Inhibition at the initiation level of eukaryotic protein synthesis by T-2 toxin

Comparative toxicokinetics of Fusarium mycotoxins in pigs and humans

Mycotoxins frequently contaminate food and feed materials, posing a threat to human and animal health. Fusarium species produce important mycotoxins with regard to their occurrence and toxicity, especially deoxynivalenol (DON), fumonisin B1 (FB1), zearalenone (ZEN) and T-2 toxin (T-2). The susceptibility of an animal species towards the effects of these toxins in part depends on the absorption, distribution, metabolism and excretion (ADME processes) of these toxins from the body. For humans, in vivo information is scarce and often animal data is used for extrapolation to humans. From a kinetic and safety point of view, the pig seems to be a promising animal model to aid in the assessment of the toxicological risk of mycotoxins to humans. Qualitatively, the ADME processes seem to be quite similar between pigs and humans. In addition, similar metabolite and excretion patterns are observed, although some quantitative differences are noticed which are subject of this review. The high sensitivity of pigs towards mycotoxins and the similar kinetics are an advantage for the use of this animal species in the risk assessment of mycotoxins, and for the establishment of legal limits of mycotoxins.

Systemic Metabolic Responses of Broiler Chickens and Piglets to Acute T-2 Toxin Intravenous Exposure

The aim of this study is to thoroughly investigate the toxicity mechanism of mycotoxin T-2 toxin and to further understand the endogenous metabolic alterations induced by T-2 toxin. To achieve this, a nuclear magnetic resonance (NMR)-based metabonomics approach was used to analyze the metabolic alterations induced by a single intravenous injection of T-2 toxin (0.5 mg/kg of body weight) in piglets and broiler chickens. A range of metabolites in the plasma, liver, kidney, and spleen of broiler chickens and plasma of piglets was changed following T-2 toxin injection. For example, a rapid increase of amino acids together with a significant reduction of glucose and lipid occurred in the plasma of broiler chickens and piglets following T-2 toxin treatment. A significant accumulation of amino acids and modulated nucleotides were detected in the liver, kidney, and spleen of T-2 toxin-treated broiler chickens. These data indicated that T-2 toxin caused endogenous metabolic changes in multiple organs and perturbed various metabolic pathways, including energy, amino acid, and nucleotide metabolism, as well as oxidative stress. We also observed elevated levels of tryptophan in the T-2 toxin-treated broiler chickens, which may explain the reported neurotoxic effects of T-2 toxin. These findings provide important information on the toxicity of T-2 toxin and demonstrate the power of the NMR-based metabonomics approach in exploring the toxicity mechanism of xenobiotics.

Integrated Transcriptional and Proteomic Analysis of Growth Hormone Suppression Mediated by Trichothecene T-2 Toxin in Rat GH3 Cells

Chronic exposure to trichothecenes is known to disturb insulin-like growth factor 1 and signaling of insulin and leptin hormones and causes considerable growth retardation in animals. However, limited information was available on mechanisms underlying trichothecene-induced growth retardation. In this study, we employed an integrated transcriptomics, proteomics, and RNA interference (RNAi) approach to study the molecular mechanisms underlying trichothecene cytotoxicity in rat pituitary adenoma GH3 cells. Our results showed that trichothecenes suppressed the synthesis of growth hormone 1 (Gh1) and inhibited the eukaryotic transcription and translation initiation by suppressing aminoacyl-tRNA synthetases transcription, inducing eukaryotic translation initiation factor 2-alpha kinase 2 (EIF2AK2) and reducing eukaryotic translation initiation factor 5 a. The sulfhydryl oxidases , protein disulfide isomerase,and heat shock protein 90 (were greatly reduced, which resulted in adverse regulation of protein processing and folding. Differential genes and proteins associated with a decline in energy metabolism and cell cycle arrest were also found in our study. However, use of RNAi to interfere with hemopoietic cell kinase (Hck) and EIF2AK2 transcriptions or use of chemical inhibitors of MAPK, p38, Ras, and JNK partially reversed the reduction of Gh1 levels induced by trichothecenes. It indicated that the activation of MAPKs, Hck, and EIF2AK2 were important for trichothecene-induced growth hormone suppression. Considering the potential hazards of exposure to trichothecenes, our findings could help to improve our understanding regarding human and animal health implications.

