The substances take advantage of the major differences between prokaryotic and eukaryotic ribosome structures which differ in their size, sequence, structure, and the ratio of protein to RNA.
The differences in structure allow some antibiotics to kill bacteria by inhibiting their ribosomes, while leaving human ribosomes unaffected. The ribosome has three sites: the A site, the P site, and the E site not shown in. The A site is the point of entry for the aminoacyl tRNA. The P site is where the peptidyl tRNA is formed in the ribosome. The E site which is the exit site of the now uncharged tRNA after it gives its amino acid to the growing peptide chain.
In general, protein synthesis inhibitors work at different stages of prokaryotic mRNA translation into proteins like initiation, elongation including aminoacyl tRNA entry, proofreading, peptidyl transfer, and ribosomal translocation , and termination.
The following is a list of common antibacterial drugs and the stages which they target. It is not known why tmRNA is essential in N. Transposon mutagenesis suggests that tmRNA is also essential in Mycoplasma species [24]. In addition, tmRNA is required for full virulence of Salmonella [23]. Recently, we have found that ssrA null mutants of the cyanobacterium Synechocystis sp. PCC are hypersensitive to several protein synthesis inhibitors [25].
This hypersensitive phenotype is also observed in Escherichia coli and Salmonella typhimurium ssrA null mutants [26] A. Vioque, unpublished; see also Fig. Vioque, unpublished. Hypersensitivity to kanamycin of a S. Wild-type cells are S. The ssrA null mutant is strain MT, which carries a Tn10d Tc r disruption cassette in the ssrA gene that confers resistance to tetracycline [23].
In our initial study, kanamycin could not be tested because the Synechocystis ssrA gene was disrupted with a kanamycin resistance cassette [25]. We have repeated these studies with a S. Therefore, the hypersensitive phenotype seems to be a general feature of cells unable to trans -translate, by inactivation of either the tmRNA or the SmpB protein. In addition, we can conclude that trans -translation has a role in cell survival in the presence of sublethal concentrations for the wild-type cells of very diverse protein synthesis inhibitors.
Is it possible to conceive a single mechanism that explains the increased resistance of cells with a functional trans -translation system to all the variety of protein synthesis inhibitors? Below we consider the different antibiotics according to their mode of action. The role of trans -translation has been studied in E.
The fact that cells carrying a mutant tmRNA that encodes a protease-resistant tag have an antibiotic resistance similar to that of the wild-type cells [26] suggests that the trans -translation system confers increased antibiotic resistance through the ribosome rescue function rather than through degradation of abnormal proteins.
In the case of other inhibitors, additional mechanisms have to be considered. As no detailed studies have been performed, we can only speculate based on the knowledge of the mechanism of action on these antibiotics. As above, trans -translation could also release the stalled ribosomes, which resume normal translation using the tag-coding region of tmRNA. The situation would be similar to the role of trans -translation when dissociating ribosomes at rare or stop codons.
We assume that at sublethal concentrations of antibiotics, only a fraction of the ribosomes less than one ribosome per mRNA engaged in translation become stalled on the mRNA, and therefore the probability that an antibiotic molecule binds again once the inhibited ribosome has been released by trans -translation is low. This simple assumption also requires that the first round of translation with tmRNA, which incorporates the non-encoded alanine, is insensitive to the presence of the antibiotic, which is displaced to allow continuous translation on the tmRNA.
It is not surprising that the resumption of translation on tmRNA would be more resistant to the antibiotic, taking into account the differences in size and structure between tRNA and tmRNA, which can imply differences in the structural transitions that occur during an elongation cycle with a normal tRNA or when resuming translation on tmRNA.
Chloramphenicol and lincomycin were shown to have a detrimental effect on growth of a ssrA null mutant of Synechocystis [25]. Similar effects have been shown for the E. In the presence of peptidyl transferase inhibitors, ribosomes become stalled with the A site occupied by an aminoacyl-tRNA. This seems to be inconsistent with the normal function of tmRNA, which requires that the ribosomal A site is free.
However, in the presence of a peptidyl transferase inhibitor, the aminoacyl-tRNA stays in the A site and its spontaneous release would give the opportunity to tmRNA to enter the A site Fig.
Recently, it has been shown that chloramphenicol also causes translational inaccuracy in vivo [28]. From the several translocation inhibitors assayed, including spectinomycin, fusidic acid and thiostrepton, only spectinomycin showed a differential effect on the ssrA null mutants of Synechocystis. In this scenario, it is even more difficult to explain the mode of action of tmRNA.
