On the mechanism of inhibitory effect of violamycin antibiotics on the transcription by bacteriophage T3-induced RNA polymerase

The effect of three components of the anthracycline antibiotic violamycin on the transcription of bacteriophage T3 DNA by bacteriophage T3-induced RNA polymerase has been investigated in a cell-free system. The glycosides of violamycin BI possess the highest inhibitory activity, whereas those of violamycin BII and violamycin A show a reduced inhibitory effect. Concentrations of violamycin BI depressing the incorporation of (3H)UMP into RNA chains have only a slight effect on the binding of the T3 RNA polymerase to T3 DNA and on the incorporation of GTP as the first nucleotide. This shows that the primary target of the antibiotic is not the initiation of the RNA synthesis. The binding of violamycin BI to T3 DNA causes a strong reduction of the elongation rate of the RNA chains.

Does the capacity of ribosomes to control translation fidelity change with age?

The stress of increased magnesium concentration and of antibiotic action was used to test for changes in the capacity of ribosomes to maintain the fidelity of translation in protein synthesis during aging. Elongation fidelity of ribosomes isolated from livers of young adult (6-10 months, n=9) and old (23-24 months, n=11) rats was assessed for poly(U) message through the error co-incorporation of leucine (Leu) with phenylalanine (Phe), using controlled rate measurements. No statistically significant difference in the error response of ribosomes from young and old animals was detected on manipulating magnesium. The maximum Phe and Leu rates occurred at approximately 8 and approximately 11 mM Mg2+, respectively; the relative error frequency (Leu/Phe ratio) at the Leu maximum was approximately 2%. The aminoglycoside paromomycin (PM) elicited a considerably higher degree of error incorporation: 12-14% Leu/Phe in the range of the Leu rate maximum, 40-60 microM paromomycin, at 9 mM Mg2+. A statistically significant increase was detected in the error frequency for ribosomes from old as compared with young animals (at p less than 0.02) between 20 and 60 microM paromomycin, the largest increase (approximately 9%) occurring at the Leu rate maximum. Also, the ribosomes from the old animals, in terms of the error frequency, were found to be significantly more sensitive to paromomycin concentration (at p less than 0.02) up to the Leu maximum. These results indicate that liver ribosomes of old rats may have a slightly decreased capacity to maintain fidelity, which can be expressed under particular conditions.

Changes in energy charge and block of protein synthesis in rabbit reticulocytes under the action of valinomycin

Valinomycin blocks protein synthesis at the elongation stage in intact reticulocyte while being without action on a cell free system. We demonstrate here that this membrane mediated inhibition is accompanied by a change of the energy charge (ATP/ADP/AMP ratio) of the cells. There seems to be an accumulation of AMP without a proportional gain in ADP. The change in energy charge in itself can be the cause of the inhibition of protein synthesis. We show also that valinomycin can cause a decrease of the energy charge at concentrations where its action as an ionophore is not seen and independent of a possible activation of the Na-K dependent ATPase.

Mechanism of translocation: effect of cognate transfer ribonucleic acids on the binding of AUGUn to 70S ribosomes

We try to mimic the unidirectional sliding-type movement of the PP-tRNA . mRNA complex with respect to the ribosome by looking at the effect of different combinations of cognate tRNAs on the stability of the 70S-AUGUn complex. The association constant for the binary complex 70S-AUGU3 was determined as 6.8 x 10(5) M-1. Addition of tRNAfMet resulted in a 67-fold increase in the association constant, which with both cognate tRNAs is revised to Kassoc = 2.2 x 10(8) M-1. Increasing the chain length of the oligonucleotide from AUGU3 to AUGU13 did not further raise the association constant. The data indicate that the stability of the 70S ribosome . mRNA interaction is governed by the presence of the cognate tRNAs and is topographically restricted to the decoding domains. Since a peptidyl group in the tRNA increases the affinity of AUGU3 for the ribosome by up to 15-fold, we conclude that the affinity of the peptidyl transfer center for the peptidyl moiety pulls the PP-tRNA . mRNA complex from the A (aminoacyl-tRNA) site to the P (peptidyl-tRNA) site. EF-G . GTP or EF-G . GMPPCP 5'-(beta, gamma-methylene)triphosphate] displace tRNAfMet from the quaternary complex 70S . AUGUn . tRNAfMet . tRNAPhe (n = 3 and 6) at Mg2+ less than 25 mM. From the amount of EF-G . GTP bound to a 70S ribosome, it follows that the elongation factor replaces the deacylated tRNA in a stoichiometric way. These data indicate that the EF-G . GTP-dependent release of the deacylated tRNA from the P site, followed by removal of EF-G . GDP from the 50S subunit, is sufficient to trigger the translocation of the mRNA . PP-tRNA complex.

