The ribosomal E site at low Mg2+: coordinate inactivation of ribosomal functions at Mg2+ concentrations below 10 mM and its prevention by polyamines

Under standard conditions (Mg2+/150 mM NH4+) ribosomes can quantitatively participate in tRNA binding at Mg2+ concentrations of 12 to 15 mM. The overall poly(U)-directed Phe incorporation and the extent of tRNA binding to either P, E or A sites decrease in a parallel manner when the Mg2+ concentration is lowered below 10 mM. At 4 mM the inactivation amounts to about 80%. The coordinate inactivation of all three binding sites is accompanied by an increasing impairment of the ability to translocate A-site bound AcPhe-tRNA to the P site. The translocation efficiency is already reduced at 10 mM Mg2+, and is completely blocked at 6-8 mM. The severe inactivation seen at 6 mM Mg2+ vanishes when the polyamines spermine (0.6 mM) and spermidine (0.4 mM) are present in the assay; tRNA binding again becomes quantitative, the total Phe synthesis even exceeds that observed in the absence of polyamines by a factor of 4. In the presence of polyamines and low Mg2+ (3 and 6 mM) two essential features of the allosteric three-site model (Rheinberger and Nierhaus, J. Biol. Chem. 261, 9133 (1986] are demonstrated. 1) Deacylated tRNA is not released from the P site, but moves to the E site during the course of translocation. 2) Occupation of the E site reduces the A site affinity and vice versa (allosteric interactions between E and A sites). The quality of an in vitro system for protein synthesis can be assessed by two criteria. First, the incubation conditions must allow a near quantitative tRNA binding. Secondly, protein synthesis should proceed with near in vivo rate and accuracy. The 3 mM Mg2+/NH4+/polyamine-system seems to be the best compromise at present between these two requirements.

Dithiothreitol and the translocation of preprolactin across mammalian endoplasmic reticulum

The translocation mode of preprolactin (pPL) across mammalian endoplasmic reticulum was reinvestigated in light of recent findings that nascent secretory polypeptides synthesized in the presence of a highly reducing environment could be translocated posttranslationally and independently of their attachment to the ribosome (Maher, P. A., and S. J. Singer, 1986, Proc. Natl. Acad. Sci. USA, 83:9001-9005). The effects of the reducing agent dithiothreitol (DTT) on pPL synthesis and translocation were studied in this respect. The translocation of pPL was shown to take place only cotranslationally. The apparent posttranslational translocation was due to ongoing chain synthesis irrespective of the presence of high concentrations of DTT. When synthesis was completely blocked, no translocation was observed in the presence or absence of DTT. The synthesis of pPL was retarded by DTT, while its percent translocation was enhanced. The retardation in synthesis was reflected in reduced rates of initiation and elongation. As a consequence of this retardation, which increases the ratio of microsomes to nascent chains, and of possible effects on the conformation of nascent pPL and components of the translocation apparatus, DTT may expand the time and chain length windows for nascent chain translocation competence.

Elongating ribosomes in vivo are refractory to erythromycin

We have studied the kinetics of erythromycin inhibition of translation in growing bacteria. In order to simplify the interpretation of our data, we have used a mutant (envA), known to have an increased permeability to several antibiotics, including erythromycin. The data clearly show that an initial stage of translation is sensitive to erythromycin, but that the elongating ribosome is insensitive to the antibiotic.

Inhibition by ganciclovir of cell growth and DNA synthesis of cells biochemically transformed with herpesvirus genetic information

The ability of LM cells, thymidine kinase-deficient LM cells (LMTK-), and LMTK- cells transformed to the LMTK+ phenotype by herpes simplex virus type 1 genetic information (LH7 cells) to anabolize the acyclovir congener ganciclovir was examined. About 50-fold more ganciclovir triphosphate was produced by LH7 cells than by either LM or LMTK- cells. Growth inhibition studies indicated that 180 and 120 microM ganciclovir were required to achieve 50% growth inhibition of LM and LMTK- cells, respectively; only 0.07 microM ganciclovir was necessary to achieve 50% inhibition of LH7 cells. DNA synthesis in the transformed cells was significantly reduced by ganciclovir treatment, whereas ganciclovir had little effect on DNA synthesis in the nontransformed cells. Alkaline sucrose gradient sedimentation analysis of transformed cellular DNA indicated that LH7 DNA synthesized in the presence of ganciclovir chased into mature DNA. Both LM and LH7 DNA synthesized in the presence of ganciclovir exhibited a concentration-dependent reduction in the rate of elongation into mature DNA. Finally, [14C]ganciclovir was incorporated internally into the growing chains of LH7 cells.

Protein synthesis inhibition by 8-oxo-12,13-epoxytrichothecenes

The Fusarium mycotoxin, 4-deoxynivalenol, is an abundant, natural contaminant of corn and wheat. 8-Oxo-12,13-epoxytrichothecenes related to 4-deoxynivalenol were synthesized; they either lacked the 7-hydroxyl but contained a hydroxyl at C-4 (7-deoxynivalenol) or lacked substituents at C-3 and C-7 (3,7-dideoxynivalenol). The ability of these synthetic analogs and their acetylated derivatives to inhibit protein synthesis by cultured mammalian cells was compared to that of 4-deoxynivalenol. Whereas the 50% inhibitory dose (ID50) for murine erythroleukemia cells was about 1 microgram/ml for 4-deoxynivalenol and 3,7-dideoxynivalenol, all of the other analogs were at least 10-fold less potent. When tested at their ID50 dose, all of the 8-oxotrichothecenes, except 4-deoxynivalenol and 3,7-dideoxynivalenol, caused polysome 'run-off', indicating that, at this dose, they are inhibitors of polypeptide chain initiation. With 4-deoxynivalenol and 3,7-dideoxynivalenol, polysomes remained at control levels indicating that these toxins prevent polypeptide chain elongation. From these results and comparisons to previous studies of 8-oxo-12,13-epoxytrichothecenes (trichothecolone, trichothecin, nivalenol and fusarenone X), trichothecenes with substituents at both C-3 and C-4 predominantly inhibit polypeptide chain initiation, whereas those lacking one substituent at either site are inhibitors of chain elongation.