Isolation and characterization of a new less-toxic derivative of the Fusarium mycotoxin diacetoxyscirpenol after thermal treatment

Trichothecenes are an important class of mycotoxins that act as potent protein synthesis inhibitors in eukaryotic organisms. The compound 4,15-diacetoxyscirpenol is highly toxic for plants and animals. Potatoes are especially prone to be contaminated with 4,15-diacetoxyscirpenol after infection with Fusarium sambucinum. In the current study, the reduction of 4,15-diacetoxyscirpenol during thermal treatment in aqueous solution was monitored. A new derivative was detected and named DAS-M1. After isolation, DAS-M1 was characterized with LC-HR-MS and LC-MS/MS and structurally elucidated with (1)H, (13)C, and 2D NMR. Potatoes were inoculated with F. sambucinum, and the infected potatoes were cooked at 100 or 121 °C, respectively. A reduction of 4,15-diacetoxyscirpenol from about 26% (1 h at 100 °C) to 100% (4 h at 121 °C) was detected by means of LC-MS/MS analysis. The effects of different pH values on the reduction of 4,15-diacetoxyscirpenol were investigated, showing higher conversion rates at more acidic pH values. In addition, the toxicity of 4,15-diacetoxyscirpenol and DAS-M1 was compared in vitro using a wheat germ transcription/translation assay and in vivo on Saccharomyces cerevisiae. The results show that the inhibitory effect of DAS-M1 on yeast growth is about 50 times lower and inhibition of protein synthesis is about 100 times lower than that of 4,15-diacetoxyscirpenol.

Transcriptomic-based bioassays for the detection of type A trichothecenes

The type A trichothecenes T-2 toxin (T-2) and HT-2 toxin (HT-2) are hazardous Fusarium products that contaminate many field crops growing in cold to temperate regions across the world. Toxicity studies in laboratory and farm animals have been used to derive a temporary tolerable daily intake (t-TDI) for the sum of T-2 and HT-2 of no more than 60 ng/kg body weight. To protect the consumers, it is now necessary to screen a large number of food samples for the presence of these poisonous fungal metabolites. Towards that goal, we discovered that the transcriptional apparatus of a human carcinoma cell line (MCF7) provides a sensitive biological sensor of type A trichothecenes. In fact, exposure of this easy-to-culture cell line to T-2 or HT-2 results in the regulation of >2,000 different transcripts with expression changes ranging from >5,000-fold gene inductions to >40-fold gene repressions. These transcriptional responses have been exploited to develop practical microchip and reverse transcription-polymerase chain reaction (RT-PCR) assays for the detection of type A trichothecenes at parts per billion levels.

T-2 Toxin: Incidence and Toxicity in Poultry

T-2 toxin is the most toxic type A trichothecene mycotoxin. It is the secondary metabolite of the Fusarium fungi, and is common in grain and animal feed. Toxic effects have been shown both in experimental animals and in livestock. It has been implicated in several outbreaks of human mycotoxicoses. Toxic effects in poultry include inhibition of protein, DNA, and RNA synthesis, cytotoxicity, immunomodulation, cell lesions in the digestive tract, organs and skin, neural disturbances and low performance in poultry production (decreased weight gain, egg production, and hatchability). Concentrations of T-2 toxin in feed are usually low, and its immunosuppressive effects and secondary infections often make diagnosis difficult. If at the onset of the disease, a change in diet leads to health and performance improvements in animals, this may point to mycotoxin poisoning. Regular control of grain and feed samples is a valuable preventive measure, and it is accurate only if representative samples are tested. This article reviews the incidence and toxic effects of T-2 toxin in poultry.

