Hemoglobin messenger ribonucleic acid translation in cell-free systems from rat and mouse liver and Landschutz ascites cells

Disease-linked microRNA-21 exhibits drastically reduced mRNA binding and silencing activity in healthy mouse liver

MicroRNAs (miRNAs) bind to mRNAs and fine-tune protein output by affecting mRNA stability and/or translation. miR-21 is a ubiquitous, highly abundant, and stress-responsive miRNA linked to several diseases, including cancer, fibrosis, and inflammation. Although the RNA silencing activity of miR-21 in diseased cells has been well documented, the roles of miR-21 under healthy cellular conditions are not well understood. Here, we show that pharmacological inhibition or genetic deletion of miR-21 in healthy mouse liver has little impact on regulation of canonical seed-matched mRNAs and only a limited number of genes enriched in stress response pathways. These surprisingly weak and selective regulatory effects on known and predicted target mRNAs contrast with those of other abundant liver miRNAs such as miR-122 and let-7. Moreover, miR-21 shows greatly reduced binding to polysome-associated target mRNAs compared to miR-122 and let-7. Bioinformatic analysis suggests that reduced thermodynamic stability of seed pairing and target binding may contribute to this deficiency of miR-21. Significantly, these trends are reversed in human cervical carcinoma (HeLa) cells, where miRNAs including miR-21 show enhanced target binding within polysomes and where miR-21 triggers strong degradative activity toward target mRNAs. Taken together, our results suggest that, under normal cellular conditions in liver, miR-21 activity is maintained below a threshold required for binding and silencing most of its targets. Consequently, enhanced association with polysome-associated mRNA is likely to explain in part the gain of miR-21 function often found in diseased or stressed cells.

A high-yield cell-free system of protein synthesis of mouse liver

1. A fractionated cell-free system of protein synthesis has been developed from mouse liver. It is composed of polysomes, "pH 5" fraction, Mg2+, K+, ATP and a ATP generating system. 2. It operates optimally at 30-37 degrees C, in the presence of 4 mM MgCl2 and 90 mM KCl. 3. Spermine is highly inhibitory, while spermidine shows a bimodal action, in that submillimolar concentrations stimulate, while millimolar concentrations inhibit protein synthesis. 4. Both spermine and spermidine show an interesting selectivity, in that, even though they inhibit incorporation of amino acids into most proteins, they stimulate incorporation into a few proteins. 5. The system can be rendered mRNA-dependent, either by preincubation or by treatment with micrococcal nuclease. In both cases globin mRNA as well as TMV RNA are faithfully translated. 6. Compared to other published mammalian fractionated cell-free systems, the mouse liver system is more efficient by approximately one order of magnitude, since the rate of incorporation of leucine per min is 30 pmol/mg protein or 435 pmol/mg RNA or 1 mol/mol ribosomes.

Optimisation of protein synthesis rates in reticulocyte lysates and reconstituted systems

Means of increasing the very low activity of a reconstituted protein synthesising system from rabbit reticulocytes were investigated. Increasing the concentration of labelled amino acid, addition of polyamines, use of Sepharose-filtered as opposed to centrifuged ribosomes, use of untreated as opposed to gel-filtered cytosol and an increase in ratio of cytosol to ribosomes all contributed to the increase in activity of the system to the point where activity was clearly consistent with initiation taking place. Similar activities could not be attained with rat liver cytosol though rat liver ribosomes incorporated well in reticulocyte cytosol. Incorporation by lysates was also found to be dependent on the concentration of the labelled amino acid added.

Interaction of cytoplasmic messenger-RNA with proteins: their possible function in the regulation of translation

In the present report we summarize recent results concerning our studies on messenger ribonucleoprotein particles and RNA-binding proteins from the cytoplasm of mouse erythroblasts and mouse Krebs II ascites cells.

