Inhibition of peptide initiation on reticulocyte ribosomes by edeine

Bunyaviridae RdRps: structure, motifs, and RNA synthesis machinery

Bunyaviridae family is the largest and most diverse family of RNA viruses. It has more than 350 members divided into five genera: Orthobunyavirus, Phlebovirus, Nairovirus, Hantavirus, and Tospovirus. They are present in the five continents, causing recurrent epidemics, epizootics, and considerable agricultural loss. The genome of bunyaviruses is divided into three segments of negative single-stranded RNA according to their relative size: L (Large), M (Medium) and S (Small) segment. Bunyaviridae RNA-dependent RNA polymerase (RdRp) is encoded by the L segment, and is in charge of the replication and transcription of the viral RNA in the cytoplasm of the infected cell. Viral RdRps share a characteristic right hand-like structure with three subdomains: finger, palm, and thumb subdomains that define the formation of the catalytic cavity. In addition to the N-terminal endonuclease domain, eight conserved motifs (A-H) have been identified in the RdRp of Bunyaviridae. In this review, we have summarized the recent insights from the structural and functional studies of RdRp to understand the roles of different motifs shared by RdRps, the mechanism of viral RNA replication, genome segment packaging by the nucleoprotein, cap-snatching, mRNA transcription, and other RNA mechanisms of bunyaviruses.

Structural domains within the HIV-1 mRNA and the ribosomal protein S25 influence cap-independent translation initiation

The 5' leader of the HIV-1 genomic RNA is a multifunctional region that folds into secondary/tertiary structures that regulate multiple processes during viral replication including translation initiation. In this work, we examine the internal ribosome entry site (IRES) located in the 5' leader that drives translation initiation of the viral Gag protein under conditions that hinder cap-dependent translation initiation. We show that activity of the HIV-1 IRES relies on ribosomal protein S25 (eS25). Additionally, a mechanistic and mutational analysis revealed that the HIV-1 IRES is modular in nature and that once the 40S ribosomal subunit is recruited to the IRES, translation initiates without the need of ribosome scanning. These findings elucidate a mechanism of initiation by the HIV-1 IRES whereby a number of highly structured sites present within the HIV-1 5' leader leads to the recruitment of the 40S subunit directly at the site of initiation of protein synthesis.

The 5' untranslated region of the human T-cell lymphotropic virus type 1 mRNA enables cap-independent translation initiation

The human T-cell leukemia virus type 1 (HTLV-1) is a complex human retrovirus that causes adult T cell leukemia and of HTLV-associated myelopathy/tropical spastic paraparesis. The mRNA of some complex retroviruses, including the human and simian immunodeficiency viruses (HIV and SIV), can initiate translation using a canonical cap-dependent mechanism or through an internal ribosome entry site (IRES). In this study, we present strong evidence showing that like HIV-1 and SIV, the 5'-untranslated region (5'UTR) of the HTLV-1 full-length mRNA harbors an IRES. Cap-independent translational activity was evaluated and demonstrated using dual luciferase bicistronic mRNAs in rabbit reticulocyte lysate, in mammalian cell culture, and in Xenopus laevis oocytes. Characterization of the HTLV-1 IRES shows that its activity is dependent on the ribosomal protein S25 (RPS25) and that its function is highly sensitive to the drug edeine. Together, these findings suggest that the 5'UTR of the HTLV-1 full-length mRNA enables internal recruitment of the eukaryotic translation initiation complex. However, the recognition of the initiation codon requires ribosome scanning. These results suggest that, after internal recruitment by the HTLV-1 IRES, a scanning step takes place for the 40S ribosomal subunit to be positioned at the translation initiation codon.

IMPORTANCE

The mechanism by which retroviral mRNAs recruit the 40S ribosomal subunit internally is not understood. This study provides new insights into the mechanism of translation initiation used by the human T-cell lymphotropic virus type 1 (HTLV-1). The results show that the HTLV-1 mRNA can initiate translation via a noncanonical mechanism mediated by an internal ribosome entry site (IRES). This study also provides evidence showing the involvement of cellular proteins in HTLV-1 IRES-mediated translation initiation. Together, the data presented in this report significantly contribute to the understanding of HTLV-1 gene expression.

