Nonribosomal proteins associated with eukaryotic native small ribosomal subunits

Isolation of Mitochondrial Ribosomes

Translation of mitochondrial encoded mRNAs by mitochondrial ribosomes is thought to play a major role in regulating the expression of mitochondrial proteins. However, the structure and function of plant mitochondrial ribosomes remains poorly understood. To study mitochondrial ribosomes, it is necessary to separate them from plastidic and cytosolic ribosomes that are generally present at much higher concentrations. Here, a straight forward protocol for the preparation of fractions highly enriched in mitochondrial ribosomes from plant cells is described. The method begins with purification of mitochondria followed by mitochondrial lysis and ultracentrifugation of released ribosomes through sucrose cushions and gradients. Dark-grown Arabidopsis cells were used in this example because of the ease with which good yields of pure mitochondria can be obtained from them. However, the steps for isolation of ribosomes from mitochondria could be applied to mitochondria obtained from other sources. Proteomic analyses of resulting fractions have confirmed strong enrichment of mitochondrial ribosomal proteins.

Human eIF3: from 'blobology' to biological insight

Translation in eukaryotes is highly regulated during initiation, a process impacted by numerous readouts of a cell's state. There are many cases in which cellular messenger RNAs likely do not follow the canonical ‘scanning’ mechanism of translation initiation, but the molecular mechanisms underlying these pathways are still being uncovered. Some RNA viruses such as the hepatitis C virus use highly structured RNA elements termed internal ribosome entry sites (IRESs) that commandeer eukaryotic translation initiation, by using specific interactions with the general eukaryotic translation initiation factor eIF3. Here, I present evidence that, in addition to its general role in translation, eIF3 in humans and likely in all multicellular eukaryotes also acts as a translational activator or repressor by binding RNA structures in the 5′-untranslated regions of specific mRNAs, analogous to the role of the mediator complex in transcription. Furthermore, eIF3 in multicellular eukaryotes also harbours a 5′ 7-methylguanosine cap-binding subunit—eIF3d—which replaces the general cap-binding initiation factor eIF4E in the translation of select mRNAs. Based on results from cell biological, biochemical and structural studies of eIF3, it is likely that human translation initiation proceeds through dozens of different molecular pathways, the vast majority of which remain to be explored.

This article is part of the themed issue ‘Perspectives on the ribosome’.

The eIF3 complex of Leishmania-subunit composition and mode of recruitment to different cap-binding complexes

Eukaryotic initiation factor 3 (eIF3) is a multi-protein complex and a key participant in the assembly of the translation initiation machinery. In mammals, eIF3 comprises 13 subunits, most of which are characterized by conserved structural domains. The trypanosomatid eIF3 subunits are poorly conserved. Here, we identify 12 subunits that comprise the Leishmania eIF3 complex (LeishIF3a-l) by combining bioinformatics with affinity purification and mass spectrometry analyses. These results highlight the strong association of LeishIF3 with LeishIF1, LeishIF2 and LeishIF5, suggesting the existence of a multi-factor complex. In trypanosomatids, the translation machinery is tightly regulated in the different life stages of these organisms as part of their adaptation and survival in changing environments. We, therefore, addressed the mechanism by which LeishIF3 is recruited to different mRNA cap-binding complexes. A direct interaction was observed in vitro between the fully assembled LeishIF3 complex and recombinant LeishIF4G3, the canonical scaffolding protein of the cap-binding complex in Leishmania promastigotes. We further highlight a novel interaction between the C-terminus of LeishIF3a and LeishIF4E1, the only cap-binding protein that efficiently binds the cap structure under heat shock conditions, anchoring a complex that is deficient of any MIF4G-based scaffolding subunit.