Polysome-dependent in vitro translation system capable of peptide chain reinitiation

A sensitive in vitro translation system has been developed which makes use of cellular polysomes as the source of mRNA and ribosomes. The soluble factors are derived from the preincubated S-30 fraction by centrifugation through a discontinuous sucrose gradient. Of the four fractions tested, fraction 1 (topmost fraction in the gradient) and fraction 2 (fraction sedimenting in 0.5 M sucrose) were stimulatory. These two fractions together yield the highest activity, corresponding to about 125 times the background incorporation. The polysome-directed system exhibits optimal activity in the range 1.8-2 mM Mg2+ and 125-175 mM KCl. The polysome-directed in vitro products exhibit a complexity comparable to the in vivo products resolved on the two-dimensional polyacrylamide gels of O'Farrell [O'Farrell, P. (1975) J. Biol. Chem. 250, 4007-4021]. The system is capable of active chain reinitiation as indicated by partial inhibition by 7-methylguanosine 5'-monophosphate and pactomycin and N-terminal end analysis of in vitro products. This system can also translate polysomes from diverse tissues such as mouse liver, rat liver, and rat brain. The levels and also the authenticity of translation of rat liver albumin and mouse liver carbamoyl phosphate synthetase I were tested by immunoprecipitation with monospecific antibodies. The results show that the major as well as the minor translation products are synthesized in this system at levels comparable to the physiological levels.

Activity of protein synthesis initiation factors in cytosol from rat liver and muscle and ascites cells

The decrease induced by appropriate initiation factors in [Mg] required for optimal polyphenylalanine synthesis primed by poly(U) has been used as an assay to test for the presence of these factors in cytosol of rat liver and muscle and of ascites cells. By calculation and from changes in [Mg] optima on addition of phosphoenolpyruvate and phosphocreatine, the [Mg2+] as opposed to total [Mg] required for polyphenylalanine synthesis has been determined. Contrary to a previous report, factors necessary for polyphenylalanine synthesis at low [Mg] appear to be present in cytosol of muscle and liver as well as that from ascites cells. The significance of these finding in relation to distribution of specific initiation factors is discussed.

mRNP proteins, initiation factors and phosphorylation

Information has been collected to stimulate interest regarding the nature and the possible role of mRNP protein phosphorylation in a cytoplasmic control mechanism for protein synthesis. It does not imply a direct relationship between mRNP protein and initiation factors. These proteins have some properties in common (e.g. molecular weight, phosphorylation, protein kinase, mRNA binding activity). We emphasize that some free mRNP may be translatable after modification of their protein by interference factors belonging to other cellular compartments. Thus, some mRNP proteins might possess initiation factor or protein synthetic activity if we accept Spirin's theory, i.e., "Eukaryotic messenger RNA and informosomes omnia mea mecum porto.

A simple procedure for the isolation and purification of protamine messenger ribonucleic acid from trout testis

Preparation of milligram quantities of purified poly(A)+ (polyadenylated) protamine mRNA from trout testis tissue was accomplished by a simple procedure using gentle conditions. This involves chromatography of the total nucleic acids isolated by dissociation of polyribosomes with 25 mM-EDTA to release messenger ribonucleoprotein particles and deproteinization of the total postmitochondrial supernatant with 0.5% sodium dodecyl sulphate in 0.25 M-NaCl by binding it to a DEAE-cellulose column. Total RNA was bound under these conditions, and low-molecular-weight RNA, lacking 18S and 28S RNA, could be eluted with 0.5 M-NaCl and chromatographed on oligo(dT)-cellulose columns to select for poly(A)+ RNA. Further purification of both the unbound poly(A)- RNA and the bound poly(A)+ mRNA on sucrose density gradients showed that both 18S and 28S rRNA were absent, being removed during the DEAE-cellulose chromatography step. Poly(A)- RNA sedimented in the 4S region whereas the bound poly(A)+ RNA fraction showed a main peak at 6S [poly(A+) protamine mRNA] and a shoulder in the 3-4S region. Analysis of the main peak and the shoulder on a second gradient showed that most of the main peak sedimented at 6S, whereas the shoulder sedimented slower than 4S. The identity of the poly(A)+ protamine mRNA was established by the following criteria: (1) purified protamine mRNA migrated as a set of four bands on urea/polyacrylamide-gel electrophoresis; (2) analysis of the polypeptides synthesized in the wheat-germ extract by starch-gel electrophoresis showed a single band of radioactivity which co-migrated exactly with the carrier trout testis protamine standard; and (3) chromatography of the polypeptide products on CM-cellulose (CM-52) showed the presence of three or four radioactively labelled protamine components that were co-eluted with the unlabelled trout testis protamine components added as carrier. The availability of large quantities of purified protamine mRNA should now permit a more thorough analysis of its physical and chemical properties.