Highly sensitive protein translation assay for trichothecene toxicity in airborne particulates: comparison with cytotoxicity assays

Screening assays for environmental mycotoxins in bulk samples currently use cytotoxicity in cell cultures, but their application to air particulate samples often lacks sensitivity and specificity for fungal spores. An assay based on inhibition of protein synthesis using translation of firefly luciferase in a rabbit reticulocyte system has been developed for the detection of trichothecene mycotoxins found in the spores of toxigenic fungi. Ethanol extracts of air particulates trapped on polycarbonate filters are ultrafiltered and applied at several dilutions to a translation reaction mixture. The activity of translated luciferase is measured directly in a luminometer, eliminating the need for radioisotopes and time-consuming sample processing. Parallel standard curves using a commercially available trichothecene provide for expression of the results in T-2 toxin equivalents per cubic meter of air. The assay can be completed in 2 h and is readily applicable to multiple samples. Comparison to the 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide cytotoxicity assay indicates a 400-fold increase in sensitivity of trichothecene detection in addition to a much higher specificity for these toxins. Initial field testing indicates a strong correlation between the measured level of toxicity and the presence of toxigenic fungi detected with microbiological methods. In conclusion, this luciferase translation assay offers a rapid and highly sensitive and specific method for quantitative detection of trichothecene mycotoxin activity in air particulate samples.

In vitro toxicity of trichothecenes on human erythroblastic progenitors

Trichothecenes are mycotoxins produced by various species of fungi which can occur on various agricultural products. Among these compounds, T-2 toxin, HT-2 toxin, diacetoxyscirpenol (DAS) and deoxynivalenol (DON) are the most naturally encountered and potent trichothecenes. Consumption of trichothecene contaminated foods by farm animals and humans leads to mycotoxicoses. Trichothecenes are known to induce haematologic disorders such as neutropenia, thrombopenia, and aplastic anemia in human and animals. The aim of our investigations is to explore the effects of trichothecenes on the haematopoietic progenitors. The four trichothecenes previously demonstrated to be strongly cytotoxic for human CFU-GM have been tested on human BFU-E. For this purpose, a culture model of human erythroblastic progenitors (BFU-E) optimized for toxicological studies was used to determine the effects of T-2, HT-2, diacetoxyscirpenol (DAS) and deoxynivalenol (DON) on red blood cell precursor proliferation and differentiation. Results showed that human BFU-E are as sensitive to trichothecenes as human CFU-GM, except for DON, in the range of concentrations tested. Differentiation of erythroblastic progenitors could be perturbed by these mycotoxins. Human erythroblastic progenitors are also a target of trichothecenes.

Ribotoxic stress response: activation of the stress-activated protein kinase JNK1 by inhibitors of the peptidyl transferase reaction and by sequence-specific RNA damage to the alpha-sarcin/ricin loop in the 28S rRNA

Inhibition of protein synthesis per se does not potentiate the stress-activated protein kinases (SAPKs; also known as cJun NH2-terminal kinases [JNKs]). The protein synthesis inhibitor anisomycin, however, is a potent activator of SAPKs/JNKs. The mechanism of this activation is unknown. We provide evidence that in order to activate SAPK/JNK1, anisomycin requires ribosomes that are translationally active at the time of contact with the drug, suggesting a ribosomal origin of the anisomycin-induced signaling to SAPK/JNK1. In support of this notion, we have found that aminohexose pyrimidine nucleoside antibiotics, which bind to the same region in the 28S rRNA that is the target site for anisomycin, are also potent activators of SAPK/JNK1. Binding of an antibiotic to the 28S rRNA interferes with the functioning of the molecule by altering the structural interactions of critical regions. We hypothesized, therefore, that such alterations in the 28S rRNA may act as recognition signals to activate SAPK/JNK1. To test this hypothesis, we made use of two ribotoxic enzymes, ricin A chain and alpha-sarcin, both of which catalyze sequence-specific RNA damage in the 28S rRNA. Consistent with our hypothesis, ricin A chain and alpha-sarcin were strong agonists of SAPK/JNK1 and of its activator SEK1/MKK4 and induced the expression of the immediate-early genes c-fos and c-jun. As in the case of anisomycin, ribosomes that were active at the time of exposure to ricin A chain or alpha-sarcin were able to initiate signal transduction from the damaged 28S rRNA to SAPK/JNK1 while inactive ribosomes were not.

