Ribosome changes during translation

Early memory consolidation window enables drug induced state-dependent memory

It is well established that newly acquired information is stabilized over time by processes underlying memory consolidation, these events can be impaired by many drug treatments administered shortly after learning. The consolidation hypothesis has been challenged by a memory integration hypothesis, which suggests that the processes underlying new memories are vulnerable to incorporation of the neurobiological alterations induced by amnesic drugs generating a state-dependent memory. The present experiments investigated the effects of amnesic drugs infused into the insular cortex of male Wistar rats on memory for object recognition training. The findings provide evidence that infusions of several amnesic agents including a protein synthesis inhibitor, an RNA synthesis inhibitor, or an NMDA receptor antagonist administered both after a specific period of time and before retrieval induce state-dependent recognition memory. Additionally, when amnesic drugs were infused outside the early consolidation window, there was amnesia, but the amnesia was not state-dependent. Data suggest that amnesic agents can induce state-dependent memory when administered during the early consolidation window and only if the duration of the drug effect is long enough to become integrated to the memory trace. In consequence, there are boundary conditions in order to induce state-dependent memory.

Novel Functionalized Selenium Nanoparticles for Enhanced Anti-Hepatocarcinoma Activity In vitro

Selenium nanoparticles loaded with an anticancer molecule offer a new strategy for cancer treatment. In the current study, anisomycin-loaded functionalized selenium nanoparticles (SeNPs@Am) have been made by conjugating anisomycin to the surface of selenium nanoparticles to improve anticancer efficacy. The prepared nanoparticles were fully characterized by transmission electronic microscopy, energy dispersive X-ray spectroscopy, Fourier-transformed infrared spectroscopy, and X-ray photoelectron spectroscopy. The results showed that anisomycin was successfully conjugated with selenium nanoparticles. The size of particles could be effectively regulated through altering the reaction concentrations of sodium selenite and anisomycin. The SeNPs@Am particles (56 nm) exhibited the greatest capacity for cellular uptake. The further study showed that SeNPs@Am entered human hepatocellular carcinoma HepG2 cells in a dose or time-dependent manner via macropinocytosis and clathrin-mediated endocytosis pathways. SeNPs@Am significantly inhibited HepG2 cell proliferation with the low cytotoxicity against normal cells, and dramatically precluded the aggression and migration of HepG2 cells. It also arrested the cell cycle progression at the G0/G1 phase through the activation of the cyclin-dependent kinase inhibitors with inhibition of CDK-2 and ICBP90, and induced the cell apoptosis through activating the caspase cascade signaling in HepG2 cells, markedly superior to anisomycin alone. The findings indicate that SeNPs@Am may be a promising drug for hepatocellular carcinoma.

Distinct stages of the translation elongation cycle revealed by sequencing ribosome-protected mRNA fragments

During translation elongation, the ribosome ratchets along its mRNA template, incorporating each new amino acid and translocating from one codon to the next. The elongation cycle requires dramatic structural rearrangements of the ribosome. We show here that deep sequencing of ribosome-protected mRNA fragments reveals not only the position of each ribosome but also, unexpectedly, its particular stage of the elongation cycle. Sequencing reveals two distinct populations of ribosome footprints, 28–30 nucleotides and 20–22 nucleotides long, representing translating ribosomes in distinct states, differentially stabilized by specific elongation inhibitors. We find that the balance of small and large footprints varies by codon and is correlated with translation speed. The ability to visualize conformational changes in the ribosome during elongation, at single-codon resolution, provides a new way to study the detailed kinetics of translation and a new probe with which to identify the factors that affect each step in the elongation cycle.

DOI:http://dx.doi.org/10.7554/eLife.01257.001

To make a protein from a gene, the gene is first transcribed to produce a molecule of messenger RNA (mRNA), which then passes through a molecular machine called a ribosome. The ribosome reads the genetic code in the mRNA in groups of three letters at a time, and each triplet of letters (or codon) represents an amino acid. The ribosome then joins the relevant amino acids together to build a protein.