Effect of homopolyribonucleotides on messenger ribonucleoprotein particles

A discrete set of polypeptides copurify with and appear to be specifically attached to mRNA from polysomes of eukaryotic cells. This report describes the effect of homopolyribonucleotides on mRNA-protein complexes separated from ribosome subunits by oligo(dT)-cellulose chromatography. It is shown that poly (U) and poly (A) can release mRNA-protein complexes adsorbed to oligo(dT)-cellulose, whereas poly (C) and poly(I) are much less effective in this process. Analysis of polyribonucleotide released material showed that poly(U) effectively dissociated the mRNA-protein complexes while poly(A) caused no or only partial derangement of these particles. The specificities seen in the polyribonucleotide effects in turn suggest a high degree of specificity in the interaction between the proteins and mRNA.

Characterization of mRNA-protein complexes from mammalian cells

In a previous report we described the use of oligo(dT)-cellulose for the isolation of mRNA-protein complexes from EDTA-dissociated polysomes extracted from normally growing or adenovirus infected KB-cells (I). Experiments presented here provide evidence that proteins involved in these complexes bind specifically to mRNA since: a) the proteins and mRNA cosediment through sucrose gradients, b) they adsorb and elute from oligo(dT)-cellulose together, and c) analysis of the products from ribonuclease digestion experiments show that the poly (A) end and a separate small fraction of the mRNA are resistant to the enzymes and attached to protein.

In-vitro translation of messenger-RNA from developing bean leaves. Evidence for the existence of stored messenger-RNA and its light-induced mobilisation into polyribosomes

Poly(A)-containing messenger RNA was purified from polyribosomes isolated from the primary leaves of 7-day-old dark-grown seedlings of Phaseolus vulgaris var. Masterpiece. Analysis of the messenger RNA on 2.4% polyacrylamide gels showed that it consists of a heterogeneous population of molecules with an average molecular weight of 500,000. The nucleotide composition of the RNA was 16.0% cytidylic acid, 39.4% adenylic acid, 21.3% guanylic acid and 23.2% uridylic acid. Based on the degree of resistance of the RNA to digestion with ribonucleases A and T1 the average length of the poly(A) sequence was calculated to be 120 nucleotides. No significant differences in mobility in polyacrylamide gels, nucleotide composition or polyadenylic acid content were found between the poly(A)-containing mRNA from polyribosomes of primary leaves of dark-grown plants and those given a 16 h white light treatment. Purified poly(A)-containing mRNA was shown to direct the incorporation of [(35)S]methionine into proteins in an in vitro protein-synthesising system from wheat germ. The protein products were fractionated according to molecular size by electrophoresis in 15% polyacrylamide/urea/SDS gels and the protein bands were detected by fluorography. Messenger RNAs directing the synthesis of three polypeptides with molecular weights of 34,000, 32,000 and 25,000 were detected in polyribosomes of plants following white light treatment. These messenger RNAs were absent, or present in much lower amounts, in polyribosomal messenger RNA from leaves of dark-grown plants, although they were present in total cell poly(A)-containing RNA. This indicates that certain messenger RNAs may be stored in the dark and that light stimulates these RNAs to engage in polyribosome formation. Continuous far-red (730 nm) irradiation for 4 h also caused the appearance of these messenger RNAs in the polyribosomes although 5 min red light followed by 4 h darkness had little effect. This suggests that phytochrome acting in the "high energy" mode, may be the photoreceptor responsible for initiating the response.