Toxicity of diphtheria toxin-related proteins produced by suppression of nonsense mutations

The Mr = 62,000 diphtheria toxin-related proteins produced from the suppression of nonsense mutations within the tox gene of corynephage beta were purified by affinity chromatography. Except for the toxin 111-sup2-62, the Mr = 62,000 polypeptides were found to have the same specific toxicity as does wild type toxin. 111-sup2-62 was found to have a prolonged lag period prior to the onset of inhibition of protein synthesis and ADP-ribosylation of elongation factor 2. 111-sup2-62 differs from wild type toxin by an amino acid substitution at a site approximatley 47,000 daltons from the NH2 terminus. The data presented provide genetic support for the Boquet-Pappenheimer model (Boquet, P., and Pappenheimer, A. M. Jr (1978) J. Biol. Chem. 251, 5770-5778) of fragment A translocation into the eukaryotic cell cytosol.

Block in the elongation of protein synthesis in rabbit reticulocyte by action of the ionophore valinomycin

In this paper we show that ribosomes isolated from cells inhibited for protein synthesis by the antibiotic ionophore Valinomycin can still support incorporation of [14C]-leucine into polypeptides in a cell-free system. The extent of this functional integrity depends upon the concentration of Valinomycin used and whether it acted as an ionophore or not. We demonstrate that the antibiotic acts on protein synthesis at the level of elongation and has no action either at the initiation level or as activating an RNase. Moreover, we show that it can inhibit protein synthesis at concentrations where its action as an ionophore cannot be detected.

Peptide elongation in rat kidney after cadmium administration

Rats received two injections (each 2.6 mg/kg body wt.) of CdCl2, and the kidneys were removed 24 h later. Postmicrosomal supernatant fractions of the homogenized kidneys were used as a source of elongation factors 1 and 2 in assays for [14C]phenylalanyl-tRNA binding to ribosomes and for peptide-bond synthesis. After purification of these preparations by precipitation with (NH4)2SO4 and gel filtration on Sephadex G-200 and G-100, elongation factor 1 activity was significantly increased. A significant increase in the activity of purified elongation factor 2 was also found. The results are discussed in relation to the reported effects of CdCl2 and of HgCl2 on renal tissue.

Glycopeptides from brain inhibit rates of polypeptide chain elongation

In previous reports, we have identified cell-surface glycopeptides from mouse cerebrum (BCSG) that inhibited protein synthesis and mitosis in several cell types. When baby hamster kidney (BHK)-21 cells were infected with vesicular stomatitis virus (a negative strand RNA virus), BCSG extensively inhibited viral protein synthesis. This inhibition was effective against both protein and glycoprotein synthesis and was independent of amino acid uptake by infected cells, synthesis of viral RNA, and degradation of viral proteins. Analysis of polyribosome profiles in uninfected BHK-21 cells indicated that the degree of cellular protein synthesis inhibition could not be attributed to activation of RNase or solely to a disruption of chain initiation. When added directly to a cell-free protein-synthesizing system derived from BHK-21 cells, BCSG was ineffective, but if the inhibitory material was first allowed to react with cells, cell-free protein synthesis was substantially reduced. When BCSG were reacted with cells for 5 min at 0 degrees C, the cells tested, BHK-21 (a BCSG-sensitive line) and murine fibrosarcoma 2237 (a BCSG-insensitive line), both effectively adsorbed the inhibitor from the medium.