Cardio-ankle vascular index in heterozygous familial hypercholesterolemia

AIM

The cardio-ankle vascular index (CAVI) is a new non-invasive marker of arterial stiffness and atherosclerosis. The purpose of this study was to compare CAVI in patients with heterozygous familial hypercholesterolemia (FH) and in healthy controls.

METHODS

82 FH subjects (27 males, 65 females), aged 53.7±13.6 years without clinical symptoms of cardiovascular diseases and 359 healthy controls (121 males, 238 females), aged 43.9±14.9 years, were examined. CAVI was measured using the system VaSera® 1500.

RESULTS

CAVI in FH patients was significantly higher (8.0±1.4) than in healthy subjects (7.5±1.3) p = 0.002; however, age, sex and BMI adjusted CAVI did not differ significantly (p = 0.061) between the FH group (7.5, CI: 7.3; 7.7) and control group (7.7, CI: 7.6; 7.7).

CONCLUSION

The study showed no significant difference in CAVI between heterozygous FH and healthy controls.

Effect of endemic fluorosis on hematological parameters

Although so many studies exist on effect of fluoride on hematological parameters in experimental animals, a few studies have been conducted to investigate the effects of chronic fluorosis on hematological parameters in humans' subjects with endemic fluorosis. So we aimed to determine the hematological parameters in patients with endemic fluorosis. The study group consisted of 60 patients with endemic fluorosis (27 females, 33 males, and mean age 33.4 ± 9.6 years). An age-, gender-, and body mass index-matched control group was composed of 34 healthy volunteers (11 females, 23 males with a mean age 32.6 ± 5.6 years). Urine fluoride levels of fluorosis patients were significantly higher than control subjects as expected (0.42 ± 0.09 vs 1.92 ± 0.14 mg/l, respectively; P < 0.001). There were no statistically significant differences between the fluorosis group and control group with respect to hematological parameters (complete blood count and ferritin). We concluded that chronic fluorosis has no effect on hematological parameters in patients with endemic fluorosis.

The immunoregulatory effects of edeine analogues in mice

The edeines analogs were tested in several in vitro and in vivo assays using the mouse model, with edeine B (peptide W1) and cyclosporine A as reference compounds. The peptides displayed moderate, stimulatory effects on concanavalin A-induced (ConA-induced) splenocyte proliferation, whereas their effects on pokeweed mitogen-induced (PWM-induced) splenocyte proliferation were inhibitory. The peptides inhibited lipopolysacharide-induced (LPS-induced) tumor necrosis factor alpha production but had little effect on interleukin 6 production. In the model of the humoral immune response in vitro to sheep red blood cells, peptide 1 was distinctly stimulatory in the investigated concentrations (1-100 μg/ml), whereas peptides 3 and 4 only stimulated the number of antibody-forming cells at the highest concentration (100 μg/ml). In the model of the delayed type hypersensitivity in vivo to ovalbumin, the peptides were moderately suppressive (3 being the most active). The reference peptide W1 stimulated ConA-induced cell proliferation at 1–10 μg/ml but was inhibitory at 100 μg/ml. It also inhibited PWM-induced cell proliferation in a dose-dependent manner. This peptide had no effect on the humoral immune response in vitro or on cytokine production, but inhibited DTH reaction in vivo. The relationship between structure and activity, and a possible mode of action of the peptides, is discussed in this paper.