Rat b/b anemia: translation of normal and anemic globin mRNA in wheat-germ cell-free system

Globin mRNAs were isolated from circulating reticulocytes both from rats carrying a homozygous, recessive mutation causing a severe thalassemia-like syndrome and from normal rats. After first identifying the rat globin chains as alpha or beta chains, the translational products primed by both polysomal and nonpolysomal mRNAs in wheat germ 30000 x g supernatant were analyzed: the ratio of alpha to beta globin mRNAs found in polysomes isolated from mutant rats is identical to the ratio of their products synthesized in vivo while the ratio of these mRNAs is quite different in the nonpolysomal fraction, the latter being enriched in alpha globin mRNA. No difference is found in the ratio of alpha and beta globin mRNAs in the polysomal and nonpolysomal RNA isolated from normal rats, both being identical to the ratio of their products synthesized in vivo. One third of the total amount of mRNA found in mutant cells is not in polysomes as compared to only 6 percent for the mRNA from normal lysates. These results suggest that a translational control mechanism is involved although the decreased globin synthesis in b/b anemia can not be fully accounted for by its operation.

Isolation and characteristics of mRNPs associated with bound polyribosomes from the ewe lactating mammary gland

The dissociation of bound polyribosomes from ewe lactating mammary gland, by EDTA or Puromycin-KC1 treatment releases Messenger Ribonucleoprotein particles (mRNP'S) which sediment in sucrose gradients as a single component having a sedimentation coefficient of about 22S. RNAs derived from mRNPs sediment between 9S and 15S and direct the synthesis of milk proteins in reticulocyte cell free systems. The mRNP protein moiety contains two major polypeptides with molecular weights of 52,000 and 72,000 daltons and two minor polypeptides with apparent molecular weights of 58,000 and 60,000 daltons.

Calf lens messenger ribonucleoprotein complexes. Characterization and comparison of template activity with corresponding mRNAs

Lens messenger ribonucleoprotein complexes have been isolated from calf lens polysomes by sucrose gradient centrifugation after puromycin-induced dissociation. A 10 S mRNA was released from a 13 S messenger ribonucleoprotein complex and a 14 S mRNA from a 19 S messenger ribonucleoprotein complex. Two major protein components with molecular weights of approx. 64 000 and 40 000 were isolated from each of the messenger ribonucleoprotein complexes after RNAase digestion. Buoyant density determinations suggest that the messenger ribonucleoprotein complexes contain approximately one mol of each major protein species per mol mRNA. In contrast to lens mRNA, lens messenger ribonucleoproteins are poor templates for transcription with avian myeloblastosis virus reverse transcriptase. Similar results were also obtained with globin messenger ribonucleoprotein containing either two major protein species (or deficient in the lower molecular weight protein species). Polynucleotide phosphorylase eliminates the reverse transcription template activity of the lens mRNA. This effect is blocked in the messenger ribonucleoprotein. Such observations suggest that at least one of the protein components associated with lens messenger ribonucleoprotein may be located in the 3'-terminal region. Only a small variation in translation activity was observed between the messenger ribonucleoproteins and their respective mRNAs.