Action of streptothricin F on ribosomal functions

The effect of streptothricin F on elongation factor-dependent and on elongation factor-free translation systems was studied. Streptothricin F inhibits factor-dependent as well as factor-free polypeptide synthesis. The results suggest that streptothricin F inhibits polypeptide synthesis via interaction with the ribosome. In partial reactions streptothricin F impairs EF-G-dependent translocation and to a lesser extent EF-Tu-dependent binding of aa-RNA to the ribosome, while it does not affect peptide bond formation significantly.

Hygromycin A, a novel inhibitor of ribosomal peptidyltransferase

In cell-free systems from Escherichia coli, hygromycin A inhibits polypeptide synthesis directed by either poly(U) or phage R 17 RNA, and the reaction of puromycin with either natural peptidyl-tRNA, or AcPhe-tRNA, or the 3'-terminal fragment of AcLeu-tRNA (C-A-C-C-A-LeuAc). In contrast, the antibiotic does no inhibit the enzymatic binding of Phe-tRNA to ribosomes or the translocation of AcPhe-tRNA. It is concluded that hygromycin A is a specific inhibitor of the peptide bond formation step of protein synthesis. The action of hygromycin A on peptidyl transfer is similar to that of chloramphenicol, an antibiotic that shares some common structural features with hygromycin A. Both antibiotics inhibit the binding of C-A-C-C-A-Leu to the acceptor site of peptidyl transferase and stimulate that of C-A-C-C-A-LeuAc to the donor site of the enzyme. Moreover, hygromycin A blocks the binding of chloramphenicol to ribosomes, indicating that the binding sites of the antibiotics may be closely related. Hygromycin A is a more potent agent than chloramphenicol and binds quite strongly to ribosomes.

Variable rate of polypeptide chain elongation in vitro. Effect of spermidine

Translation of rabbit globin mRNA and tobacco mosaic virus mRNA was studied in a wheat germ system. After a short pulse with labeled methionine, the reaction products were separated by polyacrylamide gel electrophoresis in sodium dodecyl sulfate and visualized by fluorography. Discrete bands corresponding to incomplete products were detected both with globin and tobacco mosaic virus RNA as messenger at a time when no release of peptides from tRNA could be detected. In the absence of spermidine certain intermediate products accumulated during the chase period. With increasing incubation time, the radioactivity of most bands was chased into higher-molecular-weight products. This change in size distribution was more pronounced when magnesium was partially replaced by spermidine. Evidence is presented that some of the incomplete products had in part been released from tRNA. No such release of polypeptides could be detected in experiments using the artificial messenger poly(U), indicating that non-specific peptidyl-tRNA hydrolases were not present in the system. The transient accumulation of discrete bands during the chase period indicates that in the wheat germ system polypeptide chain elongation occurs at discontinuous rates. The fact that spermidine increases the rate of elongation and the yield of full-length translation products may be due to its ability to facilitate the movement of the ribosomes beyond regions where the elongation is retarded.

Inhibition of elongation factor 2-dependent translocation by the pokeweed antiviral protein and ricin

The pokeweed antiviral protein and the toxin ricin are two plant proteins which enzymatically inactivate eukaryotic ribosomes and cause the inhibition of the elongation cycle of protein synthesis. Their mode of inhibition of protein synthesis has been attributed to either the inhibition of enzymatic binding of aminoacyl-tRNA to the acceptor site of the ribosome or its subsequent translocation to the donor site. In this communication, we show that the treatment of ribosomes with either the pokeweed antiviral protein or ricin results in the inhibition of the elongation factor 2-catalyzed translocation of acceptor site aminoacyl-tRNA to the donor site. The inhibitory effects of both proteins are identical and are partially reversed by excess concentrations of elongation factor 2.