The ribosome processes about six amino acids per second, on average, but the mRNA is not fed through at a constant rate. Instead, the ribosome changes its shape to ratchet along the mRNA from one codon to the next: it then reads the new codon and adds another amino acid to the protein. However, many of the details of this ratcheting process are not fully understood.

In this study, Lareau, Hite et al. have used a technique called ‘ribosome profiling’ to explore the movement of ribosomes along mRNA molecules. First, all of the pieces of mRNA molecules that are not protected inside a ribosome were chemically destroyed. The sequences of the protected fragments were then read and matched to the full-length gene sequences. The protected fragments came in two different sizes: some were about 28–30 letters long, and others were about 20–22 letters long. Lareau, Hite et al. suggest that these different fragment sizes occur because the ribosome switches between two shapes at each codon as it ratchets along the mRNA, and so it protects different lengths of mRNA.

In previous ribosome-profiling experiments, the fragments had all been about 28 letters long; but these experiments had used a chemical to halt the progress of the ribosomes along the mRNAs before measuring the length of the fragments. Lareau, Hite et al. argue that this chemical locks the ribosome in the same shape when it brings the ribosome to a halt, and so the protected fragments always have the same length. Further, other chemicals that halt ribosomes appear to lock this molecular machine in the other shape, and so it can only protect the shorter fragments.

The findings of Lareau, Hite et al. show that ribosomal profiling experiments can reveal much more than simply where a ribosome is on an mRNA molecule. Further study into the different stages of the ribosome ratcheting process will help uncover how the speed that a ribosome translates an mRNA into a protein can be encoded in the mRNA sequence itself.

DOI:http://dx.doi.org/10.7554/eLife.01257.002

In vitro and in vivo evaluation of anisomycin against Ehrlich ascites carcinoma

Anisomycin eminently inhibits cell proliferation in vitro. The aim of this study was to explore the potential of anisomycin to treat tumors in vivo and its mechanism(s) of action. The results showed that peritumoral administration of anisomycin significantly suppressed Ehrlich ascites carcinoma (EAC) growth resulting in the survival of approximately 60% of the mice 90 days after EAC inoculation. Enhancement of infiltrating lymphocytes was noted in the tumor tissue, which was dramatically superior to adriamycin. The growth inhibitory rate of EAC cells was enhanced with increasing concentrations of anisomycin, following an enhanced apoptotic rate. The total apoptotic rate induced by 160 ng/ml of anisomycin was higher when compared to that induced by 500 ng/ml of adriamycin. DNA breakage and nanostructure changes were also noted in the EAC cells. The levels of caspase-3 mRNA, caspase-3 and cleaved-caspase-3 proteins in the anisomycin‑treated EAC cells were augmented in a dose- and time-dependent manner, following the activation of caspase-8 and caspase-9, which finally triggered PARP cleavage. The cleaved-caspase-3, cleaved-caspase-8 and cleaved-caspase-9 proteins were mainly localized in the nuclei of the cells. These results indicate that anisomycin efficaciously represses in vitro and in vivo growth of EAC cells through caspase signaling, significantly superior to the effects of adriamycin. This suggests the potential of anisomycin for the treatment of breast cancer.

Multiple components of eIF4F are required for protein synthesis-dependent hippocampal long-term potentiation

Persistent forms of synaptic plasticity are widely thought to require the synthesis of new proteins. This feature of long-lasting forms of plasticity largely has been demonstrated using inhibitors of general protein synthesis, such as either anisomycin or emetine. However, these drugs, which inhibit elongation, cannot address detailed questions about the regulation of translation initiation, where the majority of translational control occurs. Moreover, general protein synthesis inhibitors cannot distinguish between cap-dependent and cap-independent modes of translation initiation. In the present study, we took advantage of two novel compounds, 4EGI-1 and hippuristanol, each of which targets a different component of the eukaryotic initiation factor (eIF)4F initiation complex, and investigated their effects on long-term potentiation (LTP) at CA3-CA1 synapses in the hippocampus. We found that 4EGI-1 and hippuristanol both attenuated long-lasting late-phase LTP induced by two different stimulation paradigms. We also found that 4EGI-1 and hippuristanol each were capable of blocking the expression of newly synthesized proteins immediately after the induction of late-phase LTP. These new pharmacological tools allow for a more precise dissection of the role played by translational control pathways in synaptic plasticity and demonstrate the importance of multiple aspects of eIF4F in processes underlying hippocampal LTP, laying the foundation for future studies investigating the role of eIF4F in hippocampus-dependent memory processes.