Signal recognition particle arrests elongation of nascent secretory and membrane proteins at multiple sites in a transient manner

The signal recognition particle (SRP) has been shown to target nascent secretory and membrane proteins to the endoplasmic reticulum. In the wheat germ cell-free system, SRP arrests the elongation of the nascent chains until the translational complex is docked to the endoplasmic reticulum membrane where the interaction between SRP and docking protein causes a release of the nascent chain arrest. For two secretory proteins, arrested peptides of 70 amino acids have been identified (Walter, P., Ibrahimi, I., and Blobel, G. (1981) J. Cell Biol. 91, 545-550; Meyer, D. I., Krause, E., and Dobberstein, B. (1982) Nature 297, 647-650). By using an in vitro coupled transcription-translation system, we have analyzed SRP arrest and the resulting peptides of the two secretory proteins lysozyme and granulocyte-macrophage colony-stimulating factor and the membrane protein invariant chain. SRP arrested the elongation of all three proteins at multiple sites, giving rise to ladders of arrested peptides. The size of the arrested peptides increased with the time of translation, resulting in mostly full-length pre-peptides after about 40 min. This suggests that SRP arrest in transient rather than stable. Upon addition of microsomes, the SRP arrest was released, and all the blocked peptides could be chased into mature proteins or full-length precursors.

Key role of L-alanine in the control of hepatic protein synthesis

We investigated the effects of administration of single amino acids to starved rats on the regulation of protein synthesis in the liver. Of all the amino acids tested, only alanine, ornithine and proline promoted statistically significant increases in the extent of hepatic polyribosome aggregation. The most effective of these was alanine, whose effect of promoting polyribosomal aggregation was accompanied by a decrease in the polypeptide-chain elongation time. The following observations indicate that alanine plays an important physiological role in the regulation of hepatic protein synthesis. Alanine was the amino acid showing the largest decrease in hepatic content in the transition from high (fed) to low (starved) rates of protein synthesis. The administration of glucose or pyruvate is also effective in increasing liver protein synthesis in starved rats, and their effects were accompanied by an increased hepatic alanine content. An increase in hepatic ornithine content does not lead to an increased protein synthesis, unless it is accompanied by an increase of alanine. The effect of alanine is observed either in vivo, in rats pretreated with cycloserine to prevent its transamination, or in isolated liver cells under conditions in which its metabolic transformation is fully impeded.

The anti-oestrogen drug tamoxifen is an elongation inhibitor of eukaryotic protein biosynthesis

The drug tamoxifen is widely used in the chemotherapy of breast cancer but its action is not explained completely by its anti-oestrogen properties. We now present evidence indicating that it is also a potent inhibitor of eukaryotic protein synthesis as demonstrated in Xenopus oocytes, intact reticulocytes and reticulocyte lysates. The inhibition affects general protein synthesis, is transient in oocytes and not reversed by oestrogen. The drug appears to act by inhibiting polypeptide chain elongation. This action of tamoxifen is independent of oestrogen receptors and may explain its therapeutic effectiveness in oestrogen-independent tumours.

Reduced turnover of the elongation factor EF-1 X ribosome complex after treatment with the protein synthesis inhibitor II from barley seeds

The effect of the protein synthesis inhibitor II from barley seeds (Hordeum sp.) on protein synthesis was studied in rabbit reticulocyte lysates. Inhibitor treatment of the lysates resulted in a rapid decrease in amino acid incorporation and an accumulation of heavy polysomes, indicating an effect of the inhibitor on polypeptide chain elongation. The protein synthesis inhibition was due to a catalytic inactivation of the large ribosomal subunit with no effect on the small subparticle. The inhibitor-treated ribosomes were fully active in participating in the EF-1-dependent binding of [14C]phenylalanyl-tRNA to poly(U)-programmed ribosomes in the presence of GTP and the binding of radioactively labelled EF-2 in the presence of GuoPP[CH2]P. Furthermore, the ribosomes were still able to catalyse peptide-bond formation. However, the EF-1- and ribosome-dependent hydrolysis of GTP was reduced by more than 40% in the presence of inhibitor-treated ribosomes, while the EF-2- and ribosome-dependent GTPase remained unaffected. This suggests that the active domains involved in the two different GTPases are non-identical. Treatment of reticulocyte lysates with the barley inhibitor resulted in a marked shift of the steady-state distribution of the ribosomal phases during the elongation cycle as determined by the ribosomal content of elongation factors. Thus, the content of EF-1 increased from 0.38 mol/mol ribosome to 0.71 mol/mol ribosome, whereas the EF-2 content dropped from 0.20 mol/mol ribosome at steady state to 0.09 mol/mol ribosome after inhibitor treatment. The data suggest that the inhibitor reduces the turnover of ribosome-bound ternary EF-1 X GTP X aminoacyl-tRNA complexes during proof-reading and binding of the cognate aminoacyl-tRNA by inhibiting the EF-1-dependent GTPase.

Effects of polynucleotide inhibitors on transcriptional events of influenza virions

Polynucleotide inhibitors of the influenza virion transcriptase, which is activated in vitro by detergent, can be divided into two categories. Polyribonucleotides comprised of adenine and guanine bases or adenine and uracil bases inhibit initiation but not elongation events of transcription. In contrast, polyribonucleotides containing cytidine and uracil or inosine and uracil block both initiation and elongation. Effects on elongation are apparent on addition of polynucleotides at either 5 or 10 min post-initiation. Dose-response studies with these and other polynucleotides have shown that the concentrations causing 50% inhibition of transcription vary considerably with the most potent inhibitor being the modified polymer, poly(C,S4U10).