Specificity of protein synthesis inhibitors in the inhibition of encephalomyocarditis virus replication

Effect of protein synthesis inhibitors on encephalomyocarditis virus production in L-cells was studied. Inhibition of initiation by hypertonicity, harringtonine, or pactamycin decreased viral protein synthesis to a lesser extent than that of host. Virus yield was unaffected or actually enhanced by low concentrations of these inhibitors. On the contrary, the elongation inhibitors cycloheximide, anisomycin, and emetine, shown previously to inhibit viral protein synthesis preferentially, had a greater effect on virus yield than on overall protein synthesis. These results support our earlier proposal that the antiviral activity of cycloheximide derives from its specific effect on the rate of elongation of protein synthesis, and that elongation inhibitors in general may show varying degrees of specific antiviral activity.

Marker segregation without chromosome loss at the emt locus in Chinese hamster cell hybrids

Segregation, defined as the reexpression of the recessive phenotype, has been examined in Chinese hamster cell hybrids heterozygous at the recessive emtr locus. Segregants were selected in emetine, and the role of chromosome loss in the segregation process was evaluated by detailed karyotype comparison of segregants with their hybrid parent. In emtr CHO x emt+ CHO hybrids (CHO is thought to be hemizygous at the emt locus), segregants were obtained at high frequency, and no consistent chromosome loss was found in the segregants. In hybrids made with Emtr CHO and wild-type lines other than CHO (CHW,CHL,V-79), where two wild-type alleles are thought to be present in the hybrid, segregants were obtained at much lower frequency, consistent with a two-step segregation process. These segregants revealed consistent loss of one chromosome 2 or deletion of a part of the long arm of a chromosome 2. Thus, one step in segregation seems to be chromosome loss while the other step must have a different mechanism, possibly the same mechanism that operates in the CHO x CHO hybrids. Two major conclusions can be drawn: (1) the emt gene maps to a hemizygous region of the long arm of a chromosome 2 in Chinese hamster, and (2) a segregation mechanism other than chromosome loss appears to operate with high efficiency in intraspecific hybrids.

Linkage of genetic markers emt and chr in Chinese hamster cells

Segregation analysis in somatic cell hybrids has been used to demonstrate linkage in the CHO cell line between two autosomal loci, emt (defined by mutants resistant to the protein synthesis inhibitor emetine) and chr (defined by mutants resistant to chromate). In both cases drug resistance behaves recessively in hybrids and linkage has been determined by examining the recovery of the mutant phenotypes from hybrids of the type CHO emtr chrr x CHO emt+ chr+. Of 24 independent segregants selected for resistance to emetine, all 24 were also resistant to chromate while only 18 out of 24 segregants selected for chromate resistance were also resistant to emetine. The biological significane of this "polarity effect" is currently unknown but may be related to the molecular mechanism or mechanisms by which segregation occurs in hybrid cells. In this and the accompanying paper (Worton, Duff, and Campbell) we discuss a number of possible mechanisms of segregation, of which we believe mitotic recombination to be most consistent with the available data.

Inhibition of the initiation of translation by a factor isolated from Escherichia coli cells

A low molecular weight factor isolated from Escherichia coli cells was found to inhibit protein synthesis in vitro directed by RNA of bacteriophage R-17. The factor also inhibited poly(U)-directed translation, but higher concentrations were required. When the factor was added after R-17 RNA-directed translation was initiated, the onset of inhibition was delayed. Initiation-independent translation of polysomes was not inhibited. The factor inhibited the binding of N-formylmethionyl-tRNAfMet to 70S ribosomes to form the 70S initiation complex, and it released the N-flrmylmethionyl-tRNAfMet from preformed complexes. The factor did not prevent the formation of N-formylmethionylpuromycin. It was concluded that the factor inhibits specifically the initiation of translation.