Structure activity relationship studies on the antimicrobial activity of novel edeine A and D analogues

Edeines are pentapeptide amide antibiotics composed of four nonprotein amino acids, glycine, and polyamine. They exhibit antimicrobial and immunosuppressive activities and are universal inhibitors of translation. Moreover, it was proven that the free ionizable carboxy group in the (2R, 6S, 7R)-2,6-diamino-7-hydroxyazelaic acid moiety is not essential for biological activity of these compounds. In this paper we describe the synthesis of four novel edeine A and D analogues in which the above-mentioned acid residue was replaced with the (3R, 4S)- or (3S, 4S)-4,5-diamino-3-hydroxypentanoic acid moiety. In one compound we also introduced into molecule the 3-N,N-dimethyl derivative of (S)-2,3-diaminopropanoic acid to prevent the transpeptidation process, which results in the loss of biological activity of alpha-isomers of edeines. All peptides were synthesized applying the active ester and azide methods and on the basis of the coupling of suitable N-terminal tripeptides with proper C-terminal dipeptide amides. The activities of the newly obtained edeine analogues against selected strains of bacteria and fungi are also presented.

Alternative ways to think about mRNA sequences and proteins that appear to promote internal initiation of translation

Translation of some mRNAs is postulated to occur via an internal initiation mechanism which is said to be augmented by a variety of RNA-binding proteins. A pervasive problem is that the RNA sequences to which the proteins bind were not rigorously proven to function as internal ribosome entry sites (IRESs). Critical examination of the evidence reveals flaws that leave room for alternative interpretations, such as the possibility that IRES elements might function as cryptic promoters, splice sites, or sequences that modulate cleavage by RNases. The growing emphasis on IRES-binding proteins diverts attention from these fundamental unresolved issues. Many of the putative IRES-binding proteins are heterogeneous nuclear ribonucleoproteins that have recognized roles in RNA processing or stability and no recognized role in translation. Thus the mechanism whereby they promote internal initiation, if indeed they do, is not obvious. Some recent experiments were said to support the idea that IRES-binding proteins cause functionally important changes in folding of the RNA, but the evidence is not convincing when examined closely. The proteins that bind to some (not all) viral IRES elements include a subset of authentic initiation factors. This has not been demonstrated with any candidate IRES of cellular origin, however; and even with viral RNAs, the required chase experiment has not been done to prove that a pre-bound initiation factor actually mediates subsequent entry of ribosomes. In short, the focus on IRES-binding proteins has gotten us no closer to understanding the mechanism of internal initiation. Given the aforementioned uncertainty about whether other mechanisms (splicing, cryptic promoters) might underlie what-appears-to-be internal initiation, a temporary solution might be to redefine IRES to mean "internal regulatory expression sequence." This compromise would allow the sequences to be used for gene expression studies, for which they sometimes work, without asserting more than has been proven about the mechanism.

Detection of early gene expression changes during activation of human primary lymphocytes by in vitro synthesis of proteins from polysome-associated mRNAs

The rapid increase in protein synthesis during the mitogenic stimulation of human peripheral blood lymphocyte is the result of global and specific translational control mechanisms. To study some of these mechanisms, we examined the in vitro translatability of mRNAs associated with the polyribosome fraction. Polyribosome fractions were isolated from lymphocytes after activation with ionomycin and the phorbol ester PMA. The associated PAmRNAs were translated in the presence of mRNA-depleted rabbit reticulocyte lysate and [(35)S]Met, and the protein products were analyzed by SDS--PAGE and autoradiography. There was little synthesis of protein from the PAmRNAs isolated from unactivated T cells, but the PAmRNAs isolated from activated T cells showed a rapid increase in translatability. Translation of the PAmRNAs was sensitive to edeine and m7GTP, suggesting their cap-dependent translation. With activation, the majority of proteins showed increasing in vitro translation, but two proteins, p72 and p33, were found to have increased synthesis within 30 min, which decreased in 1 h. Transcription inhibitors were used to ascertain if regulation of their expression was transcriptional or translational. To identify these proteins, we used biotinylated lysine during the in vitro translation reaction, and we extracted the biotinylated protein by using streptavidin magnetic beads. The protein product was analyzed by mass spectrometry. p33 was identified as a prohibitin-like protein (BAP37), but the identification of p72 was not found in the databases. The distinct up-regulation and down-regulation of their protein expression suggest their tightly controlled regulation during early T cell activation.