Anisomycin inhibits the behaviors of T cells and the allogeneic skin transplantation in mice

There is still a lack of a high potent and low toxic immunosuppressive drug. We accidentally found that a quite low dose of anisomycin was sufficient to block proliferation of T cells. In this study, carboxy-fluorescein diacetate-succinimidyl ester staining showed that over 10.0 ng/mL of anisomycin markedly inhibited the proliferation of T cells induced by ConA. Propidium iodide staining revealed that anisomycin led to G0/G1 arrest and blocked S phase entry stimulated by ConA or phorbol 12, 13-dibutyrate plus ionomycin. Anisomycin down-regulated remarkably the CD69 and CD25 expression on the surface of T cells. The response of T cells was repressed by treatment of anisomycin, which was partly restored by adding exogenous interleukin-2, and there was no difference between anisomycin and dexamethasone, although the used dose of the latter was 100-fold of the former. The inhibition of cytotoxicity of T cells against 7919 cells by anisomycin was observed without the direct cytotoxicity to T cells or 7919 cells. The level of transforming growth factor-beta1 fell by <80.0 ng/mL in vitro and 30.0 mg/kg of anisomycin in vivo and enhanced by more than the doses. The treatment of anisomycin prolonged the survival of the transplanted skin and depressed the delayed type hypersensitivity development and the T-cell response in the skin-transplanted mice. Moreover, the effect of its restraining allograft rejection might be superior to cyclosporine A, with relatively slight toxic signs. These results indicate anisomycin significantly inhibits the behaviors of T cells and the transplantation rejection, providing important evidence for anisomycin as a novel immunosuppressant.

Macrophage inhibitory cytokine-1 (MIC-1) and subsequent urokinase-type plasminogen activator mediate cell death responses by ribotoxic anisomycin in HCT-116 colon cancer cells

Ribosome-inactivating stresses possess a potent regulatory activity against tumor cell progression. In this study, we demonstrated that macrophage inhibitory cytokine-1 (MIC-1) and its associated signals determined the colon cancer cell response to the chemical ribotoxic stress. The ribotoxic stress agent anisomycin-induced MIC-1 gene expression which was involved in the ribotoxin-induced apoptotic pathway. MIC-1 was also a critical inducer of apoptosis-related gene products such as activated urokine-type plasminogen activator (PLAU) and PLAU receptor (uPAR). When MIC-1 or PLAU action was repressed in the tumor cells, the chemical ribotoxic stress triggered a survival-related MAP kinase such as ERK. Mechanistically, gene expression of apoptosis-mediator MIC-1 was enhanced by activating transcription factor 3 (ATF-3) via the p38 MAP kinase signaling pathway. Moreover, both promoter activity and mRNA stability of MIC-1 gene were up-regulated by ribotoxic anisomycin via the p38 MAP kinase signaling pathway. In conclusion, ribotoxic anisomycin-induced MIC-1 expression via p38-ATF3 pathway and subsequent apoptosis while suppressing survival ERK signal in the colon cancer cells. The results of this study provide mechanistic insight into tumor cell decision for death or survival pathways in response to ribosome-disrupting stresses from chemotherapeutics.