Interaction between membrane functions and protein synthesis in reticulocytes. An elongation-stage inhibitor of protein synthesis extracted from the reticulocyte membrane

A component of the reticulocyte cell membrane was found to inhibit protein synthesis severely in a reticulocyte lysate system. An investigation into the mode of action of the membrane inhibitor revealed the following facts. (1) The binding of the tertiary initiation complex (methionyl-tRNAfMet-Initiation Factor 2-GTP) to the 40S ribosomal subunit was unaffected by the membrane inhibitor. (2) The membrane component did not interfere with the binding of the 40S initiation complex to the AUG initiation codon and subsequent attachment of the 60S ribosomal subunit. (3) Elongation of the peptide chain, as assayed by peptidyl-puromycin formation, was markedly affected by the membrane inhibitor. Surprisingly, the membrane component caused a considerable increase in peptidyl-puromycin formation. (4) Reticulocyte ribosomes that had been reisolated by high-speed centrifugation, after preincubation with the membrane component, were found to be highly defective when assayed in a cell-free protein-synthesizing system. These results indicated that an extract of the reticulocyte cell membrane inhibited protein synthesis by interacting with the ribosome and thus interfered with the correct functions of the elongation stage of protein synthesis. The implications of this conclusion are discussed in the light of data showing that a highly purified preparation of the membrane inhibitor also displayed an endonucleolytic activity highly specific for 28S RNA.

Differential translation in normal and adenovirus type 5-infected human cells and cell-free systems

When uninfected or adenovirus 5-infected KB cells are exposed to hypertonic medium, the incorporation of radioactive amino acids into protein decreases in both, but more severely in the uninfected cells. Although the effect of hypertonic medium on the synthesis of specific polypeptides varies, the translation of viral polypeptides as a class is less inhibited. The same patterns of proteins are synthesized regardless of the solute used in the hypertonic medium. The mechanism by which hypertonic conditions exert their effect on whole cells was investigated in K cell-free systems. It was possible to simulate the differential patterns of protein synthesis obtained in whole cells in hypertonic medium by increasing ion concentrations in cell-free extracts which are capable of initiating polypeptide chains on exogenous templates. However, in cell lysates which only elongate proteins, the same patterns were not obtained. Certain host and viral polypeptides displayed striking responses to increased ionic conditions in whole cells and cell-free systems. The synthesis of a host 44K protein, actin, appeared to be most sensitive; lower-molecular-weight proteins were fairly resistant. Among the viral proteins, the synthesis of 100K was inhibited, but most notable was the marked resistance of the synthesis of polypeptide IX. Possible mechanisms for differential synthesis and their significance are considered.

Regulation of protein synthesis during early limitation of Saccharomyces cerevisiae

Arsenate, a competitive inhibitor with phosphate in phosphorylation reactions, has been used to lower adenine and guanine nucleotide levels in Saccharomyces cerevisiae to study nucleotide effects on protein synthesis. By measuring polysome levels, we have shown that initiation of protein synthesis is much more sensitive than elongation or termination to inhibition when the ATP/ADP, GTP/GDP ratios are low. When the arsenate-phosphate molar ratio was 0.27, protein synthesis was inhibited by about 85% and the kinetics of polysome decay was similar to that observed with the initiation inhibitor, verrucarin-76, or with the protein synthesis initiation mutant, ts187, at the restrictive temperature. With this level of arsenate, the adenylate energy charge dropped from 0.9 to 0.7 and the ATP/ADP and GTP/GDP ratios dropped from 6 to 2. The observed correlations between nucleotide ratio changes and inhibition of protein synthesis suggest that the former may be a control signal for the latter. The significance of these in vivo correlations will have to be tested with an in vitro protein synthesizing system. Higher arsenate levels resulted in even lower ATP/ADP, GTP/GDP ratios and in a slower decay of polysomes, implying that, eventually, elongation (in addition to initiation) was being inhibited.

Differential synthesis of lens proteins in the presence of m7G(5')pppG and cleavage product m7GMP in an embryonic chick lens cell lysate

The effect of m7GMP release from m7GMP-containing mRNA cap sequence m7GpppG by the embryonic chick lens m7GpppN-pyrophosphatase activity on the synthesis of lens proteins was examined in a newly developed homologous translation system derived from 15-day embryonic chick lenses. The synthesis of total lens polypeptides and delta-crystallin polypeptides, the major translation product, was inhibited 84% and 88%, respectively, by 0.5 mM m7GpppG; m7GMP (0.5 mM) inhibited total synthesis by 63% but was 33% less inhibitory toward delta-crystallin synthesis; GpppG and GMP were not inhibitors, m7GpppG inhibited met-tRNAfmet incorporation into 80S initiation complexes.