Irreversible aggregation of protein synthesis machinery after focal brain ischemia

Focal brain ischemia leads to a slow type of neuronal death in the penumbra that starts several hours after ischemia and continues to mature for days. During this maturation period, blood flow, cellular ATP and ionic homeostasis are gradually recovered in the penumbral region. In striking contrast, protein synthesis is irreversibly inhibited. This study used a rat focal brain ischemia model to investigate whether or not irreversible translational inhibition is due to abnormal aggregation of translational complex components, i.e. the ribosomes and their associated nascent polypeptides, protein synthesis initiation factors and co-translational chaperones. Under electron microscopy, most rosette-shaped polyribosomes were relatively evenly distributed in the cytoplasm of sham-operated control neurons, but clumped into large abnormal aggregates in penumbral neurons subjected to 2 h of focal ischemia followed by 4 h of reperfusion. The abnormal ribosomal protein aggregation lasted until the onset of delayed neuronal death at 24-48 h of reperfusion after ischemia. Biochemical study further suggested that translational complex components, including small ribosomal subunit protein 6 (S6), large subunit protein 28 (L28), eukaryotic initiation factors 2alpha, 4E and 3eta, and co-translational chaperone heat-shock cognate protein 70 (HSC70) and co-chaperone Hdj1, were all irreversibly clumped into large abnormal protein aggregates after ischemia. Translational complex components were also highly ubiquitinated. This study clearly demonstrates that focal ischemia leads to irreversible aggregation of protein synthesis machinery that contributes to neuronal death after focal brain ischemia.

Initiation factor eIF3 and regulation of mRNA translation, cell growth, and cancer

One important regulation of gene expression in eukaryotes occurs at the level of mRNA translation, specifically at the step of translational initiation. Deregulation at this step will cause abnormal gene expression, leading to altered cell growth and possibly cancer. Translational initiation is controlled by multiple eIFs and one of these, eIF3, is the most complex and important factor for regulation of translation. Various subunits of eIF3 have recently been implicated to play important roles in regulating translation of specific mRNAs encoding proteins important for cell growth control. The expression of these eIF3 subunits has also been found altered in various human tumors and their altered expression may cause cancer and/or affect prognosis. Although the importance of translational regulation in cell growth control and oncogenesis is being slowly recognized, more vigorous studies on the role of eIFs in oncogenesis and cancer will likely benefit diagnosis, prognosis, and treatment of human cancers.

Binding of eukaryotic initiation factor 3 to ribosomal 40S subunits and its role in ribosomal dissociation and anti-association

The multisubunit eukaryotic initiation factor (eIF) 3 plays various roles in translation initiation that all involve interaction with 40S ribosomal subunits. eIF3 can be purified in two forms: with or without the loosely associated eIF3j subunit (eIF3j+ and eIF3j-, respectively). Although unlike eIF3j+, eIF3j- does not bind 40S subunits stably enough to withstand sucrose density gradient centrifugation, we found that in addition to the known stabilization of the eIF3/40S subunit interaction by the eIF2*GTP*Met-tRNA(i)Met ternary complex, eIF3j-/40S subunit complexes were also stabilized by single-stranded RNA or DNA cofactors that were at least 25 nt long and could be flanked by stable hairpins. Of all homopolymers, oligo(rU), oligo(dT), and oligo(dC) stimulated the eIF3/40S subunit interaction, whereas oligo(rA), oligo(rG), oligo(rC), oligo(dA), and oligo(dG) did not. Oligo(U) or oligo(dT) sequences interspersed by other bases also promoted this interaction. The ability of oligonucleotides to stimulate eIF3/40S subunit association correlated with their ability to bind to the 40S subunit, most likely to its mRNA-binding cleft. Although eIF3j+ could bind directly to 40S subunits, neither eIF3j- nor eIF3j+ alone was able to dissociate 80S ribosomes or protect 40S and 60S subunits from reassociation. Significantly, the dissociation/anti-association activities of both forms of eIF3 became apparent in the presence of either eIF2-ternary complexes or any oligonucleotide cofactor that promoted eIF3/40S subunit interaction. Ribosomal dissociation and anti-association activities of eIF3 were strongly enhanced by eIF1. The potential biological role of stimulation of eIF3/40S subunit interaction by an RNA cofactor in the absence of eIF2-ternary complex is discussed.