Molecular mechanism of apoptosis and gene expressions in human lymphoma U937 cells treated with anisomycin

Anisomycin is known as a potent apoptosis inducer by activating JNK/SAPK and inhibiting protein synthesis during translation. However, only few details are known about the mechanism of apoptosis induced by this compound. The present study was undertaken to further elucidate the molecular mechanism of apoptosis and the changes of gene expression elicited by anisomycin using DNA microarrays and computational gene-expression analysis tools in human lymphoma U937 cells. Anisomycin was found to induce apoptosis in time- and concentration-dependent manner as confirmed by phosphatidylserine externalization and DNA fragmentation analysis. Furthermore, anisomycin-treated cells also showed caspase-8 activation, mitochondrial membrane potential collapse, Bid activation, caspase-3 cleavage and cytochrome c release into the cytosol. In the gene-expression analysis, six gene clusters were detected. From clusters I and II, three significant genetic networks were identified. Interestingly, many bZIP family transcription factors were observed in the up-regulated genetic networks. Moreover, the expression of protein-synthesis-related genes, such as EIF4 family proteins and ribosomal proteins, were inhibited. This finding could explain the reason why anisomycin inhibits the protein synthesis at the translation steps. These results provide novel information for understanding the molecular mechanism of apoptosis induced by anisomycin.

Protein synthesis inhibitors enhance the expression of mRNAs for early inducible inflammatory genes via mRNA stabilization

Expression of inflammatory genes is regulated at multiple steps, including transcriptional activation and mRNA stabilization. During an investigation into the requirement of de novo protein synthesis for the induction of inflammatory genes, it was revealed that protein synthesis inhibitors unexpectedly potentiated the induction of mRNAs for primary response genes, while the inhibitors suppressed the induction of secondary inducible genes as previously described. Stimulus-induced nuclear translocation and promoter recruitment of NF-kappaB, which is responsible for the transcriptional activation of many inflammatory genes, were largely unaffected by the inhibitors. Instead, these inhibitors prolonged the half-lives of all of the primary inducible mRNAs tested. Thus, these findings emphasize the important contribution of regulated mRNA longevity to gene expression induced by pro-inflammatory stimulation.

The role of protein synthesis during the labile phases of memory: revisiting the skepticism

Despite the fact that extensive evidence supports the view that phases of de novo protein synthesis are necessary for memory formation and maintenance, doubts are still raised. Skeptics generally argue that amnesia and the disruption of long-term synaptic plasticity are caused by "non-specific effects" of the reagents or approaches used to disrupt protein synthesis. This paper attempts to clarify some of these issues by reviewing, discussing and providing results addressing some of the major critiques that argue against the idea that de novo protein synthesis is necessary for the stabilization of long-term memory.

13-Deoxytedanolide, a marine sponge-derived antitumor macrolide, binds to the 60S large ribosomal subunit

13-Deoxytedanolide is a potent antitumor macrolide isolated from the marine sponge Mycale adhaerens. In spite of its remarkable activity, the mode of action of 13-deoxytedanolide has not been elucidated. [11-3H]-(11S)-13-Deoxydihydrotedanolide derived from the macrolide was used for identifying the target molecule from the yeast cell lysate. Fractionation of the binding protein revealed that the labeled 13-deoxytedanolide derivative strongly bound to the 80S ribosome as well as to the 60S large subunit, but not to the 40S small subunit. In agreement with this observation, 13-deoxytedanolide efficiently inhibited the polypeptide elongation. Interestingly, competition studies demonstrated that 13-deoxytedanolide shared the binding site on the 60S large subunit with pederin and its marine-derived analogues. These results indicate that 13-deoxytedanolide is a potent protein synthesis inhibitor and is the first macrolide to inhibit the eukaryotic ribosome.

Cisplatin-resistance modulates the effect of protein synthesis inhibitors on spermidine/spermine N(1)-acetyltransferase expression

Cisplatin (DDP)-resistance confers a deficient expression of spermidine/spermine N(1)-acetyltransferase (SSAT) gene in response to the spermine analog N(1),N(12)-bis(ethyl)spermine (BESpm) in the DDP-resistant human ovarian carcinoma cell line (C13*), compared with their parental DDP-sensitive 2008 cells. This SSAT gene deficiency is correlated with a reduced growth sensitivity to spermine analogs. This study was performed to determine whether SSAT gene expression of resistant cells was kept suppressed by labile repressor proteins developed during resistance selection. We show here that inhibitory concentrations of cycloheximide (CHX) and anisomycin (ANISO) differentially affect BESpm-induced SSAT activity in 2008 and in C13* cells in a concentration-dependent manner and allow resistant cells to reach activation levels comparable to those of the sensitive cells. Northern blot analysis revealed that both CHX and ANISO in combination with BESpm caused a synergistic BESpm-mediated accumulation of SSAT mRNA in C13* cells, with respect to each drug alone, while in 2008 cells only a slight increase was observed. The more pronounced effect of inhibitors on the SSAT activity induced by BESpm in the resistant cells was also the result of a more prolonged stabilization of SSAT mRNA and enzyme protein. By contrast, sub-inhibitory concentrations of CHX and ANISO did not significantly stimulate BESpm-induced SSAT transcription and activity. These results suggest that labile repressor proteins, related to DDP-resistance phenotype, play a regulatory role in SSAT gene expression, and further indicate that by overcoming this inhibitory control it is possible to recover BESpm response.