Biochemical and kinetic characteristics of the interaction of the antitumor antibiotic sparsomycin with prokaryotic and eukaryotic ribosomes

Using 125I-labeled phenol-alanine sparsomycin, an analogue of sparsomycin having higher biological activity than the unmodified antibiotic, we studied the requirements and the characteristics of its interaction with the ribosome. The drug does not bind to either isolated ribosomal subunits or reconstituted whole ribosomes. For sparsomycin binding to 70S and 80S ribosomes, the occupation of the peptidyltransferase P-site by an N-blocked aminoacyl-tRNA is a definitive requirement. The sparsomycin analogue binds to bacterial and yeast ribosomes with Ka values of around 10(6) M-1 and 0.6 x 10(6) M-1, respectively, but its affinity is probably affected by the character of the peptidyl-tRNA bound to the P-site. Chloramphenicol, lincomycin, and 16-atom ring macrolides compete with sparsomycin for binding to bacterial ribosomes, but streptogramins and 14-atom ring macrolides do not. Considering the reported low affinity of puromycin for bacterial ribosomes, this antibiotic is also a surprisingly good competitor of sparsomycin binding to these particles. In the case of yeast ribosomes, blasticidin is a relatively good competitor of sparsomycin interaction, but anisomycin, trichodermin, and narciclasin are not. As expected, puromycin is a poor competitor of the binding in this case. The results from competition studies carried out with different sparsomycin analogues reveal, in some cases, a discrepancy between the drug ribosomal affinity and its biological effects. This suggests that some intermediate step, perhaps a ribosomal conformational change, is required for the inhibition to take place.

mRNA poly(A) tail, a 3' enhancer of translational initiation

To evaluate the hypothesis that the 3' poly(A) tract of mRNA plays a role in translational initiation, we constructed derivatives of pSP65 which direct the in vitro synthesis of mRNAs with different poly(A) tail lengths and compared, in reticulocyte extracts, the relative efficiencies with which such mRNAs were translated, degraded, recruited into polysomes, and assembled into messenger ribonucleoproteins or intermediates in the translational initiation pathway. Relative to mRNAs which were polyadenylated, we found that nonpolyadenylated [poly(A)-]mRNAs had a reduced translational capacity which was not due to an increase in their decay rates, but was attributable to a reduction in their efficiency of recruitment into polysomes. The defect in poly(A)- mRNAs affected a late step in translational initiation, was distinct from the phenotype associated with cap-deficient mRNAs, and resulted in a reduced ability to form 80S initiation complexes. Moreover, poly(A) added in trans inhibited translation from capped polyadenylated mRNAs but stimulated translation from capped poly(A)- mRNAs. We suggest that the presence of a 3' poly(A) tail may facilitate the binding of an initiation factor or ribosomal subunit at the mRNA 5' end.

mRNA poly(A) tail, a 3' enhancer of translational initiation

To evaluate the hypothesis that the 3' poly(A) tract of mRNA plays a role in translational initiation, we constructed derivatives of pSP65 which direct the in vitro synthesis of mRNAs with different poly(A) tail lengths and compared, in reticulocyte extracts, the relative efficiencies with which such mRNAs were translated, degraded, recruited into polysomes, and assembled into messenger ribonucleoproteins or intermediates in the translational initiation pathway. Relative to mRNAs which were polyadenylated, we found that nonpolyadenylated [poly(A)-]mRNAs had a reduced translational capacity which was not due to an increase in their decay rates, but was attributable to a reduction in their efficiency of recruitment into polysomes. The defect in poly(A)- mRNAs affected a late step in translational initiation, was distinct from the phenotype associated with cap-deficient mRNAs, and resulted in a reduced ability to form 80S initiation complexes. Moreover, poly(A) added in trans inhibited translation from capped polyadenylated mRNAs but stimulated translation from capped poly(A)- mRNAs. We suggest that the presence of a 3' poly(A) tail may facilitate the binding of an initiation factor or ribosomal subunit at the mRNA 5' end.