Identification of homologous ribosomal proteins in HeLa cells and in Tetrahymena pyriformis. A study of proteins binding 5-S RNA and acidic proteins released from 40-S subunits by EDTA

Tetrahymena pyriformis 60-S ribosomal subunits treated with EDTA release a 7-S particle containing 5-S RNA and a 36000-Mr protein that is similar to mammalian 5-S-RNA-binding protein L5 in molecular weight, in two-dimensional acrylamide gel mobility, and in peptide pattern as generated by a simple, one-dimensional acrylamide gel technique. Human and T. pyriformis 40-S ribosomal subunits, treated with buffers lacking magnesium or containing EDTA, release varying amounts of two large acidic proteins. We have identified these released proteins by two-dimensional gel electrophoresis.

Phosphorylation in vivo of non-ribosomal proteins from native 40 S ribosomal particles of Krebs II mouse ascites-tumour cells

Four non-ribosomal proteins from native 40 S ribosomal subunits with mol.wts. of 110 000, 84 000, 68 000 and 26 000 were phosphorylated in vivo when ascites cells were incubated in the presence of [32P]Pi. The 110 000-, 84 000- and 26 000-dalton proteins are identical with phosphorylated products from native 40 S subunits after phosphorylation in vitro by a cyclic nucleotide-independent protein kinase. Phosphoserine was the major phosphorylated amino acid of the proteins phosphorylated in vivo and in vitro.

Interaction of a limited set of proteins with different mRNAs and protection of 5'-caps against pyrophosphatase digestion in initiation complexes

A variety of 5'-3H-methyl-labeled, oxidized viral mRNAs were used as probes for detecting in wheat germ initiation complexes proteins that interact with, and can be cross-linked to, the 5'-cap structure. A limited and reproducible set of specific proteins was obtained with the different mRNAs. The binding of these proteins to the 5'-end of mRNA apparently results in protection against nucleotide pyrophosphatase digestion of the cap even in initiation complexes in which the 5'-end is susceptible to pancreatic RNase digestion. Cross-linked proteins from mammalian initiation complexes comigrated with several of the subunits of similarly treated eIF-3. A model for cap binding protein interaction with mRNA cap during initiation of translation is suggested.

Regulation of translational initiation ina temperature-sensitive mutant of BHK cells

The role of various essential components in mammalian cell protein chain initiation, such as the 40S ribosomal subunit and the initiation factors, have been studied largely in fractionated systems and little is known about the mechanisms of their regulation. The process was studied in an unfractionated system derived from temperature-sensitive (ts) 422E cells. The mutant cells fail to assemble 60S ribosomal subunits at the nonpermissive temperature with little effect on the production of 40S ribosomal subunits. Although the resulting imbalance in ribosomal subunits has little immediate effect on the overall rate of protein synthesis, the nascent 40S subunits in mutant cells appear to be markedly unstable at the nonpermissive temperature. The initial step in protein synthesis, initiation factor elF-2-promoted binding of [35S]Met-tRNAf Met to 40S ribosomal subunits, remains unimparied in ts 422E cells at the nonpermissive temperature. However, most of the nascent 40S ribosomal subunits in ts 422E cells band at a higher density, suggesting their failure to bind initiation factor elF-3. These newly synthesized 40S subunits in the mutant cells are unstable, and may be degraded before their appearance in the polysomes.

Ribosomal proteins of the dimorphic fungus, Mucor racemosus

Ribosomal proteins of the dimorphic fungus Mucor racemosus were isolated and characterized by 2-dimensional gel electrophoresis. Proteins from ribosomes of the yeast and mycelial phase were compared, and were found to be qualitatively indistinguishable. The only consistent difference in the patterns of proteins was in a protein of the 40S subunit, S-6. This protein was phosphorylated in yeast and hyphae forms, but not in asexual sporangiospores. Studies on protein S-6 showed that it contained 3 phosphate residues per molecule of protein when maximally phosphorylated. In this form 3 different tryptic peptides were shown to contain a single phosphoserine. The S-6 protein also existed in forms containing 1 or 2 phosphates per molecule, depending on growth conditions.

Postribosomal complexes containing eukaryotic initiation factor eIF-2

Eukaryotic initiation factors are found in the post-ribosomal subunits. We have analyzed the factor activities from the supernatant by means of zonal centrifugation followed by Sepharose-heparin affinity chromatography. They exist both as free factors, sedimenting in a broad range from 4 to 7S, and complexed with other protein(s) with a sedimentation value of 16-20S. This complexed fraction contains besides eIF-2 another activity which exhibits a profound stimulation on amino acid incorporation in crude lysates and appears to counteract the heme-regulated inhibitor.