Anisomycin selectively desensitizes signalling components involved in stress kinase activation and fos and jun induction

Anisomycin, a translational inhibitor secreted by Streptomyces spp., strongly activates the stress-activated mitogen-activated protein (MAP) kinases JNK/SAPK (c-Jun NH2-terminal kinase/stress-activated protein kinase) and p38/RK in mammalian cells, resulting in rapid induction of immediate-early (IE) genes in the nucleus. Here, we have characterized this response further with respect to homologous and heterologous desensitization of IE gene induction and stress kinase activation. We show that anisomycin acts exactly like a signalling agonist in eliciting highly specific and virtually complete homologous desensitization. Anisomycin desensitization of a panel of IE genes (c-fos, fosB, c-jun, junB, and junD), using epidermal growth factor (EGF), basic fibroblast growth factor, (bFGF), tumor necrosis factor alpha (TNF-alpha), anisomycin, tetradecanoyl phorbol acetate (TPA), and UV radiation as secondary stimuli, was found to be extremely specific both with respect to the secondary stimuli and at the level of individual genes. Further, we show that anisomycin-induced homologous desensitization is caused by the fact that anisomycin no longer activates the JNK/SAPK and p38/RK MAP kinase cascades in desensitized cells. In anisomycin-desensitized cells, activation of JNK/SAPKs by UV radiation and hyperosmolarity is almost completely lost, and that of the p38/RK cascade is reduced to about 50% of the normal response. However, all other stimuli produced normal or augmented activation of these two kinase cascades in anisomycin-desensitized cells. These data show that anisomycin behaves like a true signalling agonist and suggest that the anisomycin-desensitized signalling component(s) is not involved in JNK/SAPK or p38/RK activation by EGF, bFGF, TNF-alpha, or TPA but may play a significant role in UV- and hyperosmolarity-stimulated responses.

Comparison of the in vitro activities of quassinoids with activity against Plasmodium falciparum, anisomycin and some other inhibitors of eukaryotic protein synthesis

Using the inhibition of incorporation of [3H]hypoxanthine as an index of viability of malaria parasites, it was shown that a chloroquine-sensitive strain of Plasmodium falciparum (T9-96) and a chloroquine-resistant strain (K1) did not differ in their sensitivities to the quassinoids ailanthinone, bruceantin and chaparrin. Similarly, there were no differences between the strains in their sensitivities to the protein synthesis inhibitors anisomycin, deacetylanisomycin, cephalotaxine, homoharringtonine, cycloheximide, puromycin and puromycin aminonucleoside. The IC50 values derived for ailanthinone and bruceantin, cycloheximide, homoharringtonine and puromycin were in the nanomolar range, whereas those for the anisomycins, cephalotaxine and the aminonucleoside of puromycin were micromolar or greater. Those drugs tested which contain an ester moiety (ailanthinone, bruceantin, anisomycin, homoharringtonine) were more active than the related drugs (chaparrin, deacetylanisomycin, cephalotaxine) that do not. Cross-resistance to inhibitors of protein synthesis appeared not to accompany resistance to chloroquine.

tRNA binding to programmed ribosomes increases the ribosomal affinity for tuberactinomycin O

The binding of 14C-labelled tuberactinomycin O was analysed in equilibrium dialysis cells. The ionic conditions and the concentration of the labelled drug used in the binding assays allowed the binding of just one drug molecule per non-programmed ribosome. Under these conditions, the occupation of the ribosomal P-site by deacylated tRNAPhe in the presence of poly(U) increased the amount of [14C]tuberactinomycin O bound by a factor of two. Kanamycin, gentamicin and neomycin reduced the binding of tuberactinomycin O, whereas chloramphenicol, tetracycline, streptomycin and puromycin had no effect. A stimulation of the binding of tuberactinomycin O was found upon addition of erythromycin.

