Inhibitors of protein synthesis

Interferon, double-stranded RNA, and protein phosphorylation

We reported earlier that the addition of double-stranded RNA and ATP increases the endonuclease activity more in an extract of Ehrlich ascites tumor cells which have been treated with an interferon preparation than in a comparable extract from control cells. We report here that the addition of double-stranded RNA to an extract from Ehrlich ascites tumor cells which have been treated with an interferon preparation [or with the interferon inducer poly(I)-poly(C)] promotes the phosphorylation by [gamma-32P]ATP of at least two proteins: P1 (molecular weight of 64,000) and P2 (molecular weight of 37,000). Double-stranded RNA also promotes the phosphorylation of at least one (i.e., P1) of these two proteins in an extract from cells which have not been treated with interferon, but the extent of phosphorylation is much smaller. Double-stranded RNA which has been degraded by RNase III, or DNA, does not promote the phosphorylation.

Specificity and properties of the destabilization, induced by initiation factor IF-3, of ternary complexes of the 30-S ribosomal subunit, aminoacyl-tRNA and polynucleotides

Initiation factor IF-3 causes the destabilization of preformed ternary complexes of 30-S ribosomal subunit, codons and aminoacyl-tRNAs or peptidyl-tRNA. This destabilization is dilution-dependent and affects all ternary complexes with the exception of those containing the initiator fMet-tRNA, which remain more resistant to IF-3-induced destabilization under the various conditions studied. Several possible reasons for this specificity have been examined. It was found that the basis for the specificity is not: (a) an intrinsic greater stability of the ternary complexes containing fMet-tRNA, (b) the amoung of aminoacyl-tRNA bound to the ribosome, (c) the conditions under which the ternary complex is made or (d) the formylation of the amino group. On the other hand, the nature of the polynucleotide in response to which the ternary complex is formed was found to influence the amount of aminoacyl-tRan bound to the ribosome, and to some extent the amount of aminoacyl-tRNA which can be relased. The ternary complex containing the mischarged initiator tRNA fVal-tRNAfMet displays greater resistance to the IF-3-induced destabilization than the complex containing fVal-tRNAVal. These results indicate that the specificity of the IF-3 activity is due to the special structural feature of the initiator tRNA molecule and to some extent to the nature of the polynucleotide. The IF-3-induced destabilization of ternary complexes was found to be little affected by variations in reaction conditons, so that this IF-3 activity can be used to measure the stoichiometric binding of IF-3 to the ribosome over a broad range of pH and K+ and Mg2+ concentrations. Several antibiotics have been tested for their capacity to interfere with this reaction; only high concentrations of tetracycline blocked this IF-3 activity.

Specific inhibition of translocation by tubulosine in eukaryotic polysomes

The alkaloid tubulosine inhibits the process of peptide chain elongation by eukaryotic polysomes by specifically preventing the elongation-factor-2-dependent step of translocation. Tubulosine does not affect either the elongation-factor-1-dependent binding of aminoacyl-tRNA or peptide bond formation. The site of action of tubulosine appears to be independent from the ribosomal site involved in cycloheximide action, since the alkaloid is active in blocking polymerization and enzymic translocation by polysomes from a yeast mutant resistant to cycloheximide. Furthermore tubulosine does not affect the non-enzymic translocation which takes place in the presence of high potassium ion concentrations, whereas this reaction is strongly inhibited by cyclohemimede. The different steps of translocation are discussed to explain the reactions blocked by tubulosine and cycloheximide.

Antibiotics and compounds affecting tanslation by eukaryotic ribosomes. Specific enhancement of aminoacyl-tRNA binding by methylaxnthines

The mode and site of action of inhibitors of translation (initiation, elongation and termination of protein synthesis) in eukaryotic systems is reviewed. The isolation and characterization of a factor is described that binds Ac-Phe-tRNA to form a complex made up of binding factors, Ac-Phe-tRNA, and ribosome. The binding of Ac-Phe-tRNA probably occurs at the ribosomal site involved in the binding of the initiator substrate Met-tRNAF. The effect of inhibitors of the intitiation phase of protein synthesis on the nonenzymic Ac-Phe-tRNA binding to ribosomes is investigated. The two sites translocation model for translation in eukaryotic cells is presented and the effects of inhibitors on the various steps of protein synthesis are determined empirically. The site of action of inhibitors of peptide bond formation at the ribosomal peptidyl transferase center is elucidated. The action of inhibitors of translocation is sutdied in model cell-free systems from human cells. In addition, a number of methylxanthines are shown to enhance the elongation phase in polypeptide synthesis by stimulating the enzymic binding of aminoacyl-tRNA. The effect of caffeine, theophylline and its derivatives are shown to be fairly specific and dependent on the ribosome concentration. Aminophylline is shown to have a similar effect but also enhances aminoacyl-tRNA synthetase activity at low Mg++ concentrations, probably displacing the optimal concentration of Mg++ in the reaction. This second effect of aminophylline appears to be due to the ethylenediamine moiety of aminophylline since it is also observed in the presence of different polyamines but not in the presence of caffeine or theophylline.

The photochemical inactivation of peptidyl transferase activity

The photochemical oxidation of the 50-S ribosomal subunit results in a rapid irreversible loss of peptidyl transferase activity. The first-order rate of inactivation occurring during the first forty minutes suggests that a single reactive group is being inactivation exhibits a maximum at pH 7.5. Erythromycin at a low concentration (0.04 mumol) affords significant protection. Puromycin also exerts a protective effect but at higher concentrations. Chloramphenicol, sparsomycin and lincomycin did not exert a protective effect. The loss in catalytic activity was not accompanied by a loss in substrate binding affinity of the donor and acceptor substrates.