Identification of initiation factors and ribosome-associated phosphoproteins by two-dimensional polyacrylamide gel electrophoresis

A two-dimensional polyacrylamide gel electrophoresis procedure has been used to identify initiation factors rapidly in the high-salt-wash fraction from reticulocyte ribosomes. Initiation factors are identified by relative mobility and by co-electrophoresis with purified factors. A creatine phosphate/ATP/GTP/Pi exchange system is described which has been used to maintain [gamma-32P]ATP and [gamma-32P]GTP at constant specific activity in the cell-free protein-synthesizing system. Phosphorylated proteins associated with the protein-synthesizing complex have been identified using a combination of the two procedures. The salt-wash fraction contains eight major phosphorylated proteins and a number of minor ones. Two phosphorylated proteins are observed to comigrate with two of the three subunits of eukaryotic initiation factor 2 (eIF-2), the initiation factor involved in binding Met-tRNAf onto the 40-S subunit and promoting dissociation of 80-S ribosomes. eIF-4B, one of the proteins involved in binding mRNA to 40-S subunits is also phosphorylated. The remainder of phosphorylated proteins in the high-salt-wash fraction are not previously characterized initiation factors and have not been identified further. Two of the six phosphoproteins associated with the salt-washed ribosomes comigrate with ribosomal proteins; one is the major phosphorylated protein in 40-S ribosomal subunits, the other is an acidic protein.

Localization of eukaryotic initiation factor 3 on native small ribosomal subunits

The localization of eukaryotic initiation factor 3(eIF-3) on native small ribosomal subunits has been established by electron microscopy through a comparison of native small ribosomal subunits with derived subunits and with native subunits stripped of eIF-3. Small subunits derived from reticulocyte ribosomes by the puromycin/KCl method are seen in electron micrographs as elongated particles, divided by a heavily stained partition into approximately one-third and two-third domains. Most particles (60-70%) observed in electron micrographs of native small subunit preparations resemble derived small subunits, but have an additional mass attached to one side, thus producing profiles with a three-lobed appearance. The mass measures approximately 160 x 100 x 60 A, and its particle weight is estimated to be about one-third to one-half that of a 40S subunit. The site of attachment of the additional mass is located on a prominence extending from the central part of the small subunit and is separated by a cleft from the smaller third of the subunit. The remaining particles in preparations of native subunits resemble the profiles seen in electron micrographs of derived subunits. After removal of eIF-3 by treatment with high concentrations of salt, profiles observed in electron micrographs of washed, native subunits were indistinguishable from those of derived subunits. Since removal of eIF-3 coincided with removal of a mass of the correct molecular weight, subunits with the three-lobed appearance are identified as native small subunits carrying eIF-3.

Free fibosomes and growth stimulation in human peripheral lymphocytes: activation of free ribisomes as an essential event in growth induction

During the initial ten hours of growth in lymphocytes stimulated by phytohemagglutinin, the cells are converted from a state in which over 70% of all ribosomes are inactive free ribosomes, to one in which over 80% of ribosomes are in polysomes or in native ribosomal subunits. In this initial period, there was a neglible increase in total ribosomal RNA due to increased RNA synthesis, and abolition of ribosomal RNA synthesis with low concentrations of actinomycin D did not interfere with polysome formation. Therefore, the conversion is accomplished by the activation of existing free ribosomes rather than by accumulation of newly synthesized particles. The large free ribosome pool of resting lymphocytes is thus an essential source of components for accelerated protein synthesis early in lymphocyte activation, before increased synthesis can provide a sufficient number of new ribosomes. Free ribosomes accumulate once more after 24 to 48 hours of growth, when RNA and DNA synthetic activity are maximal. This reaccumulation of inactive ribosomes at the peak of growth activity may represent preparation for a return to the resting state where cells are again susceptible to stimulation. Activation of free ribosomes to form polysomes appears to involve modification of at least two steps: (a) dissociation of free ribosomes with stabilization as native subunits, and (b) adjustment of a rate-limiting step at initiation.