Interactions of elongation factor 2 (EF-2) with guanine nucleotides and ribosomes. Binding of periodate-oxidized guanine nucleotides to EF-2

Interactions of rat liver elongation factor 2 (EF-2) with guanine nucleotides and ribosomes were studied by equilibrium dialysis and sedimentation methods. GDP (Kd = 0.5 microM) or GDP-Mg2+ (Kd = 1.57 microM) displayed a higher affinity in the formation of a binary complex with EF-2 than GTP (Kd = 2.68 microM), GTP-Mg2+ (Kd = 2.77 microM), or guanosine 5'-[beta, gamma-methylene]triphosphate (GuoPP[CH2]P) (Kd = 24.0 microM). NaIO4-oxidized guanine nucleotides (oGDP) (Kd = 38 microM) and oxidized/reduced guanine nucleotides (orGDP) (Kd = 27 microM) had lower affinites to the binding site on EF-2 than those of GDP or GTP. However, the binding of oGDP, oGTP or oGuoPP[CH2]P to EF-2 resulted in the formation of a stable product which could be recovered by the nitrocellulose filter technique or by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. In the presence of ribosomes and EF-2 the formation of a new binding site (or a different conformation of the binding site) with a higher affinity for GuoPP[CH2]P-Mg2+ (Kd = 0.26 microM) than fof GDP-Mg2+ (kd = 9.3 microM) became apparent. The presence of ribosomes thus appeared to favor the formation of a complex involving guanosine triphosphates. Adenosine diphosphate ribosylated EF-2 (ADP-Rib-EF-2) in its turn could bind to the ribosome with high affinity even without guanosine nucleotides (Kd = 0.18 microM). GuoPP[CH2]P increased to some extent the affinity of ADP-Rib-EF-2 for its ribosomal binding site (Kd = 0.05 microM).

Accessibility of ribosomal proteins to lactoperoxidase-catalyzed iodination following phosphorylation and during subunit interaction

Lactoperoxidase-catalyzed iodination was employed as a probe to monitor conformational change in 40-S ribosomal subunits from rat liver. Using this probe, it was observed that phosphorylation of protein S6 resulted in no detectable change in the iodination pattern of 40-S subunit proteins. These results suggest that the conformation of the small subunit remains unaltered following phosphorylation. On the other hand, the differences noted in the iodination pattern between 40-S ribosomal proteins derived from isolated subunits and those from 80-S monosomes, suggest that the 40-S subunit undergoes a conformational change during association with the 60-S subunit. Following 40-S and 60-S subunit association, proteins S2, S3, S5, S6, S8, S10 and S14 became less accessible to iodination. It is suggested that these proteins may be located at the interface between the 40-S and 60-S subunits.

Binding of [3H]narciclasine to eukaryotic ribosomes. A study on a structure-activity relationship

[3H]Narciclasine is a specific inhibitor of peptide bond formation on eukaryotic ribosomes and binds to 60-S ribosomal subunits. Binding of [3H]-narciclasine to yeast ribosomes is inhibited by many other inhibitors of peptide bond formation including anisomycin, several sequiterpene antibiotics (trichodermin, trichothecin, fusarenon X and verrucarin A) several Cephalotaxus alkaloids (harringtonine, homoharringtonine and isoharringtonine), several Amaryllidaceae alkaloids (pretazettine, haemanthamine, lycorine, pseudolycorine and dihydrolycorine) and the narciclasine derivatives trans-dihydronarciclasine, trans-dihydronarciclasine acetonide and isonarciclasine. Binding is also inhibited, although to a very small extent, by methylnarciclasine and cisdihydronarciclasine. In contrast, no inhibition of [3H]narciclasine binding was observed in the presence of certain other inhibitors of peptide bond formation including blasticidin S, gougerotin, sparsomycin and puromycin.