Interaction of tRNA with the A and P sites of rabbit-liver 80S ribosomes and their 40S subunits

The interaction between tRNA and rabbit liver 80S ribosomes and 40S subunits was studied using a nitrocellulose membrane filtration technique. Binding of the different tRNA forms (aminoacyl-, peptidyl- or deacylated) to poly(U)-programmed 40S subunits and 80S ribosomes was found to be a cooperative process. The association constants of AcPhe-TRNA(Phe) for the A and P sites of 80S ribosomes and the cooperativity constant were measured at different temperature and Mg2+ concentration. The AcPhe-tRNA(Phe) association constant for the P site was shown to be between 2 x 10(7) M-1 and 2 x 10(8) M-1 at 25-37 degrees C and 5-20 mM Mg2+, while the affinity for the A site was 10-100-fold lower. The cooperativity constant was shown to decrease with the increase of incubation temperature and the decrease of Mg2+ concentration. The affinity of AcPhe-tRNA(Phe) for the A site of 80S ribosomes was shown to depend upon the codon specificity of tRNA at the P site. The cooperativity of the tRNA interaction with 80S ribosomes was suggested to be mostly contributed by the association with the 40S subunit and result from the correct codon-anticodon pairing at the P site. The data presented imply a codon-anticodon interaction at the P site of eukaryotic 80S ribosomes.

Effect of subacute dosage of fluoride on male mice

A sublethal concentration (one-tenth of the LD50) of fluoride (F) (5.2 mg F/kg body weight) was administered to Swiss albino mice (male) daily for 35 days. These mice showed a decrease in body weight gain, and food and water consumption. A significant decrease in red blood cell counts and an increase in white blood cell counts were seen in fluoride-administered mice. These animals also showed a decline in albumin, total protein, cholesterol, glucose and alkaline phosphatase activity in the serum. The fluoride content significantly increased in different organs of these animals. Sperm did not show any abnormalities due to fluoride toxicity.

Shiga toxin from Shigella dysenteriae 1 inhibits protein synthesis in reticulocyte lysates by inactivation of aminoacyl-tRNA binding

Inhibition of the peptide elongation cycle of eukaryotic protein synthesis by Shiga toxin from Shigella dysenteriae 1 was examined in toxin-treated reticulocyte lysate mixtures. Peptidyl transferase activity of toxin-treated ribosomes was measured by following the decrease in peptidyl-tRNA concentrations when puromycin was added after incubation with toxin. Concentrations of [3H]leucine-labeled peptidyl-tRNA were measured by extraction with cetyltrimethylammonium bromide. The data suggest that Shiga toxin inhibited aminoacyl-tRNA binding. Toxin-treated ribosomes retained peptidyl transferase activity, and toxin did not block translocation. Furthermore, no inhibition of initiation of protein synthesis could be observed. Finally, Shiga toxin had no detectable nucleolytic effect on polysomal 28S rRNA, nor was hydrolysis of 5.8S or 5S rRNA observed.

Increased ribosomal affinity for mRNA causes resistance to edeine in a mutant of Saccharomyces cerevisiae

The effect of edeine on the translation of mRNA or poly(U)-directed polyphenylalanine synthesis has been studied in an edeine-resistant mutant of Saccharomyces cerevisiae under three different experimental conditions: in the whole lysate system, in a micrococcal-nuclease-treated lysate, and in a high-salt-treated lysate. The results indicate that translation of messenger is more resistant to edeine in the whole lysate than in the depleted lysates; these observations suggest that resistance to edeine is associated with the presence of endogenous mRNA. It is shown that 40S mutant subunits have a higher affinity for polysomal RNA than 40S wild-type subunits. Since the mRNA binding is inhibited by 7-methylguanosine 5'-monophosphate, the interaction between polysomal RNA and 40S ribosomes is specific for mRNA. The data demonstrate that in each of the depleted lysates, with edeine initially present, the formation of the 80S initiation complex is inhibited. However, edeine inhibition of [3H]methionine binding to 80S ribosomes is overcome completely in the mutant extract by preincubation of this lysate with polysomal RNA. The results indicate that the mutant may carry a specific change in a messenger-binding factor or in a ribosomal protein thereby permitting an increased stability of the messenger-ribosome complex which consequently results in an increased resistance of the mutant lysate to edeine.