Simian virus 40-specific ribosome-binding proteins induced by a nondefective adenovirus 2-simian virus 40 hybrid

We have studied the intracellular distribution of the two simian virus 40-specific proteins, with apparent molecular weights of 56,000 and 42,000, detectable in human KB cells infected by a nondefective adenovirus 2-simian virus 40 hybrid, Ad2+ND2. After a 20-min pulse of [35S]methionine, about two-thirds of the newly synthesized 56K protein and one-third of the 42K protein were found localized on the plasma membrane. The remainder of each protein was found in the cytoplasm, whereas the nuclear fraction was virtually free of either component. A significant portion of both proteins present in the cytoplasmic fraction was complexed to the 40S ribosomal subunits and was not removed by treatment with 0.5 M KCl. Moreover, the portion that was found free in the cytoplasm could bind preferentially and quantitatively to purified 40S ribosomes in vitro, leading us to propose that these simian virus 40 proteins may act as translational control elements in cells.

Studies on native ribosomal subunits from rat liver. Purification and characterization of a ribosome dissociation factor

A population of free, native ribosomal 40S subunits, that do not react with 60S subunits to form 80S ribosomes, has been identified in the postmicrosomal fraction of rat liver homogenates. A protein (IF-3) has been purified from high salt (0.88 M KCI) extracts of native 40S subunits by gradient centrifugation and by ammonium sulfate fractionation; it prevents the reassociation of subunits and to a limited extent dissociates ribosomes to subunits. The activity is measured by ultracentrifugation of the reaction products on linear sucrose gradients, or with an assay developed in this laboratory that couples dissociation with the 60S-specific peptidyltransferase reaction; the latter procedure measures the amount of 60S subunits released from ribosomes or remaining in incubations in the presence of IF-3. Dissociation factor activity is recovered from most of the particles that are resolved by zonal centrifugation of the total "native subunits" obtained from the postmicrosomal fraction; the highest concentration of IF-3, however, appears to be associated with native 40S subunits. The purified dissociation factor IF-3 is composed of about ten polypeptides and the molecular weight is estimated to be between 500 000 and 700 000, on the basis of glycerol and cesium chloride gradient centrifugation. When purified 40S subunits react with IF-3 or when 80S ribosomes are dissociated by IF-3, a product is formed which is dependent on the concentration of the protein factor and has the characteristics of a 40SIF-3 complex; centrifugation of the complex on sucrose and cesium chloride gradients suggests that the complex consists of 1 equiv of each of the two components. Although dissociation factor IF-3 appears to react in a specific manner with free or ribosome-associated 40S subunits, the reaction with subunits differs in several respects from that with ribosomes. The dissociation factor also appears to interact with 60S subunits but multiple complexes are formed, some with more than 1 IF-3 equiv per 60S particle. The IF-3 converts 40S dimers (55S particles) to the 40S-IF-3 complex and dissociates free, native 80S particles present in the postmicrosomal fraction, but it does not affect polysome-associated ribosomes engaged in protein synthesis.

Initiation of protein synthesis in eukaryotes. Binding to Sepharose-heparin and partial purification of initiation factors from Krebs II ascites cells

By means of affinity chromatography of lysates from Krebs II ascites cells and rabbit reticulocytes on Sepharose-heparin an active fraction of initiation factors has been obtained. The fraction is eluted from the column at 350 mM KCl using a linear gradient and displays a number of activities, i.e. binding of Met-tRNAfMet to form a ternary complex with GTP; transferring this complex to 40-S subunits in an A-U-G-independent step and finally coupling of the 40-S initiation complex to the 60-S subunit, a reaction which is completely A-U-G-dependent. Moreover, MettRNA is bound into the P-site as is indicated by its puromycin sensitivity. The method is very suitable for large-scale preparation. Further purification and characterization of the factors have been carried out on DEAE-cellulose and phosphocellulose columns. Evidence is presented that the polysomes present in a lysate that has been passed through the Sepharose-heparin column can only complete their nascent chains, initiation of new polypeptides is completely dependent on addition of initiation factors.