On the fine structure of polyribosomes

The ultrastructural morphology of ribosomes was studied in tissue sections of rat uterus using defferent fixatives (acrolein formaldehyde, acetic acid, methanol-acetic acid, OsO4) after various pre-incubations in solutions of different osmolarity, electrolyte content, and pH. In addition, whole-mount ribosomes of spread cytoplasm of epithelial cells were examined. The results indicate: Ribosomes are "thickenings" in the course of a cytoplasmic RNP-network which extends in three dimensions between nucleus and plasma membranes. This network consists of fibrils which vary in width depending on intrinsic and extrinsic factors. Often the fibril width is approx. 100 A. In places where the fibril is folded up into ribosomal "granules" and strands the width may be 300 A. Each fibril seems to be composed of two elementary filaments, 10 A up to 40 A thick. Estimation of filament length within a ribosomal "granule" allowed the conclusion that only a fraction of a 45 S-RNA filament is folded up into a "ribosome" in cytological preparations. No morphological evidence was found for clefts within ribosomes or single messenger RNA filaments "piercing" ribosomes. Filaments of membrane-bound ribosomes appearently show continuities with fibrils within the cytoplasmic matrix on the one side and with filaments within the cisternae of the endoplasmic reticulum on the other. Ribosomal filaments of stromal cells also seen to have filamentous continuities with extracellular fibrils. This morphological evidence opens new perspectives concerning the role of RNA filaments in protein synthesis.

Cooperative and antagonistic interactions of peptidyl-tRNA and antibiotics with bacterial ribosomes

There is a single-site interaction of [methylene-14C]thiamphenicol and [methylene-14C]chloramphenicol with run-off ribosomes with dissociation constants Kd = 6.8 micronM and Kd = 4.6 micronM respectively. Similar affinities for the antibiotics are observed in polysomes totally deprived of nascent peptides, or bearing nascent peptides on the A-site. However, two types of interaction are observed in endogenous polysomes with some ribosomes bearing nascent peptides on the P-site and other in the A-site. The lower-affinity bindings (dissociation constants Kd = 6.4 micronM and Kd = 1.5 micronM for thiamphenicol and chloramphenicol respectively) are due to the ribosomes bearing nascent peptides on the A-site. The higher-affinity bindings (dissociation constants Kd = 2.3 micronM and Kd = 1.5 micronM for thiamphenicol and chloramphenicol, respectively) are due to the ribosomes bearing nascent peptides on the P-site. Therefore binding of nascent peptides to the A-site does not affect the affinities of thiamphenicol and chloramphenicol for the ribosome. On the other hand interaction of the nascent peptides with the P-site of the ribosomes increases the affinities of both antibiotics for the ribosome. Thiamphenicol and chloramphenicol are thus good inhibitors of peptide bond formation in ribosomes and polysomes. Their affinities are increased precisely when the peptidyl-tRNA is placed in the P-site preceeding the peptide bond formation step, which is specifically blocked by the antibiotics. There is a single-site interaction per ribosome for [35S]thiostrepton, which does not appear to be affected by the attachment to the ribosomes of mRNA, tRNA and nascent peptides either to the A or the P-site. [N-methyl-14C]Lincomycin, [N-methyl-14C]erythromycin, [G-3H]streptogramin B and [G-3H]-streptogramin A bind to run-off ribosomes and polysomes totally free from nascent peptides. However, these antibiotics do not interact with ribosomes bearing nascent peptides either in the A or the P-site and therefore are not active on preformed polysomes. Thus lincomycin and streptogramin A only interact with free ribosomes and 50-S subunits and block the early rounds of peptide bond formation prior to polysome formation. Erythromycin and streptogramin B do not inhibit either initiation or the first round of peptide bond formation. However, erythromycin and streptogramin B, prebound to the ribosome, block peptide elongation probably by steric hindrance with the growing oligopeptide chain when this reaches a certain critical length.

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.

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.