[Study of a protein complex from the brain which reduces actin viscosity. II. The complex inhibits elongation of actin filaments at the end preferable for polymerization and fragments the filaments]

Functional properties of the protein complex from bovine brain that shortens actin filaments are described. In the presence of Ca2+ complex shortens actin filaments and increases the initial rate of actin polymerization. In the absence of free calcium ions the complex loses its accelerating effect on actin polymerization, but still possesses actin filament shortening activity. Neither phalloidin nor tropomyosin prevent the shortening of actin filaments induced by the protein complex. Therefore the protein complex causes the fragmentation of actin filament. The data on actin polymerization in the presence of F-actin nuclei have indicated that the protein complex inhibits the elongation step of actin polymerization. The analysis of elongation in the presence of both the protein complex and cytochalasin D has demonstrated that the inhibition occurs on the fast-growing end of actin filaments.

Changes in the rate of translation with reactivation of delayed implanting mouse embryos

The transient embryonic diapause associated with delayed implantation in mice is characterized by decreases in the rates of synthesis of RNA and protein as well as a cessation of development. The present experiments were undertaken to examine the possibility that controls on protein synthesis at the level of translation of mRNA provide a regulatory mechanism in this situation. Rates of peptide chain elongation were determined in dormant embryos as well as in embryos that were reactivated either in vivo by estradiol-17 beta or by incubation in vitro. In dormant embryos the rate of peptide elongation was found to be approximately half that in active embryos. Although this change in translational efficiency appears to be sufficient to account for previously observed differences in overall rates of protein synthesis in dormant and reactivated embryos, the possibility that some changes also occur at the level of transcription during reactivation is not ruled out.

Effect of streptomycin on the stoichiometry of GTP hydrolysis in a poly(U)-dependent cell-free translation system

The technique of Sepharose-bound template translation has been used to estimate the stoichiometry of GTP hydrolysis during peptide elongation in the presence of streptomycin. The presence of streptomycin has been shown to have no great effect on the elongation rate and the stoichiometry of GTP hydrolysis during codon-specific peptide elongation in the poly(U)-directed translation system: the molar ratio of hydrolysed GTP to incorporated phenylalanine was about 2. At the same time streptomycin exerted a significant effect during misreading when a ribosome-bound peptide in the poly(U)-programmed system was elongated by leucine or isoleucine residues: the miselongation was stimulated and hence the ratio of hydrolysed GTP per peptide bond was strongly reduced, as compared with the excessive GTP hydrolysis which is characteristic of the misreading system in the absence of streptomycin [(1984) FEBS Lett. 178, 283-287]. The conclusion has been made that streptomycin blocks the stage of correction ('proof-reading') following GTP hydrolysis during EF-Tu-dependent aminoacyl-tRNA binding.

Differential effects of parathyroid hormone and somatomedin-like growth factors on the sizes of proteoglycan monomers and their synthesis in rabbit costal chondrocytes in culture

In the proteoglycans extracted from rabbit costal chondrocytes in culture, two populations of proteoglycans were distinguished by density gradient centrifugation under dissociative conditions. The major component was the faster sedimenting population (proteoglycan I), the putative 'cartilage-specific' proteoglycans, and the minor component was the slower sedimenting population (proteoglycan II). The monomeric size of proteoglycan I was closely related to the differentiation-state of chondrocytes and was a good marker of the differentiated chondrocytes. Treatment of the cultures with parathyroid hormone (PTH) induced an increase in the monomeric size of proteoglycan I. This increase was ascribed to an increase in the molecular size of the glycosaminoglycan chain in proteoglycan I. On the other hand, somatomedin-like growth factors, such as multiplication-stimulating activity (MSA) and cartilage-derived factor (CDF), did not affect the size of proteoglycan I, while they markedly stimulated the synthesis of proteoglycan I. In contrast, treatment with nonsomatomedin growth factors, such as fibroblast growth factor (FGF) and epidermal growth factor (EGF), resulted in not only a decrease in glycosaminoglycan synthesis but also a slight decrease in size of proteoglycan I. However, synthesis and size of proteoglycan II were little affected by these agents. Thus, the present study clearly shows that PTH and somatomedin-like growth factors have differential functions in bringing about the expression of the differentiated phenotype of chondrocytes: PTH influences chain elongation and termination of glycosaminoglycans in proteoglycan I, while somatomedin-like growth factors affect primarily the synthesis and secretion of proteoglycan I.

Experimental methyl mercury neurotoxicity: locus of mercurial inhibition of brain protein synthesis in vivo and in vitro

Brain cell-free protein synthesis is inhibited by methyl mercury chloride (MeHg) following in vivo or in vitro administration. In this report, we have identified the locus of mercurial inhibition of translation. Intraperitoneal injection of MeHg (40 nmol/g body wt) induced variable inhibition of amino acid incorporation into the post-mitochondrial supernatant (PMS) harvested from the brain of young (10-20-day-old) rats. No mercurial-induced disaggregation of brain polyribosomes nor change in the proportion of 80S monoribosomes was detected on sucrose density gradients. No difference in total RNA was found in the PMS. Initiation complex formation was stimulated by MeHg, as detected by radiolabelled methionine binding to 80S monoribosomes following continuous sucrose density gradient centrifugation. After micrococcal nuclease digestion of endogenous mRNA, both in vivo and in vitro MeHg inhibited polyuridylic acid-directed incorporation of [3H]phenylalanine. However, the in vivo inhibition was no longer observed when [3H]phenylalanyl-tRNAPhe replaced free [3H]phenylalanine in the incorporation assay. The formation of peptidyl[3H]puromycin revealed no difference from controls. There was significant mercurial inhibition of phenylalanyl-tRNA Phe synthetase activity in pH 5 enzyme fractions derived from brain PMS of MeHg-poisoned rats. These experiments revealed that the apparent MeHg inhibition of brain translation in vivo and in vitro is due primarily to perturbation in the aminoacylation of tRNA and is not associated with defective initiation, elongation, or ribosomal function.