Heterogeneity of native ribosomal 60-S subunits in Ehrlich ascites tumor cells cultured in vitro

Native large ribosomal subunits in cultured Ehrlich ascites tumor cells analyzed by high-resolution CsCl isopycnic centrifugation consist of at least two classes of particles with densities of 1.57 g/cm3 (LI) and 1.59 g/cm3 (LII), respectively. A wash with 0.5 M KCl converts LI into particles with the density of LII particles. Incubation of derived large subunits (density 1.59 g/cm3) with 0.5 M KCl wash of reticulocyte ribosomes leads to the formation of particles with the density of LI particles. A protein with a molecular weight of 57000 present in the high-KCl wash of 60-S native subunits was virtually absent in the KCl wash of 40-S subunits or polyribosomes suggesting that specific protein factors may be present on some native 60-S subunits. Possible functions of these protein factors are discussed.

Purification and characterization of initiation factor IF-E3 from rabbit reticulocytes

Initiation factor IF-E3 from rabbit reticulocytes was isolated from a high salf extract of ribosomes prepared according to the procedure of Schreier and Staehelin (J. Mol9 Biol, 73, 329-349, 1973). The factor was highly purified from the crude extract by ammonium sulfate fractionation, sucrose gradient centrifugation, salf gradient elution from DEAE-cellulose and phosphocellulose columns, and glycerol gradient centrifugation. IF-E3 stimulated cell-free protein synthesis dependent on an exogenous globin mRNA fraction 4- to 5-fold. The factor under nondenaturing conditions behaved as a large multipolypeptide complex, but was separated into 11 major protein components by two-dimensional polyacrylamide gel electrophoresis with urea and sodium dodecyl sulfate. The stoichiometry and molecular weights (range: 28,000-140,000) of the IF-E3 proteins were determined. None of the components corresponded to ribosomal proteins found in high salt-washed ribosomes. 14CH3-IF-E3 was prepared by reductive alkylation without detectable loss of its initiation factor activity, and bound stoichiometrically to 40S ribosomal subunits, but not to 60S or 80S ribosomes. 14CH3-IF-E3 isolated from the 40S complex contained only nine of the 11 original protein components.

Competition between cellular and viral mRNAs in vitro is regulated by a messenger discriminatory initiation factor

Encephalomyocarditis viral RNA has previously been shown to outcompete host cellular mRNA has for translation in vitro in crude and fractionated protein synthesizing systems. In the present communication it is shown that the competition is regulated by an initiation factor or complex of factors, and not the 40S initiation complex per se. The factor primarily involved is the murine equivalent of a component present in a partially purified preparation of rabbit initiation factor M3. Both the murine and rabbit factors are clearly messenger discriminatory.

Specificity of the protein kinase activity associated with the hemin-controlled repressor of rabbit reticulocyte

Highly purified preparations of hemin-controlled repressor of rabbit reticulocyte contain a 3':5'-cyclic AMP-indenpendent protein kinase activity that phosphorylates the low-molecular-weight (about 38,000) polypeptide chain of the initiation factor that forms a ternary complex with GTP and Met-tRNAf. These preparations also phosphorylate several polypeptide components of reticulocyte 40S ribosomal subunits. However, no significant levels of phosphorylation are observed when casein, histones, Artemia salina 40S ribosomal subunits, or other initiation factor fractions are used as substrates although high levels of phosphorylation are obtained with cruder preparations of the repressor. An antibody to these highly purified preparations of repressor has been obtained from the serum of immunized goats. Preincubation with immune goat IgG results in the neutralization of the inhibitory activity of the repressor, while normal IgG has no effect. Preincubation with immune IgG also abolishes the protein kinase activity responsible for the phosphorylation of the initiation factor and reticulocyte 40S subunits. Histone phosphorylation by crude repressor preparations, on the other hand, is unaffected by preincubation with immune IgG.