Differential inhibition of protein synthesis: a possible biochemical mechanism of thalidomide teratogenesis

A theory concerning the chemical and biochemical mechanisms of thalidomide teratogenesis is presented. A considerable body of evidence suggests that the glutarimide ring of thalidomide may exert its biological activity because of its resemblance to the imide pyrimidines thymine and uracil. In addition to the glutarimide ring, thalidomide contains a moderately reactive phthalimide moiety, which allows the spontaneous formation of various glutarimide derivatives in fetal tissues. A model is proposed in which the phthalimide group reacts with small nucleophiles, most likely the polyamines, to produce a derivative(s) having a similar biochemical potential to that of cycloheximide, a glutarimide which is a powerful inhibitor of the elongation phase of protein synthesis. Interference in the elongation phase results in the selective inhibition of the translation of messages which have a high translational efficiency. Evidence is reviewed concerning the differential inhibition or protein synthesis by cycloheximide and the effects of this inhibition on various biochemical and biological processes which are critical during development and differentiation. A similar biochemical activity by the putative thalidomide derivative(s) could explain its extreme teratogenic potential. A number of parallels between the biological effects of thalidomide and cycloheximide are discussed which support the idea that a similar biochemical activity is involved. The theory readily explains many of the observed biological effects of thalidomide including the large difference between fetal and adult toxicity. In addition, evidence is reviewed which suggests that the teratogenic properties of a number of drugs which are structurally related to thalidomide may have a common chemical basis due to the similarity of their imide core structures to thymine and uracil.

An edeine resistant mRNA-dependent protein synthesis system from a Saccharomyces cerevisiae mutant

A cell-free protein synthesizing system from a mutant of Saccharomyces cerevisiae translated exogenous mRNA in the presence of 2 microM edeine, while a similar system from wild-type strain was completely inhibited by the drug. The mutant ribosomes showed an affinity for [125I]edeine comparable to the wild-type ribosomes, thereby suggesting that these macromolecules alone were not responsible for the edeine-resistant capacity of the mutant.

Assembly in vitro of a spanning membrane protein of the endoplasmic reticulum: the E1 glycoprotein of coronavirus mouse hepatitis virus A59

The E1 glycoprotein of coronavirus mouse hepatitis virus A59 was synthesized in vitro by translation of viral mRNA in the presence of dog pancreatic microsomes. Its disposition in the membrane was investigated by digestion with proteases and by selective NH2-terminal labeling. The protein spans the membrane, but only small portions from the NH2 and COOH terminus are exposed respectively in the lumenal and cytoplasmic domains; the bulk of the molecule is apparently buried in the membrane. The protein lacks a cleavable leader sequence and does not acquire its characteristic O-linked oligosaccharides in rough microsomes. It may enter the membrane at any stage during synthesis of the first 150 amino acid residues. These unusual features of the protein might help to explain why it is not transported to the cell surface in vivo but remains in intracellular membranes, causing the virus to bud there.

Modulation of protein synthesis in a cell-free system derived from rat brain by corticotropin (ACTH), magnesium, and spermine

Modulation of protein synthesis by fragments of the ACTH molecule has been studied in a cell-free system obtained from subcortical brain tissue of rats. Both the activity of the protein-synthesizing system and its sensitivity to ACTH-like peptides appeared to be highly dependent on the Mg2+ and spermine concentrations. At optimal Mg2+ concentrations (4 mM) the peptide sequences ACTH(1-24) and (11-24) were both inhibitory, the latter being the more active. The inhibitory effect was reduced or abolished at higher (suboptimal) concMg2+ concentrations. Spermine, like ACTH, inhibited protein synthesis at the optimal Mg2+ concentration. However, at lower Mg2+ concentrations spermine had a stimulatory effect and maximal activity was obtained at 0.75-1.0 mM Mg2+. In the presence of spermine (60 microM) and Mg2+ (0.75 mM), a half-maximal inhibition of protein synthesis was obtained with a peptide concentration of 5 microM. A structure-activity study showed that the peptides ACTH(7-16)-NH2, (11-24), (5-18, 17Lys 18Lys)-NH2 and (15-24) were active in inhibiting protein synthesis, whereas the fragments ACTH(1-16)-NH2 and (17-24) were inactive. The results are discussed in terms of an interaction between ACTH, Mg2+, and spermine, and intracellular processes involved in protein synthesis.