Hormonal regulation of transcription of rDNA: glucocorticoid effects upon initiation and elongation in vitro

Various parameters of transcription of cloned mouse rDNA have been examined using extracts from control P1798 cells and from cells treated 24 h with 0.1 microM dexamethasone. Highly purified RNA polymerase I from either source catalyzes nucleotidyl transfer (elongation) at a rate of approximately 30 nucleotides/sec. Extracts from hormone-treated cells are capable of forming stable, preinitiation complexes. The rates of stable complex formation are the same in extracts from control and hormone-treated cells. Nevertheless, initiation of transcription does not occur in extracts from hormone-treated cells. Initiation in such extracts may be restored by the addition of a partially purified RNA polymerase I initiation factor, designated TFIC. The data indicate that initiation by RNA polymerase I is a multi-step process. The first step involves the formation of stable, preinitiation complexes, as demonstrated by a number of groups. Initiation, per se, requires an additional protein, TFIC. Glucocorticoids and perhaps other mitogenic agents regulate transcription of rDNA by influencing the amount or activity of TFIC.

Efficient modification of E. coli RNA polymerase in vitro by the N gene transcription antitermination protein of bacteriophage lambda

The N gene protein of bacteriophage lambda prevents termination of transcription by E. coli RNA polymerase. We describe here the conditions of a cell-free reaction system in which pure N stimulates net transcription up to tenfold and therefore nearly stoichiometrically modifies transcribing RNA polymerase molecules. The reaction contains micrococcal nuclease-treated S100 extract derived from E. coli and a plasmid template DNA containing the lambda early promoter PL, the N utilization site nutL, and the Rho-dependent terminator tL1. Stimulation by N in this system is specific and biologically relevant since it is absent with vector pBR322 DNA and with extracts derived from E. coli strains bearing the nusA1 and nusE71 mutations known to block N function in vivo. We use the system to provide further evidence that ribosomes are not necessary for N function and to demonstrate the direct involvement in N function of the NusA protein of E. coli.

Effect of ribavirin triphosphate on primer generation and elongation during influenza virus transcription in vitro

These studies examine the effect of ribavirin triphosphate (RTP) on two replicative functions associated with influenza virus nucleocapsids, primer generation and its subsequent elongation. To study primer generation influenza virus cores were added to beta-globin mRNA in the presence of only [32P]GTP. To examine elongation, ATP and CTP were added to the reaction mixture to permit limited elongation, and products from both reactions were separated on polyacrylamide gels and quantified. Under these conditions, the 50% inhibitory concentration of RTP for primer generation was 3.0 mM, and the 50% inhibitory concentration for elongation was 0.6 mM. RNA polymerase activity associated with cores isolated from clinical strains of influenza A and B viruses reacted as did the laboratory strain of influenza virus and was equally susceptible to inhibition by RTP.

The mechanisms of action for beta-lactam antibiotics and inhibitors of bacterial protein synthesis

The replication of bacteria, when unchecked, may lead to morbidity or mortality in a susceptible host. The majority of antimicrobial agents either modify or inhibit the synthesis of key substances and are reflected by morphologic, if not lethal, changes. Activity occurs during synthesis of the bacterial cell wall and synthesis of bacterial protein. These two events are described in terms of the local mechanism of action of different classes of antibiotics. The role of antibacterial agents in the replication of bacteria should be clearly understood before antibiotic therapy is administered.

The control of accuracy during protein synthesis in Escherichia coli and perturbations of this control by streptomycin, neomycin, or ribosomal mutations

This review surveys the different experimental approaches which describe the binding of tRNA to mRNA-programmed ribosomes and the control of tRNA selection. This selection is best described by the two-step model proposed by Hopfield and demonstrated by Thompson and his collaborators. The model involves a first control at the initial reversible binding of tRNA to the ribosome and a second control, the proofreading control, which promotes rejection of the incorrect tRNA from a high-energy intermediate during the transition from the initial to the final binding state. Streptomycin, neomycin, and ribosomal fidelity mutations appear to affect both control steps. Their effect can be related to the location of the mutated ribosomal proteins and to the conformational changes induced in the ribosome by the misreading agents. An alteration of the first control probably results from a distortion of the codon-anticodon interaction, while an alteration of the second control may be caused by a change in the association between ribosomal subunits.

Heparin as an inhibitor of L-arginyl-tRNA: protein arginyltransferase

Heparin, a naturally occurring mucopolysaccharide, was found to be a potent inhibitor of L-arginyl-tRNA:protein arginyltransferase [EC 2.3.2.8]. It was demonstrated that heparin was not an acceptor of arginine, but an inhibitor competing with arginyl-tRNA. The Ki value of heparin was 1.5 microM, while the Km value of arginyl-tRNA was 0.5 microM. The inhibition was specific for heparin among the mucopolysaccharides examined. The N-sulfate groups were indispensable for heparin to inhibit the arginyltransferase activity. Arginyltransferases from various origins were commonly inhibited by heparin. Polyamines such as spermine and spermidine were also found to inhibit the transferase activity, competing with arginyl-tRNA. An extremely high specific activity of 2.3 mumol/min/mg was achieved by the application of these inhibitors to the purification of the transferase by affinity chromatography.

Effect of protein synthesis inhibitors on the fidelity of translation in eukaryotic systems

Factors influencing the accuracy of poly(U)-directed poly(Phe) synthesis in a wheat germ and in a reticulocyte system were studied. Addition of preformed phenylalanyl-tRNA, as well as increasing the ratio of poly(U) to ribosomes, significantly enhanced the poly(Phe) synthesis and concurrently reduced the misincorporation of leucine. The protein synthesis inhibitors cycloheximide, abrin and ricin had little or no effect on the misreading when the system was supplemented with 100 microM phenylalanyl-tRNA, but they reduced the relatively high error rate observed when the poly(U) system was not supplemented with the cognate substrate. Raising the incubation temperature enhanced the accuracy to the same extent whether or not ricin was present i.e., at widely different rates of elongation. The results show that the translational accuracy is not linked to the elongation rate as such. Translational inhibitors affect the fidelity by influencing the kinetics of the system. In systems containing limiting concentrations of cognate substrate, translational inhibitors will cause an increase in the limiting aminoacyl-tRNA species and thereby increase fidelity.