Effect of modeccin on the steps of peptide-chain elongation

Modeccin inhibits polypeptide-chain elongation catalysed by Artemia salina (brine shrimp) ribosomes by inactivating the 60 S ribosomal subunit. Among the individual steps of elongation, peptide-bond formation, catalysed by 60 S peptidyltransferase, is unaffected by the toxin, whereas the binding of EF 2 (elongation factor 2) to ribosomes is strongly inhibited. Modeccin does not affect the poly(U)-dependent non-enzymic binding of either deacylated tRNAPhe or phenylalanyl-tRNA to ribosomes. The inhibitory effect of modeccin on the EF 1 (elongation factor 1)-dependent binding of phenylalanyl-tRNA is discussed, since it is decreased by tRNAPhe, which stimulates the binding reaction. The analysis of the distribution of ribosome-bound radioactivity during protein synthesis shows that modeccin consistently inhibits the radioactivity bound as long-chain peptides, but depending on the experimental conditions, can leave unchanged or even greatly stimulates the radioactivity bound as phenylalanyl-tRNA and/or short-chain peptides. It is concluded that, during the complete elongation cycle, modeccin does not affect the binding of the first aminoacyl-tRNA to ribosomes, but inhibits some step in the subsequent repetitive activity of either EF 1 or EF 2. The results obtained indicate that the mechanism of action of modeccin is very similar to that of ricin and related plant toxins such as abrin and crotin.

Inhibition of elongation steps of protein synthesis at reduced potassium concentrations in reticulocytes and reticulocyte lysate

We treated intact reticulocytes with nystatin to make them permeable to small cations and varied the concentration of K+ in the medium; the ionic strength was kept constant by adjusting Na+. As the intracellular K+ was lowered, the rate of protein synthesis decreased. In a reticulocyte lysate, the dependence of the rate of protein synthesis on K+ concentration (at constant ionic strength) was similar to that seen in nystatin-treated cells. We observed no loss of polysomes at low K+. Even when initiation was blocked with aurintricarboxylic acid, decreased K+ inhibited protein synthesis. Analysis of the kinetics of synthesis showed that lowering the K+ concentration inhibits the rate of elongation of nascent chains, but has little effect on the relative rate of initiation of chains.

The different effects of N-ethylmaleimide and iodoacetamide on the activity of rat liver 60S subunits for peptide bond elongation

The activity of 60S subunits of rat liver ribosomes in poly(U)-dependent polyphenylalanine synthesis was inhibited by incubation with N-ethylmaleimide. However, when 60S subunits were incubated with iodoacetamide, their activity decreased only slightly. Furthermore, iodoacetamide-pretreated 60S subunits became insensitive to N-ethylmaleimide. Similar results were obtained for the activity of EF-2-dependent GTPase of 60S subunits. As a whole, the labeling patterns of ribosomal proteins on two-dimensional gel electrophoresis were similar for 60S subunits labeled with both 14C-labeled sulfhydryl reagents, although the extent of labeling of some proteins was somewhat different. These results indicate that the SH groups in the 60S subunits are not directly involved in the activities of the subunits described above.