Effects of steroids on association of T-2 toxin with mammalian cells

The effects of steroids on the association of T-2 toxin with cultured cells were evaluated. Preincubating cells with certain steroids led to a time- and concentration-related increase in total T-2-cell association. At maximally effective concentrations, the increase in association was 300-500%. This effect required a preincubation at 37 degrees C for a minimum of 10 min and was completely reversible after 20-30 min. Steroid treatment increased the rate of toxin-cell association and decreased the rate of dissociation. The effect was elicited by progesterone, estradiol, testosterone and diethylstilbestrol, but not by several other steroids tested. Binding of T-2 to isolated ribosomes was not altered by the steroids. We speculated that steroids somehow alter the state of ribosomal aggregation or assembly such that more toxin can bind after entering the cell.

In vivo effects of T-2 mycotoxin on synthesis of proteins and DNA in rat tissues

Rats were given an ip injection of T-2 mycotoxin (T-2), the T-2 metabolite, T-2 tetraol (tetraol), or cycloheximide. Serum, liver, heart, kidney, spleen, muscle, and intestine were collected at 3, 6, and 9 hr postinjection after a 2-hr pulse at each time with [14C]leucine and [3H]thymidine. Protein and DNA synthesis levels in rats were determined by dual-label counting of the acid-precipitable fraction of tissue homogenates. Rats given a lethal dose of T-2, tetraol, or cycloheximide died between 14 and 20 hr. Maximum inhibition of protein synthesis at the earliest time period was observed in additional rats given the same lethal dose of the three treatments and continued for the duration of the study (9 hr). With sublethal doses of T-2 or tetraol, the same early decrease in protein synthesis was observed but, in most of the tissues, recovery was seen with time. In the T-2-treated rats. DNA synthesis in the six tissues studied was also suppressed, although to a lesser degree. With sublethal doses, complete recovery of DNA synthesis took place in four of the six tissues by 9 hr after toxin exposure. The appearance of newly translated serum proteins did not occur in the animals treated with T-2 mycotoxin or cycloheximide, as evidenced by total and PCA-soluble serum levels of labeled leucine. An increase in tissue-pool levels of free leucine and thymidine in response to T-2 mycotoxin was also noted. T-2 mycotoxin, its metabolite, T-2 tetraol, and cycloheximide cause a rapid inhibition of protein and DNA synthesis in all tissue types studied. These results are compared with the responses seen in in vitro studies.

Binding of T-2 toxin to eukaryotic cell ribosomes

The binding of radiolabeled T-2 to eukaryotic ribosomes was studied. The toxin bound to ribosomes in a time-, temperature- and concentration-dependent manner. The binding was saturable (0.3 nM), reversible at 37 degrees (half-time approximately 2.5 hr) and specific. The stoichiometry was one toxin molecule bound per ribosome. Binding of T-2 appeared to stablize the toxin recognition site to thermal degradation. A synthetically derived epimer of T-2 bound to the same ribosomal site as authentic T-2, but apparently with lower affinity. Two other trichothecene toxins tested blocked the binding of T-2 to ribosomes in a manner reflecting their protein synthesis inhibitory potencies. Anisomycin blocked the binding of T-2 to both isolated ribosomes and cells, whereas emetine blocked binding only to cells. Our data, together with that in the accompanying paper (Middlebrook JL and Leatherman DL, Biochem Pharmacol 38: 3093-3102, 1989), suggest that T-2 interaction with CHO cells is best viewed as a free, bidirectional movement of toxin across the plasma membrane and specific high-affinity binding to ribosomes.