G-418, an elongation inhibitor of 80 S ribosomes

The mode of action of the aminoglycoside G-418 was studied in wheat-germ, cell-free translation systems programmed with rat-liver polyadenylated RNA. Incorporation of amino acids into protein was effectively inhibited by G-418 in the microM concentration range. The inhibition pattern obtained was not uniform. The synthesis of polypeptides with higher molecular weights was more inhibited than that of smaller polypeptides. An identical inhibition pattern within a similar range of concentrations was obtained with cycloheximide, a known elongation inhibitor. Translation activity was abolished when the wheat-germ 80 S ribosomes were removed and could be partially reconstructed upon addition of the ribosomes. Incubation with G-418 prior to isolation yielded ribosomes defective in their reconstruction ability. The inhibition pattern was not uniform and exhibited again the same relationship between the size of a polypeptide and the extent of inhibition of its synthesis. Therefore, we suggest that in wheat-germ, cell-free translation systems G-418 affects the 80 S ribosomes and inhibits the elongation cycle.

Cell-free translation of calf type III collagen. Effect of magnesium on ribosome movement during elongation

Calf aortic smooth muscle cell cultures produce both type III and type I collagen. Polyadenylated mRNA species purified from these cells direct the synthesis of prepro-alpha 1(III), prepro-alpha 1(I), and prepro-alpha 2(I) in a rabbit reticulocyte cell-free system. These polypeptides were identified by specific immunoprecipitation, cyanogen bromide peptide mapping, and bacterial collagenase digestion. Lower molecular weight collagenase susceptible polypeptides were also produced in translation reactions incubated under conditions optimized for incorporation of radiolabeled amino acids. Their presence did not appear to result from ribonuclease or protease involvement or from premature termination. Increasing the Mg2+ concentration in the translation system significantly reduced the production of these lower molecular weight species. Pulse-chase experiments indicate that the time required for completion of full length preprocollagen at the high Mg2+ concentration is greatly decreased compared to the low concentration. Additional experiments suggest that the incomplete collagen polypeptides result from pausing of ribosome movement during elongation. The relative synthesis of type III and type I chains was examined as a function of mRNA concentration in the cell-free system. At levels of RNA above saturation, the relative production of type III decreased with respect to type I. These data suggest that the ability of the alpha 1(III) mRNA to initiate translation is less efficient than the mRNAs of alpha 1(I) and alpha 2(I).

Resistance to fusidic acid in Escherichia coli mediated by the type I variant of chloramphenicol acetyltransferase. A plasmid-encoded mechanism involving antibiotic binding

Plasmid-encoded fusidic acid resistance in Escherichia coli is mediated by a common variant of chloramphenicol acetyltransferase (EC 2.3.1.28), an enzyme which is an effector of chloramphenicol resistance. Resistance to chloramphenicol is a consequence of acetylation of the antibiotic catalysed by the enzyme and the failure of the 3-acetoxy product to bind to bacterial ribosomes. Cell-free coupled transcription and translation studies are in agreement with genetic studies which indicated that the entire structural gene for the type I chloramphenicol acetyltransferase is necessary for the fusidic acid resistance phenotype. The mechanism of resistance does not involve covalent modification of the antibiotic. The other naturally occurring enterobacterial chloramphenicol acetyltransferase variants (types II and III) do not cause fusidic acid resistance. Steady-state kinetic studies with the type I enzyme have shown that the binding of fusidic acid is competitive with respect to chloramphenicol. The inhibition of polypeptide chain elongation in vitro which is observed in the presence of fusidic acid is relieved by addition of purified chloramphenicol acetyltransferase, and equilibrium dialysis experiments with [3H]fusidate and the type I enzyme have defined the stoichiometry and apparent affinity of fusidate for the type I enzyme. Further binding studies with fusidate analogues, including bile salts, have shown some of the structural constraints on the steroidal skeleton of the ligand which are necessary for binding to the enzyme. Determinations of antibiotic resistance levels and estimates of intracellular chloramphenicol acetyltransferase concentrations in vivo support the data from experiments in vitro to give a coherent mechanism for fusidic acid resistance based on reversible binding of the antibiotic to the enzyme.

Mode of action of bottromycin A2: effect of bottromycin A2 on polysomes

When bottromycin A2 was added to an in vitro protein synthesis system carried out by naturally occurring polysomes, it inhibited protein synthesis effectively. Examination of the 3 steps of peptide chain elongation revealed that the binding of aminoacyl-tRNA to the polyribosomes was inhibited by bottromycin A2. In contrast, we concluded that the peptide bond formation and the translocation steps in this system were not inhibited by bottromycin A2 on the basis of the following observations: (1) The break-down of polysomes, which is dependent on EFG, puromycin and RR (ribosome releasing) factor, was insensitive to bottromycin A2; (2) The puromycin dependent release of polypeptide from polysomes, with or without EFG, was not inhibited by bottromycin A2. Thus bottromycin specifically interferes with proper functioning of the A sites of polysomes. This is consistent with the results obtained using the model system with synthetic polynucleotides.

Resistance to inhibitors of mammalian cell protein synthesis induced by preincubation in hypertonic growth medium

Incubation of L-929 cells in growth medium containing excess NaCl induces cellular protein synthesis to become resistant to the inhibitory action of growth medium made hypertonic by the addition of Na-acetate, KCl, choline chloride, D-alanine, or sucrose. Hypertonic preincubation also induces resistance to the inhibitory action of the drugs dimethyl sulfoxide, ethanol, and L-1-tosylamido-2-phenylethyl chloromethyl ketone, but not to the action of NaF or the inhibitors of polypeptide chain elongation, emetine and cycloheximide. Induction is independent of the solute used to make the induction medium hypertonic and is not mediated by an elevated intracellular Na+ concentration. In a separate series of experiments, it was determined that the formation of functional methionyl-tRNAf.40 S ribosomal complexes is not impaired by exposure of uninduced cells to hypertonic growth medium. The combined results are evaluated with respect to the mechanism of action of hypertonic growth medium in the inhibition of polypeptide chain initiation and the adaptive mechanisms that animal cells have evolved to deal with alterations in their extracellular environments.

