTP contained in the 308 preinitiation complex. There are two tRNA... 

The answer is c. (Murray, pp 452—467. Scriver, pp 3—45. Sack, pp 1—40. Wilson, pp 101-120.) Prokaryotic ribosomes have a sedimentation coefficient of 70S and are composed of SOS and 30S subunits. Eukaryotic cytoplasmic ribosomes, either free or bound to the endoplasmic reticulum, are larger—60S and 40S subunits that associate to an SOS ribosome. Nuclear ribosomes are attached to the endoplasmic reticulum of the nuclear membrane. Ribosomes in chloroplasts and mitochondria of eukaryotic cells are more similar to prokaryotic ribosomes than to eukaryotic cytosolic ribosomes. Like bacterial ribosomes, chloroplast and mitochondrial ribosomes use a formylated tRNA. In addition, they are sensitive to many of the inhibitors of protein synthesis in bacteria. 

Many antibiotics inhibit protein synthesis in bacteria. They do so by binding to ribosomes, which are different to those in human cells. Ribosomes are essential for protein synthesis because they read the code on messenger RNA. This ensures that amino acids are assembled in the correct order to make a protein. Drugs that bind to ribosomes inhibit cell growth and are therefore bacteriostatic. 

MECHANISM OF ACTION Chloramphenicol inhibits protein synthesis in bacteria, and to a lesser extent, in eukaryotic cells. It binds reversibly to the SOS ribosomal subunit (near the binding site for the macrolide antibiotics and clindamycin). The drug prevents the binding of the amino acid-containing end of the aminoacyl tRNA to the acceptor site on the SOS ribosomal subunit. The interaction between peptidyltransferase and its amino acid substrate is blocked, inhibiting peptide bond formation (Figure 46-2). 

Recall What are two major similarities between protein synthesis in bacteria compared to eukaryotes What are two major differences ... 

Similarities between protein synthesis in bacteria and protein synthesis in eukaryotes same start and stop codons same genetic code same chemical mechanisms of synthesis interchangeable tRNAs. Major differences in prokaryotes, the Shine-Dalgarno sequence and no introns in eukaryotes, the 5 -cap and 3 -tail on mRNA and introns have been spliced out. 

Table 21-3. Antibiotic Inhibition of Protein Synthesis in Bacteria...

Several antibiotics that decrease protein synthesis in bacteria also decrease protein synthesis in mitochondria. 

Thiopeptide antibiotics, thiazolyl peptides, naturally occurring sulfur-containing, highly modified, macrocyclic peptides. They share a number of structural motifs, including several heterocycles such as thiazoles, a dehydropiperidine, a pyridine, oxazoles, and indoles. Nearly aU of the thiopeptide antibiotics act as inhibitors of protein synthesis in bacteria. They are secondary metabolites produced by actino-mycetes, largely by the genus Streptomyces. A representative member of this family is thiostrepton [M. C. Bagleyetal., Chem. Rev. 2005, 105, 685]. 

EXAMPLE 9.33 Do different inhibitors act on analogous targets in protein synthesis in bacteria and eukaryotes ... 

A number of workers have shown that novobiocin inhibits protein synthesis in bacteria. For example, it inhibits /S-galactosidase synthesis in both Staph, aureus and E. coli [48, 64]. Although M-protein synthesis in a Group A streptococcus is not inhibited by novobiocin [70], the inhibition of protein synthesis in Strept. faecium has been attributed to the inhibition of tRNA synthesis [23]. Novobiocin inhibits protein synthesis in Staph, aureus [26], but as the inhibition of other macro-molecules also occurs to a similar extent, this effect may not be the primary action of the antibiotic. The antibiotic also inhibits protein synthesis in E. coli [33, 71] but this inhibition appears much later than the observed inhibition of deoxyribonucleic acid (DNA) synthesis. 

Another example of a well-established class of natural products containing the thiazole heterocycle is the thiopeptide antibiotics. Often referred to as thiazolyl peptides, most of these highly modified, macrocyclic peptides inhibit protein synthesis in bacteria. They display a high inhibition... 

Thiopeptide antibiotics are a class of highly modified macrocyclic sulfur-containing peptides, and nearly all the thiopeptide antibioties identified to date inhibit protein synthesis in bacteria. The Bohlmann-Rahtz pyridine synthesis is a useful methodology in the synthetic approach for thiopeptide antibiotics.For example, one-step Bohlmann-Rahtz assembly of 189 and 190 in the presence of ammonium acetate in acetic acid at reflux afforded the corresponding pyridine-thiazole cores of thiopeptide antibiotics 191 in 63% yield.The TBS protecting group was also replaced by an acetate, probably as a consequence of acid-promoted cleavage and consequent Fischer-type esterification of the liberated alcohol. 

The selectivity of the tetracyclines, so much used in treating bacterial infections in mammals, depends on a similarly favourable distribution the bacteria concentrate these antibiotics whereas mammalian cells do not. Concentration of tetracyclines by bacteria (both Gram-positive and -negative types) was found to be a function of the cytoplasmic membrane (Franklin, 1971). Because of this difference, tetracyclines inhibit ribosomal protein synthesis in bacteria at doses which do not affect it in higher organisms (Franklin, 1963b, 1966). This selectivity depends on the intactness of the mammalian cell membrane, because isolated ribosomes (rat liver was used) were found to be as subject to inhibition of protein synthesis as bacterial ones were (Franklin, 1963a Section 11.8). 

Post-transcriptional control of protein synthesis in bacteria seems to be confined to the possible existence of messenger or cistron-specific initiation factors and their modification after phage infection. In animal cells, however, where the site of transcription is physically separated by the nuclear membrane from the protein synthesizing apparatus in the cytoplasm, there is die possibility of more complex forms of control, and some of these will be discussed lielow. 

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