Rapid reduction of prostaglandin 15-hydroxy dehydrogenase activity in rat tissues after treatment with protein synthesis inhibitors

1 The capacity of rat kidneys to metabolize prostaglandins is rapidly lost when de novo protein synthesis is reduced by cycloheximide and puromycin. Enzyme activity declines to about half control values 45-75 min after a single dose of cycloheximide. Prostaglandin metabolism by the lung is also diminished. 2 These inhibitors apparently act by preventing the synthesis of new prostaglandin 15-hydroxy dehydrogenase enzyme. 3 The RNA synthesis actinomycin D has no effect on metabolism. 4 It is concluded that prostaglandin dehydrogenase is a short-lived enzyme in the cell whose replacement depends upon de novo protein (but not RNA) synthesis. The implications of this finding are discussed.

The mode of action of griseoviridin at the ribosome level

The antibiotic griseoviridin binds to the larger subunit of Escherichia coli ribosomes blocking the interaction between the 3' terminal end of peptidyl-tRNA and the donor site of the peptidyl transferase centre. Griseoviridin inhibits binding of chloramphenicol, thiamphenicol, lincomycin, erythromycin and streptogramin A to bacterial ribosomes. Moreover griseoviridin protects the ribosomal binding site of gougerotin from the drastic conformational changes taking place in the presence of ethanol. Griseoviridin is also able to interact with eukaryotic ribosomes as shown by its effects on model systems and on anisomycin, trichodermin and gougerotin binding studies. Nevertheless, griseoviridin affinity for the 80-S type ribosomes (yeast or human) is two orders of magnitude smaller than with E. coli ribosomes. The inhibitory spectrum and mode of action of griseoviridin on ribosomes is compared to that of antibiotics of the streptogramin A group and found to be essentially the same.

Early days of antimicrobialherapy

Sporangiomycin and micrococcin inhibit the binding of aminoacyl-transfer ribonucleic acid into the ribosomal A site in intact bacterial protoplasts. They also prevent the assembly of [ribosome-elongation factor G-guanine nucleotide] complexes in vitro and compete with [³⁵S]thiostrepton for ribosomal binding sites. We conclude that micrococcin and sporangiomycin block the ribosomal A site in the vicinity of the complex guanosine triphosphatase center and so resemble thiostrepton in their modes of action.

Quantitative binding of antibiotics to ribosomes from a yeast mutant altered on the peptidyl-transferase center

Quantitative binding studies of [G-3H]anisomycin and [acetyl-14C]trichodermin to sensitive and resistant 80-S ribosomes from yeasts are described in this work. A single mutation, most probably affecting the ribosome peptidyl transferase centre, appears to have pleiotropic effects on the ribosome leading to resistance to trichodermin and anisomycin and to an increased sensitivity to sparsomycin. Resistance to trichodermin is due to a reduced affinity of ribosomes from the mutant for the antibiotic. Ribosomes from the sensitive strain (Y 1661 bind [acetyl-14C]trichodermin with a dissociation constant of 0.99 muM while those from the resistant one (TR1) bind [acetyl-14C]trichodermin with a dissociation constant of 15.4 muM. Similar results are obtained when the binding of [acetyl-14C]trichodermin to Y 166 and TR1 60-S subunits is studied. The mutant TR1 is also resistant to anisomycin. Although trichodermin and anisomycin bind to the ribosome at mutually exclusive sites, the higher affinity binding of [G-3H]anisomycin that is responsible for the inhibition of the peptidyl transferase center is practically identical for Y 166 and TR1 ribosomes. Therefore, the mutation in the ribosome leading to resistance to trichodermin and anisomycin decreases the affinity for trichodermin but not for anisomycin. Trichodermin, trichothecin and fusarenon X inhibit the binding of [G-3H]anisomycin to TR1 ribosomes to a lower extent than to Y 166 ribosomes, suggesting that the resistance of TR1 ribosomes to the effects of trichothecin and fusarenon X is caused by a decrease in the affinity of the ribosomes for these drugs, as was seen with trichodermin. On the other hand, verrucarin A inhibits [G-3H]anisomycin binding to Y 166 and TR1 ribosomes to a similar extent and therefore its affinity for the ribosome does not appear to be affected by the mutation leading to resistance. Trichothecin, trichodermin and fusarenon X appear to have a common binding site on the 60-S ribosomal subunits, which overlaps or is closely linked to the binding sites of anisomycin and verrucarin A.

Effects of ricin on the ribosomal sites involved in the interaction of the elongation factors

The effects of ricin on the different steps of the elongation cycle of protein synthesis in a rabbit reticulocyte cell-free system are studied in this paper. The toxin most probably acts by catalytically inactivating the ribosomes, since a single molecule of the toxin can inactivate 300 ribosomes for poly(U)-directed phenylalanine incorporation. The effect of the toxin on the ribosome is irreversible. Ricin specifically inhibits elongation-factor-1-dependent aminoacyl-tRNA binding to ribosomes but has no effect on the non-enzymic binding of aminoacyl-tRNA. Ricin also inhibits formation of the complex elongation-factor-2 - ribosome - nucleotide with GTP, GDP or GMP-P(CH2)P. However, the toxin has no effect on translocation. These apparently conflicting results are discussed in this study.