The storage of globin mRNA during the inhibition of protein synthesis by heme deprivation

The inhibition of protein synthesis in reticulocytes and their lysates caused by heme-deprivation is reversible on restoration of an optimal heme concentration. Inhibition is accompanied by the disaggregation of polyribosomes and the accumulation of components of the translational mechanism. By determining the fate of labeled globin 9S mRNA added to an unfractionated reticulocyte lysate cell-free system, we find that normal cellular mRNA accumulates during inhibition in 20S and 48S complexes and in a complex which sediments just ahead of the 80S ribosome dimer OD260 peak (designated as greater than or equal to 80S complex)1. The 20S and greater than 80S complexes are the major pools of stored mRNA which is readily translated if optimal heme conditions are restored. In the 48S complex, however, the mRNA remains non-functional, and the complex is abortive, probably as a result of deacylation of the Met.tRNAf.

Structure-function relationships of 12,13-epoxytrichothecene mycotoxins in cell culture: comparison to whole animal lethality

Nineteen 12,13-epoxytrichothecene mycotoxins were tested for their relative capabilities to inhibit protein synthesis in Vero cells and rat spleen lymphocytes. Although the lymphocytes were generally more sensitive to the mycotoxins, good correlation existed between the relative potencies of the various trichothecenes in the two cell systems. The most potent mycotoxins (T-2, verrucarin A and roridin A) have acetyl side groups on, or a hydrocarbon chain between, carbons 4 and 15 of the basic ring structure. Loss of side groups from either of these positions or an isovaleryl group at carbon 8 resulted in reduced protein synthesis inhibition (T-2 to HT-2, neosolaniol or diacetoxyscirpenol). Any combination of loss from all three positions (T-2 triol, T-2 tetraol, 15-monoacetyl DAS, scirpentriol, fusarenon X and deoxynivalenol) further weakens their effect. Reduction of the hydroxyl groups to hydroxides, forming verrucarol and deoxyverrucarol, reduced their effectiveness by over a thousand-fold compared to the most potent mycotoxins. Addition of side groups resulted in reduced effectiveness only when an acetyl group was added to the carbon 3 position of T-2 (acetyl T-2) and deoxynivalenol (3-acetyl deoxynivalenol) or on substitution of an epoxide across the 9,10 carbons of diacetoxyscirpenol (beta-epoxide DAS). Effects of combining these and other mycotoxins were additive and showed no synergism or competition for binding to the active site. When in vitro effects of the mycotoxins were compared with results from whole animal lethality tests, several of the trichothecenes were weak inhibitors of protein synthesis in vitro but had in vivo toxicities similar to that of T-2 toxin. Thus, the in vitro cell response of a given trichothecene is not always an accurate predictor of toxicity in whole animals.

Effects of trichothecenes (T-2 toxin) on protein synthesis in vitro by brain polysomes and messenger RNA

The effects of T-2 toxin on protein synthesis were tested in two reticulocyte lysate in vitro systems pretreated with micrococcal nuclease. One of the test systems contained purified globin mRNA and was initiation dependent. The other contained rat brain polysomes and incorporated amino acids by an elongation dependent process. T-2 toxin inhibited the translation of globin mRNA at all concentrations tested, from 10(-8) M to 10(-4) M. Rat brain polysomes were much less sensitive to T-2 toxin than globin mRNA. While high concentrations of the toxin (10(-4) M) led to partial inhibition of protein synthesis by polysomes, low concentrations (10(-8) M and 10(-6) M) stimulated protein synthesis. Comparison of the above results with those obtained by other workers suggest that the T-2 toxin may inhibit not only the initiation step of translation, but also elongation and termination, depending upon the concentration of the toxin and the nature of the translation system. A similar mechanism may operate for all the trichothecene toxins that exert their effect through binding to ribosomal peptidyl transferase.