Amiloride, protein synthesis, and activation of quiescent cells

Amiloride is known to inhibit both influx of sodium ions and activation of quiescent cells by growth factors. The coincidence of these effects has been cited to support the proposal that influx of sodium ions acts as a mitogenic signal. Although it was noted that amiloride inhibited protein synthesis, this was attributed to an action on transport of amino acids, particularly those coupled to sodium fluxes. We find, however, that amiloride directly inhibits polypeptide synthesis in a reticulocyte lysate. In Swiss 3T3 cells, concentrations of amiloride and of cycloheximide that are nearly matched in their degree of inhibition of protein synthesis, produce about the same degree of inhibition of transit of cells from G0 to S. Inhibition of protein synthesis is sufficient to explain the effect of amiloride on mitogenesis; the drug, therefore, is not suitable for testing the hypothesis that sodium influx is a mitogenic signal.

Insulin affects only initiation and not elongation in protein synthesis in soleus muscles of lean and obese mice

The effect of insulin on polypeptide chain initiation and elongation has been studied in soleus muscles isolated from lean and goldthioglucose-obese mice. Insulin increased the amount of radioactivity present in nascent chains by approximately 30% in muscles from both lean and obese mice, indicating that it stimulates peptide chain initiation. In contrast, elongation rates, estimated by measurement of half transit time, were similar in basal conditions and insulin-treated muscles of lean and obese animals. Thus, insulin increased the initiation without modifying the elongation rates. Obesity did not affect either basal rates of initiation and elongation or the effect of insulin.

Probes of eukaryotic DNA-dependent RNA polymerase II-I. Binding of 9-beta-D-arabinofuranosyl-6-mercaptopurine to the elongation subsite

9-beta-D-Arabinofuranosyl-6-mercaptopurine (ara-6-MP) was used to affinity-label wheat germ DNA-dependent RNA polymerase II (or B) (nucleosidetriphosphate:RNA nucleotidyltransferase, EC 2.7.7.6). This nucleoside analogue was found to be a competitive inhibitor with respect to [3H]UMP incorporation. Natural substrates protected the enzyme from inactivation by ara-6-MP when the enzyme was preincubated with excess concentrations of substrates, suggesting that the inhibitor binds at the elongation subsite. The inhibitor bound the catalytic center of the enzyme with a stoichiometry of 0.6:1. The sulfhydryl reagent, dithiothreitol, reversed the inhibition by ara-6-MP, suggesting that the 6-thiol group of the inhibitor was interacting closely with an essential cysteine residue in the catalytic center of the enzyme. Chromatographic analysis of the pronase-digestion products of the RNA polymerase II-ara-6-MP complex also showed that ara-6-MP had bound a cysteine residue. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the denatured [6-35S]ara-6-MP-labeled RNA polymerase II revealed that over 80% of the radioactivity was associated with the IIb subunit of the enzyme.

The effects of chronic hyperphenylalaninaemia on mouse brain protein synthesis can be prevented by other amino acids

A prolonged elevation in the concentrations of circulating phenylalanine was maintained in newborn mice by daily injections of phenylalanine and a phenylalanine hydroxylase inhibitor, alpha-methylphenylalanine. The result of this chronic hyperphenylalaninaemia was an accumulation of vacant or inactive monoribosomes that persisted for 18 h of each day. An elongation assay in vitro with brain postmitochondrial supernatants demonstrated that, in addition, there was an equally prolonged decrease in the rates of polypeptide-chain elongation by the remaining brain polyribosomes. Analyses of the free amino acid composition in the brains of hyperphenylalaninaemic mice showed a loss of several amino acids from the brain, particularly the large, neutral amino acids, which are co- or counter-transported across plasma membranes with phenylalanine. When a mixture of these amino acids (leucine, isoleucine, valine, threonine, tryptophan, tyrosine, methionine) was injected into hyperphenylalaninaemic mice, there was an immediate cessation of monoribosome accumulation in the brain and there was no inhibition of the rates of polypeptide-chain elongation. Although the concentrations of the large, neutral amino acids in the brain were partially preserved by treatment of hyperphenylalaninaemic mice with the amino acid mixture, the elevated concentrations of phenylalanine remained unaltered. The amino acid mixture had no detectable effect on brain protein synthesis in the absence of the hyperphenylalaninaemic condition.

Differential effect of chronic ethanol administration on rates of protein synthesis on free and membrane-bound polysomes in vivo in rat liver during dependence development

Administration of ethanol thrice daily to rats in amounts sufficient to induce a high degree of physical dependence resulted in a 20% decrease in the rate of protein synthesis on liver membrane-bound polysomes in vivo after 3 days of treatment without affecting the rate on free polysomes. The inhibition was attributable to a decrease in the rate of polypeptide elongation as evidence by comparable decreases in nascent chain synthesis and completed protein release without any change in leucine uptake by liver. Chronic ethanol treatment did not affect the quantity or distribution of free and membrane-bound polysomes, the DNA concentration, or the weight of liver. The inhibition of protein synthesis on membrane-bound polysomes cannot, therefore, be readily ascribed to ethanol-induced nutritional deficiencies or to some nonspecific toxic effect of ethanol.