Quantitative study of interaction of deacylated tRNA with Escherichia coli ribosomes. Role of 50 S subunits in formation of the E site

The 30 S subunit contains 2 sites for tRNA binding (Phe-tRNA, AcPhe-tRNA, tRNAPheOH) with the functional properties of D and A sites of the 70 S ribosome after attachment of 50 S subunit. The third (E) site specific for deacylated tRNA is introduced into 70 S ribosome by its 50 S subunit. The E-site binding of tRNAPheOH is not sensitive to either tetracycline and edeine, and practically codon-independent. The affinity constant of tRNAPheOH for the E site is 2-3 orders of magnitude lower than that for the D site.

Inhibition of eukaryotic protein chain initiation by vanadate

Vanadate inhibits protein chain initiation in rabbit reticulocyte lysates. The evidence that supports this conclusion is as follows: (i) the biphasic kinetics of inhibition in which protein synthesis is maintained at the control rate for 1-2 min is followed by an abrupt decline in the rate of synthesis; (ii) inhibition is associated with a marked disaggregation of polyribosomes and a concomitant increase in 80S ribosomes; and (iii) vanadate concentrations that inhibit protein chain initiation do not inhibit polypeptide chain elongation or the aminoacylation of tRNA. In partial reactions of protein chain initiation, vanadate concentrations that inhibit protein synthesis have no detectable effect on the formation of eukaryotic initiation factor eIF-2-promoted ternary complex with Met-tRNAf and GTP and on the assembly of 40S ribosomal subunit-Met-tRNAf complexes. On the addition of mRNA, the 40S ribosomal subunit-Met-tRNAf complexes also are transformed into 80S ribosome-mRNA-Met-tRNAf complexes, termed 80S initiation complexes. In vanadate-treated samples, however, these 80S initiation complexes are defective and unable to proceed beyond this step.

Cytoplasmic polyadenylate processing events accompany the transfer of mRNA from the free mRNP particles to the polysomes in Physarum

The relationship between the mRNA in the polysomes and the free cytoplasmic messenger ribonucleoprotein of Physarum polycephalum was studied by microinjection techniques. Labeled free cytoplasmic ribonucleoprotein, prepared from donor plasmodia, was microinjected into unlabeled host plasmodia, and its fat was followed in the host ribonucleoprotein particles. Approximately one-half of the poly(A)-containing RNA [poly(A)+RNA] that originated from the microinjected particles was incorporated into the host polysomes by normal translational processes within 1 hr. Very short poly(A) sequences (approximately 15 nucleotide residues) were found in these poly(A)+RNA molecules. These short poly(A) sequences were sensitive to digestion with micrococcal nuclease, suggesting that they were not associated with protein. Because the poly(A)+RNA molecules of the microinjected free cytoplasmic mRNP had originally contained poly(A) sequences 50-65 nucleotides long and were associated with protein extensive poly(A) degradation and poly(A).protein complex dissociation must have occurred during their incorporation into the polysomes or during their translation. These results demonstrate a precursor-product relationship between free cytoplasmic mRNP and polysomal mRNA and suggest that the incorporation process in Physarum is accompanied by structural modifications in the poly(A) region of mRNA. They also imply that the polysome is a site for disruption of the poly(A).protein complex and poly(A) degradation.