Pulvomycin, an inhibitor of protein biosynthesis preventing ternary complex formation between elongation factor Tu, GTP, and aminoacyl-tRNA

Pulvomycin and the synonymous antibiotics labilomycin and 1063-Z are shown to inhibit prokaryotic protein synthesis by acting on elongation factor Tu (EF-Tu): in the presence of the antibiotic, the affinity of EF-Tu for guanine nucleotides is altered, the EF-Tu.GDP/GTP exchange is catalyzed, and the formation of the EF-Tu.GTP complex is stimulated. Hydrolysis of GTP by EF-Tu, induced by aminoacyl-tRNA, ribosomes, and mRNA or by kirromycin, is inhibited by pulvomycin. As shown by Millipore filtration, chromatographic analysis, and hydrolysis protection experiments, pulvomycin prevents interaction between aminoacyl-tRNA and EF-Tu.GTP to yield the ternary complex aminoacyl-tRNA.EF-Tu.GTP. Thus, enzymatic binding of aminoacyl-tRNA to ribosomes is blocked.

Comparison of initiation rates of encephalomyocarditis virus and host protein synthesis in infected cells

The relative initiation rates for encephalomyocarditis virus mRNA and host mRNA's in infected cells were measured using two independent techniques. In both cases the results showed that viral mRNA initiates at a much higher rate than host mRNA'S. This difference was observed midway in the infectious cycle, well before virus-induced cytopathic effects (leakage of low-molecular-weight metabolites, failure to exclude trypan blue) were apparent. These results confirm that encephalomyocarditis viral mRNA is a more efficient initiator than host mRNA's in vivo, as has previously been demonstrated in in vitro experiments.

Specificity of interferon action in protein synthesis

Inhibitors of elongation steps in protein synthesis such as cycloheximide and anisomycin mimic interferon treatment in that they specifically inhibit the synthesis of certain viral proteins. These specific effects are seen only at very low concentrations of the antibiotics, under conditions where host cellular protein synthesis, as well as cell viability, are not severely reduced. A qualitatively as well as quantitatively close correlation between the effects of the two types of agents has been established for encephalomyocarditis virus, vesicular stomatitis virus and murine leukemia virus protein synthesis. It is concluded that one of the primary mechanisms of interferon action may be a nonspecific retardation of one or more elongation steps, and that this may be sufficient to account for its effects on the replication of certain viruses such as encephalomyocarditis and vesicular stomatitis viruses.

Decreased activity of peptide-elongation factors after treatment with cholesterol esterase

1. Peptide-elongation factors were purified from rat liver and treated with cholesterol esterase and phospholipase A2 immobilized on Sepharose 4B. 2. Binding of L-[3H]-phenylalanyl-tRNA to 40S ribosomal subunits was decreased by approx. 70% and to polyribosomes by 30% in the presence of the binding factor incubated with cholesterol esterase. Treatment of this factor with immobilized phospholipase A2 decreased the binding to smaller ribosomal subunits by only about 15%. 3. Poly(U)-dependent phenylalanine polymerization by ribosomal subunits was decreased to approx. 30% of its original value by treatment of both elongation factors with cholesterol esterase. 4. The normal activity of esterase-treated elongation factor in both the binding reaction and peptide-elongation assay was fully recovered by the addition of cholesteryl 14-methyl-hexadecanoate. 5. Different classes of lipids present in peptide-elongation factor 1 have apparently different functions. Whereas phospholipids are required to maintain the strcture of heavy aggregates of this factor, the presence of cholesteryl 14-methylhexadecanoate is obviously necessary for the normal function of peptide-elongation factors.

Inhibition of globin synthesis by rabbit reticulocyte rRNAs

The effect of concentration of rabbit reticulocyte rRNAs on the translation of rabbit globin mRNAs was studied by using a rabbit reticulocyte lysate system. Globin synthesis was studied using L-[U-14C]leucine. Both the 18S and 28S rRNA inhibited globin synthesis. The 18S rRNA inhibited the synthesis of alpha-globin chain more than that of beta chain, but the 28S rRNA inhibited the synthesis of alpha- and beta-chains almost equally. Nascent chains prelabelled with [14C]Leu or f[35S]Met were released at various concentrations of rRNAs. Release of the nascent chains was not inhibited at various concentrations of rRNAs. The ratios [14C]alpha/[14C]beta and [35S]alpha/[35S]beta in the released chains were almost constant at various concentrations of rRNAs. It therefore appears that the inhibition of globin synthesis by these rRNAs was at the step of initiation.