Regulation of the peptide elongation reaction on uterine ribosomes by estrogens

Administration of 17 beta-estradiol to ovariectomized mature rats for 1 h induces an increased capacity of subsequently isolated uterine ribosomes to synthesize protein in a cell-free protein synthesis system. The increased rate of protein synthesis can be ascribed to an effect of estrogen on the rate of peptide elongation rather than synthesis of additional new peptides. The increased rate of peptide elongation is dependent upon the dose of estradiol over the range of 0.1 to 10 micrograms/animal, and exhibits hormone specificity; 17 beta-estradiol, diethylstilbesterol, estrone and estriol but not 17 alpha-estradiol, progesterone, dihydrotestosterone or corticosterone will induce the response. Removal of ribosome associated proteins by extraction with 0.5 M KCl results in activation of protein synthesis by uterine ribosomes from control rats to rates that are equal to that of ribosomes from estrogen-stimulated rats suggesting that ribosomes from control animals are in an inhibited state. The KCI extracted ribosomal factors from control animals inhibit the synthesis of protein by salt-washed uterine ribosomes when added back to the ribosomes prior to assay and the inhibitory properties of these factors are greater if derived from ribosomes of control rather than 1 h estradiol-treated rats. The extracted inhibitor is inactivated by heat, is insensitive to treatment with N-ethylmaleimide, is insensitive to micrococcal nuclease and is reversible. The early activation of uterine ribosomes by estrogen appears to result from either the removal or inactivation of a ribosome associated-peptide elongation reaction, inhibitory factor.

Bactericidal synergism between beta-lactams and aminoglycosides: mechanism and possible therapeutic implications

This work, presented as the introductory lecture of the symposium, reviews the mechanisms of antibiotic action of beta-lactams and of aminoglycosides from a historical perspective and gives emphasis to the features relevant to their synergistic interaction. These features include the limiting effect of penetration of aminoglycosides on the rate of killing; the ability of beta-lactams to increase that penetration, even under conditions where they cannot cause a lethal damage to the cell envelope; and the ability of aminoglycosides to be lethal with a smaller amount of cell growth than is required for killing by beta-lactams. The paper also briefly discusses theoretical arguments favoring a broader use of bactericidal synergism in clinical practice.

Effects of cadmium on mouse liver polyribosome function

The distribution of ribosomes between their various functional states (native subunits, unprogrammed ribosomes, monoribosomes and polyribosomes) was analyzed in livers of cadmium-treated (CdCl2, 20 umol/kg, l h) and control mice. The ribosomes with double-labelled RNA were separated by sucrose density-gradient centrifugation in zonal rotor, and monomeric ribosome fraction was subsequently isolated and analyzed by selective dissociation of unprogrammed ribosomes. The analysis shows that the increase in monomeric ribosomes, occurring during polyribosome disaggregation in livers of cadmium-treated mice, entirely due to the increase of the unprogrammed ribosome fraction at the expense of polyribosomes. Protein synthetic activity of polyribosomes in vivo in livers of cadmium-treated and control mice were compared by double labelling of nascent and soluble polypeptides with [14C]-and [3H]-leucine. Incorporation of radioactivity was relatively higher in nascent polypeptides and lower in soluble polypeptides in cadmium-treated as compared with control mice. The results indicate that cadmium inhibits protein synthesis in livers of mice, affecting both the rate of initiation and the rate of elongation, but decreasing the former more than the latter.

Purification and characterization of an inhibitor protein with cytochalasin-like activity from bovine adrenal medulla

A protein preparation with cytochalasin-like activity has been obtained from bovine adrenal medulla. Analysis by electrophoresis in SDS-polyacrylamide gel and chromatography in a Sephacryl S-200 column indicated that the inhibitor activity coincided with a 90 000 dalton polypeptide. The inhibitor decreased high-affinity binding of [3H]cytochalasin B to actin nuclei, apparently by competing with the drug for the same binding site. At substoichometric levels, the inhibitor had a potent effect on actin filament elongation and on actin-dependent gelation of cell extracts in vitro. These results suggest that the inhibitor may be involved in the control of actin filament assembly and interaction in the adrenal medulla.

Translocation of proteins across the endoplasmic reticulum III. Signal recognition protein (SRP) causes signal sequence-dependent and site-specific arrest of chain elongation that is released by microsomal membranes

The previously observed (Walter, et al. 1981 J. Cell Biol. 91:545-550) inhibitory effect of SRP selectively on the cell-free translation of mRNA for secretory protein (preprolactin) was shown here to be caused by a signal sequence-induced and site-specific arrest in polypeptide chain elongation. The Mr of the SRP-arrested nascent preprolactin chain was estimated to be 8,000 corresponding to approximately 70 amino acid residues. Because the signal sequence of preprolactin comprises 30 residues and because approximately 40 residues of the nascent chain are buried (protected from protease) in the large ribosomal subunit, we conclude that it is the interaction of SRP with the amino-terminal signal peptide of the nascent chain (emerged from the large ribosomal subunit) that modulates translation and thereby causes an arrest in chain elongation. This arrest is released upon SRP-mediated binding of the elongation-arrested ribosomes to the microsomal membrane, resulting in chain completion and translocation into the microsomal vesicle.

Diphtheria toxin. Site and configuration of ADP-ribosylation of diphthamide in elongation factor 2

Diphtheria toxin inactivates protein synthesis elongation factor 2 by catalyzing the ADP-ribosylation of a novel derivative of histidine, diphthamide, in the protein (Van Ness, B. G., Howard, J. B., and Bodley, J. W. (1980) J. Biol. Chem. 255, 10710-10716). In this report, we describe experiments involving nuclear Overhauser enhancement NMR spectroscopy which were undertaken to elucidate the site of ADP-ribosylation of diphthamide and the configuration of the glycosidic bond formed by the toxin. The essential result of these experiments is that, in ribosyl-diphthamide obtained by enzymatic digestion of ADP-ribosyl-elongation factor-2, the H-5 imidazole proton is near the R-4 proton of ribose. This result and others are consistent with the interpretation that diphtheria toxin covalently attaches ADP-ribose to the imidazole N-1 of diphthamide via an alpha-glycosidic linkage.