Regulation of protein synthesis in mitotic HeLa cells

Mitotic HeLa cells (M cells) synthesize protein at about 25% of the rate of S phase cells. This decrease in protein synthesis is due to a reduction in the rate of initiation. However, extracts prepared from M cells are almost as active in protein synthesis as S cell extracts. Both cell extracts are quite active in in vitro initiation of protein synthesis. Moreover, two steps in initiation, binding of Met-tRNAf to 40S ribosomal subunits and binding of mRNA to ribosomes, show similar activity in both extracts. The difference in protein synthesizing activity observed in vivo is largely eliminated in the preparation of cell-free systems. The ribosomes of M cells contain small mol wt RNA, which inhibits protein synthesis in vitro. This RNA, which has possibly a nuclear origin, may be a cause of the reduction in the rate of protein synthesis in M cells.

The mode of action of alpha sarcin and a novel assay of the puromycin reaction

A new technique was developed for measuring the amount of peptidyl-tRNA in a protein-synthesizing system in vitro. By this technique the course of the puromycin reaction may be followed and the modes of action of various inhibitors of protein synthesis readily determined. We conclude that the polypeptide alpha sarcin inhibits the binding of aminoacyl-tRNA into the ribosomal 'A' site, that sparsomycin inhibits the peptidyl transferase reaction and that cycloheximide may block translocation.

Regulation of protein synthesis in rabbit reticulocyte lysates: characteristics of inhibition of protein synthesis by a translational inhibitor from heme-deficient lysates and its relationship to the initiation factor which binds Met-tRNAf

In heme-deficient reticulocyte lysates a translational inhibitor which regulates protein synthesis is formed or activated. To define the mechanism of action of the translational inhibitor (RI), RI was partially purified. We have utilized the isolated RI to examine its relationship to the translational inhibitor formed in situ in heme-deficiency, some quantitative aspects of inhibition of protein synthesis, and the relationship of RI concentration to the initiation factor (IF-MP) which forms a ternary complex with Met-tRNAf and GTP (IF-MP-Met-tRNAf-GTP). The results demonstrate that the activity of isolated RI is related to the in situ heme-deficiency inhibitor by several criteria: (a) the biphasic kinetics of inhibition manifested by RI in lysates containing optimal levels of hemin are very similar to those observed in heme-deficiency, i.e., an initial period in which several rounds of protein synthesis proceed at the control rate followed by an abrupt decline in the rate of protein synthesis. (b) Both inhibitions are accompanied by the disaggreagation of polyribosomes with a concomitant increase in 80S ribosomes. (c) Both inhibitions are reversed by IF-MP. The isolated RI blocked protein synthesis in lysates at temperatures ranging from 15 degrees to 30 degrees. Although the rate of protein synthesis was a function of the temperature of incubation, the number of rounds of protein synthesis prior to shut-off was essentially the same at various temperatures. When RI was added to lysates, at increasing intervals after the start of incubation, the period of synthesis before shut-off (lag) progressively decreased. The inhibition of protein synthesis by RI was immediately reversed by the addition of IF-MP. The extent of reversal increased with increasing concentrations of IF-MP; at low levels of RI almost complete reversal of inhibition by IF-MP was obtained. However, at high levels of RI which did not appreciably increase the degree of inhibition of protein synthesis, equivalent amounts of IF-MP were less effective in reversing inhibition. These results suggest that the inhibition of protein synthesis by the isolated inhibitor involves the initiation factor IF-MP.

The binding sites for tRNA on eukaryotic ribosomes

We have studied the non-enzymic binding of phe-tRNA to ribosomes from rat liver using deacylated tRNA to inhibit binding to the P-site and puromycin (5 x 10-minus3M) to inhibit binding to the A-site. We conclude that at a low concentration of magnesium ions (10mM) phe-tRNA is bound only at the A-site of 80S irbosomes, whereas at a high concentration of magnesium ions (40mM) phe-tRNA is also bound at the P-site. Studies with edeine indicate that, during non-enzymic binding of phe-tRNA, eukaryotic ribosomes (in contrast to prokarotic ribosomes) have the A-site of the 60S subunit and the initiation site of the 40S subunit juxtaposed. This may account for the differences observed, in formation of diphenylalanyl-tRNA and phenylalanyl-puromycin, between phe-tRNA bound non-enzymically to the P-sites of eukaryotic and prokaryotic ribosomes.