Purification and characterization of two initiation factors required for maximal activity of a highly fractionated globin mRNA translation system

Two additional initiation factors (IF-M4 and IF-M5) have been purified and characterized both physically and biologically. IF-M4 is active as a single polypeptide chain with a molecular weight of 48,000. In contrast, IF-M5 is active as a complex with a molecular weight of about 500,000 and consists of seven major and several minor polypeptide components. Analysis of IF-M5 in two polyacrylamide gel electrophoresis systems indicated that one of the major polypeptide chains of IF-M5 was the 35,000 dalton subunit of IF-MP. This analysis also revealed that IF-M2A, IF-M3, and elongation factor 2 were present as minor components. Both IF-M4 and IF-M5 are required to achieve maximal activity in an assay system dependent on exogenous globin mRNA, but neither factor has been observed to stimulate model reactions that utilize artificial templates [poly(U) or AUG].

Purification of myosin translational control RNA and its interaction with myosin messenger RNA

Myosin messenger ribonucleoprotein-translational control ribonucleic acid (mRNP-tcNA) from myosin mRNPs found in embryonic chick muscle has been further purified by Dowex chromatography and, from a number of controls, it is suggested that this small RNA is not an artifact produced through the degradation of RNA during its isolation. This highly purified myosin mRNP-tcRNA is shown to have a molecular weight of 10 000 on formamide-acrylamide gels, and reacts stoichometrically (on a 1:1 mole ratio) with myosin mRNA. The stoichiometric interaction between myosin mRNA and myosin mRNP-tcRNP is demonstrated by ists ability to increase the nuclease resistance of the messenger, as well as inhibit its translation in a cell-free amino acid incorporating system.

Poliovirus-induced inhibition of polypeptide initiation in vitro on native polyribosomes

The inhibition of HeLa cell protein synthesis by poliovirus was studied by examining initiation in vitro on endogenous host polyribosomes. At an early stage, before major viral RNA replication and protein synthesis begins, the initiation of translation on cellular mRNA is strongly inhibited. Fractionation of extracts from infected cells shows that the lesion is associated mainly with the crude polyribosome fraction. The cellular mRNA appears unchanged and is as active as mRNA from control cells in stimulating incorporation. The native ribosomal subunits and KCl-washed polyribosomes from the infected cells are also active. Only the ribosomal wash fraction prepared from the inhibited polyribosomes had reduced activity. However, the reduction in the ribosomal wash activity measured in a reconstructed system is not as large as the inhibition seen with "native" polyribosomes. The results indicate that a viral induced inhibition is probably associated with the ribosomal wash fraction, but the reconstructed system is not equivalent to the "native" inhibited system.

Fluctuation in activity of the molecular forms of cellular DNA polymerase during infection by SV40

Infection of BSC-1 cells by SV40 brings about an increase of 7--11-fold in DNA polymerase activity, found in the nuclei and cytoplasm, respectively. The overall ratio between activites of DNA polymerase beta (3.1S) and DNA polymerase alpha (5.5S) remains fairly constant throughout infection. However,there is a large increase in DNA polymerase alpha2 (7.1S) in the cytoplasm, and its appearance in the nuclei late in infection. The addition of 1 M NaCl to infected cytoplasm,causes an aggregation of DNA polymerase alpha into a higher sedimenting form (9.8S), termed DNA polymerase alpha3. DNA polymerase alpha1, alpha2 and alpha3 are different molecular forms of the same enzyme, as can be seen by their similar inhibition by N-ethyl-maleimide, heparin and NaCl. However, this new activity, alpha3, is stimulated by dithiothreitol to a greater extent at pH 9.30 than at pH 7.94. The conformational changes induced in DNA polymerase and its increase in activity during infection with SV40 are discussed.

Polypeptide chain initiation in eukaryotes: mechanism of formation of initiation complex

Artemia salina ribosomal subunits and highly purified reticulocyte initiation factors (IF) are used to study the mechanism of formation of the puromycin-sensitive initiation complex Met-tRNAi-80S ribosome-AUG. A complex with equimolar amounts of 40S subunit, GTP, and Met-tRNAi is formed at low Mg2+ concentration with a requirement for IF-MP (homogeneous) but not AUG or other factors. An 80S complex is formed only upon the further addition of AUG, IF-M2A, and IF-M2B, but not of either factor alone. This complex contains no GTP or GDP. A 40S complex, which cannot be converted to an 80S one, is formed when the nonhydrolyzable analog GMPPCP is substituted for GTP. IF-M2A has no effect on the formation of this complex, but IF-M2B enhances its formation.