Formation of a 22S mRNA X rRNA X protein complex during translation of globin messenger RNA

Poly(A)-rich RNA was isolated from rabbit reticulocyte polyribosomes by affinity chromatography on oligo(dT)-cellulose and fractionated in sucrose gradients under non-denaturing conditions. Most of the translatable RNA sedimented in sucrose gradients both as free 9S mRNA and as a 22S complex containing 18S ribosomal RNA and a protein of Mr 66 000. The complex was characterized by identification of the translation products. Experiments with both labelled globin mRNA and Mr-66 000 protein indicate that the complex is not an artefact, but rather that it is formed during the initiation of protein synthesis. The Mr-66 000 protein appears to be a component of the 48S pre-initiation complex and recycles before 80S complex formation.

The effects of T-2 toxin on alveolar macrophage function in vitro

T-2 toxin, a metabolite of several Fusarium species, is a mycotoxin of the trichothecene family which occurs in a variety of grains. Previous work in our laboratory showed that T-2 toxin is highly toxic to rat alveolar macrophages in vitro at submicromolar concentrations. The present investigation was undertaken to study the basis of the cytotoxic effects observed. The following parameters of macrophage function were measured: macromolecular synthesis, release of 51Cr, cellular ATP, phagocytosis, and alveolar macrophage "activation." The incorporation of radiolabeled leucine into acid-precipitable molecules was significantly inhibited within 1 hr of treatment at sublethal concentrations, although amino acid uptake was unaffected. Cell volume and release of 51Cr was unaffected by 0.1 microM T-2 toxin after 6 hr but evidence of significant leakage was seen after 18 hr treatment. The capacity of alveolar macrophages to phagocytize Saccharomyces cerevisiae and 3H-Staphylococcus aureus was significantly reduced whereas binding of 3H-S. aureus to the macrophage was not. Macrophage activation with endotoxin (Escherichia coli lipopolysaccharide) and mitogen-generated lymphokines, as monitored by incorporation of [14C]glucosamine, was significantly altered at 0.01 microM T-2 toxin. Thus, the data clearly demonstrate that T-2 is toxic to alveolar macrophage function in vitro and suggest that the primary mechanism of this toxicity is related to the inhibition of protein synthesis.

Inhibition, by trichothecene antibiotics, of brain protein synthesis and fever in rabbits

1. To test further the hypothesis that brain protein synthesis is necessary for fever, three structurally similar trichothecene antibiotics were injected into the cerebral ventricles of rabbits. They were 3,15-diacetoxy-12-hydroxytrichothec-9-ene (DAHT), 3,15-didesacetyl-calonectrin (DDAC) and T-2 toxin. Their actions on hypothalamic incorporation of [14C]leucine and fever were compared. 2. DDAC (60 micrograms) and T-2 toxin (10 micrograms) strongly inhibited leucine incorporation and fever. DAHT (60 micrograms) did not diminish fever and had a smaller effect upon leucine incorporation. 3. The findings strengthen considerably earlier suggestions that brain protein synthesis is an essential step in pyrogenesis.

Preparation and partial characterization of cell-free protein-synthesizing extracts from Tetrahymena pyriformis

1. We have examined methods necessary for preparing post-mitochondrial supernatants from Tetrahymena pyriformis strain HSM that are capable of efficient cell-free protein synthesis. 2. The requirements for optimum synthesis in these extracts are described. 3. Data relating to the kinetics of protein synthesis and the initiation capacity of these supernatants are presented.