Mechanism of eukaryotic protein synthesis inhibition by brusatol

The mechanism by which brusatol inhibits protein synthesis in rabbit reticulocytes has been investigated. When added to reticulocyte lysates, brusatol inhibits endogenous protein synthesis only after a lag of 2-4 min at 30 degrees C. During this period 80 S ribosomes accumulate. Brusatol is equally effective in inhibiting endogenous protein synthesis in lysates and poly(U)-directed polyphenylalanine synthesis with runoff ribosomes. In fractionated reticulocyte systems, brusatol does not inhibit formation of the ternary, 40 S, and 80 S initiation complexes, but does inhibit the reaction of puromycin with initiation complexes containing [35S]Met-tRNAf. These data suggest that brusatol inhibits the peptidyl transferase elongation reaction of protein synthesis, but can do so only after one round of protein synthesis has been completed. Thus, the mechanism of action of brusatol in the rabbit reticulocyte system is very similar to the effects previously reported for bruceantin in a yeast system.

Factor requirements for the tritin inactivation of animal cell ribosomes

Wheat germ contains (tritin) that efficiently inhibits protein synthesis in cell-free extracts from animal cells but not from wheat germ. Tritin has been purified to apparent homogeneity and shown to block enzymatically polypeptide chain elongation. We have extended these studies to examine more closely then mechanism of tritin inactivation of animal cell ribosomes. Here we provide evidence suggesting that ATP and tRNA, previously though to be tritin co-factors, function in the inhibition by altering the conformation of the ribosome to a form susceptible to tritin attack. Tritin treatment does not inhibit the binding of aminoacyl-tRNA to ribosomes, but it does partially reduce the ribosome binding of elongation factor 2. Tritin inhibition appears to affect the core ribosome and not ribosome-associated factors. However, the core ribosome itself is not a suitable substrate for tritin attack; a factor(s) is removed from ribosomes by high salt washing which is required for the tritin-induced inhibition. The ability to be inhibited is restored when ascites cell core ribosomes are supplemented with factors from either ascites cells or wheat germ. In contrast, neither ascites cell factors nor wheat germ factors will promote a significant tritin-induced inhibition of core ribosomes from wheat germ. This indicates that the specificity of tritin inhibition resides primarily at the level of eukaryotic core ribosome.

Effects of antibiotics, N-acetylaminoacyl-tRNA and other agents on the elongation-factor-Tu dependent and ribosome-dependent GTP hydrolysis promoted by 2'(3')-O-L-phenylalanyladenosine

GTP hydrolysis on elongation factor (EF) Tu . ribosome complexes has been assayed in the presence of 2'(3')-O-L-phenylalanyladenosine (AdoPhe), i.e. the 3'-terminal portion of Phe-tRNAPhe. Several requirements of the reaction have been characterized. Maximal activity is observed at 60-120 mM NH4Cl and 5-15 mM magnesium acetate. The reaction requires the free sulfhydryl group of EF-Tu normally implicated in aminoacyl-tRNA binding. Intact EF-Tu cannot be replaced by a large tryptic fragment of EF-Tu (Mr 39,000) that retains the ability to bind guanosine nucleotides. The aminoglycoside antibiotics, neomycin C and several kanamycins and gentamicins, stimulate the AdoPhe-promoted GTPase. Surprisingly, however, other closely related antibiotics, like neomycin B, paromomycin and ribostamycin, are ineffectual, thus indicating subtle differences in the actions of these antibiotics. AcPhe-tRNAPhe, bound to the ribosomal A-site, stimulates the AdoPhe-promoted GTPase, but this compound or AcTyr-tRNATyr, present in unbound form, strongly inhibits the reaction. These results suggest that N-blocked aminoacyl-tRNAs form ternary complexes with EF-Tu . GTP, which have not been previously detected because of their low stability.

An analysis of rates of polypeptide chain elongation in avian liver explants following in vivo estrogen treatment. I. Determination of average rates of polypeptide chain elongation

Average rates of polypeptide chain elongation have been determined in cockerel liver explants on 4 successive days following an in vivo injection of 17 beta-estradiol. Incorporation of [3H]leucine by the explants is linear for at least 24 h, and the rate of protein synthesis increases significantly after estrogen injection. The explants synthesize and secrete serum albumin. B-apolipoprotein, and the phosphoprotein vitellogenin at relative rates which are similar to those reported for liver in vivo. Using this system, changes in the average rates of polypeptide chain elongation have been analyzed as a temporal sequence following a single injection of 17 beta-estradiol into cockerels. For this, average ribosome half-transit times were determined by measuring the kinetics of transfer of labeled polypeptides from polysomal-bound to released polypeptides. The data revealed a dramatic effect of estradiol on the average ribosome half-transit time with a maximum increase of 4.6-fold; however, the average size of polypeptides synthesized by explants at the peak of induction increased only 15% when compared to uninduced liver explants. These findings indicate that injection of estradiol results in large changes in the actual rates at which amino acids are added to the growing nascent polypeptide chain; that is, rates of polypeptide chain elongation. Therefore, translation-level regulation of protein synthesis in cockerel livers plays a significant part in determining the magnitude of the response to hormone stimulation.

Speed-accuracy relationships during in vitro and in vivo protein biosynthesis

Poly U-directed incorporation of phenylalanine and leucine into polypeptide has been described in at least 50 papers since 1961. In general, high translation activities are associated with high accuracies, and vice-versa. Moreover, a vast body of independent experimental data (effect of ethanol, temperature, urea, aminoglycosides, etc... on protein synthesis) put together here suggests that, in many circumstances, speed and accuracy of elongation are correlated. This result is to be contrasted with the view that the speed and the fidelity of protein synthesis are two opposing parameters. In this report, recent experimental data on the nature and effect of ribosomal ambiguity (ram) and streptomycin resistance (Strr) mutations are reexamined. Models on the action of streptomycin and other misreading-inducing antibiotics, as well as long-standing ideas on the control of misreading in mammalian systems are critically evaluated. An explanation is provided for the long-befuddling data on the action of gentamicin.