Translocation can occur in the absence of EF-G at a slow rate [30]. The ssrA null mutants of Synechocystis are also more sensitive than wild-type cells to erythromycin, spiramycin and tylosin. These antibiotics sterically block the peptide exit tunnel [31 , 32]. There are reports that conclude that erythromycin induces dissociation of peptidyl-tRNA [33]. If peptidyl-tRNA is released from the ribosome, what would be the role of trans -translation? One provocative speculation is that ribosomes blocked by macrolides dissociate the peptidyl-tRNA and bind alanyl-tmRNA to initiate translation on the tag-coding sequence of tmRNA.
In this respect, it is also interesting to consider that in vitro, the alanyl-tmRNA from E. This interaction could be functionally relevant, facilitating the resumption of translation on the tag-coding sequence of tmRNA.
It is plausible to speculate that the stress caused by the presence of sublethal concentrations of protein synthesis inhibitors could increase the requirement for tmRNA. In this scenario, no specific models would be required to explain the hypersensitivity of ssrA mutants to these inhibitors.
Indeed, in Bacillus subtilis , tmRNA is required for growth under several different stresses unrelated to protein synthesis [35]. However, this seems not to be the case in Synechocystis , where no differences were detected between wild-type and ssrA null mutants under other different stressful conditions than the presence of protein synthesis inhibitors [25]. In addition, ssrA null mutants have increased sensitivity only to a subset of all the protein synthesis inhibitors tested, suggesting that specific mechanisms of trans -translation action can be relevant.
Independently of the mechanism by which trans -translation contributes to survival in the presence of protein synthesis inhibitors, it is clear that inactivation of trans -translation has a synergistic effect with protein synthesis inhibitors on promoting inhibition of cell growth.
This effect is more significant with aminoglycoside antibiotics that induce misreading, such as kanamycin and streptomycin.
Specific drugs that inactivate trans -translation could be developed and used in combination with aminoglycosides as a more effective tool against pathogenic bacteria.
Moreover, the effective concentration of the protein synthesis inhibitor can be reduced in the presence of a trans -translation inhibitor, reducing side effects on health. The rational development of trans -translation inhibitors requires knowledge of the structure of the macromolecules involved. An important step forward has been the recent publication of the solution structure of SmpB [36].
Of course, inhibitors of trans -translation could be directly useful against those pathogens in which this mechanism is essential, such as N. Ribosome inhibitors could be a useful tool as a probe to understand the mechanism of trans -translation just as they have been extremely useful in the elucidation of the normal protein synthesis process. The increased sensitivity to protein synthesis inhibitors in different bacterial cells defective in trans -translation suggests that tmRNA could be an attractive target for the development of new specific drugs.
Eukaryotes do not use trans -translation, but rather an unrelated mechanism to deal with stalled ribosomes blocked on defective mRNAs [37]. Moreover, tmRNA is not known in animal cells [1].
Therefore, a specific drug directed against trans -translation is expected to have low toxicity. The potential therapeutic use of inhibitors of trans -translation can be extended to inhibitors of bacterial RNase P, because RNase P processing is required for the biosynthesis of tmRNA [8]. Recently several such inhibitors have been developed that work at low concentrations [38]. Therefore, protein synthesis inhibitors and RNase P inhibitors may also have a synergistic effect on promoting pathogenic bacterial inhibition.
We apologize to our colleagues who were not cited in this minireview because of space limitations. We thank Robert Sauer and Michael Mahan for strains.
Research in A. Williams K. Nucleic Acids Res. Google Scholar. Muto A. Ushida C. Himeno H. Trends Biochem. Karzai A. Roche E. Sauer R. Nature Struct. Gillet R. Felden B. Withey J. Friedman D. Susskind M. EMBO J. SmpB tagging and ribosome rescue complex. USA 98 , — Komine Y. Kitabatake M. Yokogawa T. Nishikawa K. Inokuchi H. USA 91 , — Rudinger-Thirion J. Giege R. RNA 5 , — Tadaki T. Contact a health care provider if you have questions about your health.
How do genes direct the production of proteins? From Genetics Home Reference. Topics in the How Genes Work chapter What are proteins and what do they do? Can genes be turned on and off in cells? What is epigenetics? How do cells divide? How do genes control the growth and division of cells?
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