Two-step binding of eukaryotic ribosomes to brome mosaic virus RNA3

Although brome mosaic virus RNA3 has only one translatable cistron, it can bind two 80S ribosomes at initiation. One ribosome binds at the first AUG codon (base 92-94). The other binds nearer the 5' end at an entry or holding site. Disome formation is thus unrelated to a silent cistron approximately 1,000 bases downstream.

Structural requirements for the inhibitory action of 12,13-epoxytrichothecenes on protein synthesis in eukaryotes

1. The inhibitory actions of ten trichothecene antibiotics were investigated, in reticulocyte cell-free systems synthesizing protein in vitro, by studying polyribosome profiles and kinetics of amino acid incorporation in the presence or absence of the drugs. 2. The modes of action observed were critically dependent on the drug concentrations used, but the antibiotics tested could be divided into four distinct groups, each exerting a characteristic inhibitory response. 3. The inhibitory action observed in every case was controlled by the chemical structure of the individual trichothecene and in particular was closely related to the nature of the substituent groups present on C-3, C-4, C-8 and C-15 of the molecule.

Inhibition of initiation, elongation, and termination of eukaryotic protein synthesis by trichothecene fungal toxins

The 12,13-epoxytrichothecenes, specific inhibitors of protein synthesis in eukaryotes, can be subdivided further in terms of their mode of action. In addition to the I-type (initiation inhibitors) and E-types (elongation inhibitors), we found that some E-types apparently exhibit inhibition of chain termination at low concentrations. The nature of substituents on C4 may determine the type of inhibitory activity observed.

Inhibition of translation in eukaryotic systems by harringtonine

The Cephalotaxus alkaloids harringtonine, homoharringtonine and isoharringtonine inhibit protein synthesis in eukaryotic cells. The alkaloids do not inhibit, in model systems, any of the steps of the initiation process but block poly(U)-directed polyphenylalanine synthesis as well as peptide bond formation in the fragment reaction assay, the sparsomycin-induced binding of (C)U-A-C-C-A-[3H]Leu-Ac, and the enzymic and the non-enzymic binding of Phe-tRNA to ribosomes. These results suggest that the Cephalotaxus alkaloids inhibit the elongation phase of translation by preventing substrate binding to the acceptor site on the 60-S ribosome subunit and therefore block aminoacyl-tRNA binding and peptide bond formation. However, the Cephalotaxus alkaloids do not inhibit polypeptide synthesis and peptidyl[3H]puromycin formation in polysomes. Furthermore, these alkaloids strongly inhibit [14C]trichlodermin binding to free ribosomes but hardly affect the interaction of the antibiotic with yeast polysomot interact with polysomes and therefore only inhibit cycles of elongation. This explains the polysome run off that has been observed by some workers in the presence of harringtonine.

Competition between trichodermin and several other sesquiterpene antibiotics for binding to their receptor site(s) on eukaryotic ribosomes

1. Of the five sesquiterpene antibiotics tested and found to inhibit protein synthesis in yeast spheroplasts, trichothecin, trichodermol or trichodermin stabilized polyribosomes whereas, in contrast, verrucarin A or T-2 toxin induced 'run off' of polyribosomes with a corresponding increase in 80S monoribosomes. The effect of fusarenon X on the system could not be determined as the drug failed to enter the cells. 2. [acetyl-14C]Trichodermin bound to yeast polyribosomes with a dissociation constant of 2.10 muM and to yeast 'run off' ribosomes with a dissociation constant of 0.72 muM. 3. Trichothecin, trichodermol, fusarenon X, T-2 toxin and verrucarin A competed with [acetyl-14C]trichodermin for binding to its receptor site on 'run off' ribosomes. The observed competition was quantitatively similar for all drugs tested. In contrast, the five drugs competed to different extents with trichodermin for binding to its receptor site on polyribosomes. Thus trichothecin competed with relative efficiency, whereas verrucarin A competed poorly, and the other drugs occupied intermediate positions between these two extremes. 4. Studies were also carried out with yeast 'run off' ribosomes prepared from both a wild-type strain and a strain resistant to trichodermin. Competition experiments between verrucarin A and [3H]anisomycin indicated that verrucarin A bound to 'run off' ribosomes from the mutant strain less efficiently than to those from the wild-type.

Initiation of the polypeptide chain by reticulocyte cell-free systems. Survey of different inhibitors of translation

In order to elucidate the mechanism of action of inhibitors that block the initiation of protein synthesis in mammalian systems, we have studied the following steps: (a) formation of the ternary complex Met-tRNAr-IF-E2-GTP, (b) binding of the initiator Met-tRNAf to the 40-S ribosomal subunit in the presence of initiation factors and dependent or not on the addition of mRNA, (c) formation of the initiation complex with 80-S ribosomes and (d) formation of the first peptide bond. Adrenochrome, aurintricarboxylic acid, polydextran sulphate, pyrochatechol violet and showdomycin block the formation of the ternary complex Met-tRNAf-IF-E2-GTP. Edeine A1, aurintricarboxylic acid and polydextran sulphate block the binding of the mRNA to the 40-S ribosomal subunit. Pactamycin induces the formation of stable smaller initiation complexes which are unable to go through the subsequent steps of initiation. Stimulation of the binding of the initiator Met-tRNAf to the 80-S ribosome in the presence of initiation factors is observed with sparsomycin and antibiotics of the sesquiterpene family (verrucarin A, trichodermin and trichothecin). However, these antibiotics block the reaction of the bound Met-tRNAf with puromycin. Narciclasine has no effect on the binding of the initiator to the ribosome but strongly blocks its reaction with puromycin. We have developed a simple technique to detect the Met-tRNAf-40-S-subunit-poly(A, G, U) initiation complexes by chromatography on Sepharose 6B columns. The requirements for the formation of such complexes measured by this technique and its comparison with the sucrose gradient centrifugation method are described.

Prevention, by ribosome-bound nascent polyphenylalanine chains, of the functional interaction of t-2 toxin with its receptor site

1. The inhibitory effects of T-2 toxin and trichodermin on poly(U)-directed polyphenylalanine synthesis were studied by using cell-free systems from reticulocytes. Conditions for amino acid incorporation were carefully chosen in an attempt to ensure that the large majority of poly(U) chains bound only one ribosome engaged in protein synthesis and that all such ribosomes carried nascent polyphenylalanine chains containing approximately the same number of residues. 2. Cell-free systems were allowd to synthesize polyphenylalanine, and T-2 toxin and trichodermin were added to the incorporation mixtures at various times. Irrespective of the time of addition, trichodermin (50 mug/ml) inhibited polyphenylalanine synthesis by approx. 70%. In contrast, although T-2 toxin (40 mug/ml), when added at early incubation times, could inhibit polyphenylalanine synthesis with a maximum of 50%, the drug had no effect on the system when added after a critical time-period. 3. It is concluded that although both T-2 toxin and trichodermin can inhibit peptide-bond formation on ribosomes at the level of the peptidyl transferase catalytic centre the presence, on ribosomes, of nascent polyphenylalanine chains above a certain critical chain length excludes T-2 toxin from functional interaction with its receptor site.

Inhibition of protein synthesis in reticulocyte lysates by trichodermin

1. The effect of trichodermin as an inhibitor of eukaryotic protein synthesis was studied in a reticulocyte cell-free system. 2. Trichodermin at a concentration of 25 mug/ml inhibits total protein synthesis instantaneously and stabilizes polyribosome profiles. Conversely, at a concentration of 0.25 mug/ml the drug inhibits total protein synthesis by only 70-75% and allows 30-35% breakdown of the polyribosomes in the system. These effects were compared with those produced by two other drugs (pactamycin and anisomycin) examined under conditions identical with those used for trichodermin.