Globin mRNA translation on Artemia salina ribosomes with components from Friend leukemia cells

Globin mRNA can be translated with relatively high efficiency in a fractionated cell-free system containing ribosomes prepared from cytst of Artemia salina. These ribosomes have unusually low endogenous activity for peptide synthesis in the absence of added mRNA. The system requires components from the postribosomal supernatant and from the 0.5 M KCl ribosomal wash fraction. Both these fractions were derived from either rabbit reticulocytes or unstimulated Friend leukemia cells that produce little or no hemoglobin. The activity of mRNA and enzyme fractions from rabbit reticulocytes and Friend leukemia cells were tested in this system in vitro for their ability to direct the synthesis of the alpha and beta chains of globin. The alpha:beta chain ratio synthesized from mRNA in the rabbit reticulocyte salt wash fraction was 4:1. The corresponding value for the 9-S mRNA fraction from the salt-washed reticulocyte ribosomes was 1:4, thus these two fractions appear to provide sources enriched in either alpha or beta globin mRNA. Under all conditions tested, the ratio and amounts of peptides formed in vitro appear to reflect mRNA composition. Globin mRNA from dimethysulfoxide-stimulated Friend leukemia cells when translated in vitro produced alpha and beta chains in a ratio of 1:1. These peptides are formed in the same ratio in the intact cells.

Translation of poly(riboadenylic acid)-enriched messenger RNAs from the yeast, Saccharomyces cerevisiae, in heterologous cell-free systems

Poly(riboadenylic acid) [poly(A)] enriched messenger RNAs from yeast have been used to direct the synthesis of yeast polypeptides in mouse Krebs II ascites and wheat embryo extracts. Both cell-free systems, synthesize polypeptides over a molecular weight range of 10,000-100,000. Autoradiograms of sodium dodecyl sulfate-polyacrylamide slab gels used to fractionate [35S]methionine-labeled in vitro products reveal that about 25 major bands (each of them possibly representing multiple polypeptides) are produced by each cell-free system. Each of these coelectrophoreses with a major polypeptide labeled in vivo or in a yeast lysate. These results suggest that cell-free translational machinery from eukaryotes is not able to discriminate in an all or none fashion against messenger RNAs which are available to it. While yeast poly(A)-enriched messenger RNA directs the synthesis polypeptides over approximately the same molecular weight range in both cell-free systems, the wheat germ system directs the incorporation of 45 times the amount of [3H]serine into Cl3CCOOH-precipitable polypeptides. This is in contrast to the 2.5-fold more efficient translation of hemoglobin mRNA in the wheat embryo extract. Thus, the extract from mammalian cells is able to translate mRNA from a lower plant with a much lower efficiency than it translates hemoglobin mRNA, and at a lower efficiency than is observed using a cell-free system from wheat embryos. This indicates that the wheat embryo system is the one of choice for translation of yeast messenger RNA.

The cell-free translation of Rauscher leukemia virus RNA into high molecular weight polypeptides

Rauscher leukemia virus (RLV) 65-S RNA, 35-S mengovirus RNA and reticulocyte A-rich RNA each stimulated cell-free protein synthesis in a JLS-V5 cell derived S-30 system. rRNA, however, was not stimulatory in this system. Of the stimulated protein products only those synthesized in response to added RLV RNA were immune-precipitable with anti-RLV rabbit serum. Furthermore, cell-free incubations with pactamycin at a concentration which specifically inhibits initiation and not elongation prevented the stimulation of amino acid incorporation in response to added RLV RNA. Analysis of the polypeptides synthesized by the cell-free system in response to reticulocyte A-rich RNA, showed them to be globin-like and, therefore, also mRNA specific. The RLV RNA-directed product included at least two classes of polypeptides (mol. wts of 140 000-185 000 and 50 000-75 000) both of which were larger than the group specific polypeptides of mature virions. None of the internal structural polypeptides of mature virions were synthesized in response to RLV RNA. The large molecular weight, viral-specific polypeptides are candidate precursor polyproteins which may represent the translational products of a polycistronic mRNA with a single initiation site.

Synthesis of alpha- and beta-globin directed by messenger ribonucleoprotein from rabbit reticulocytes

The translation of globin messenger ribonucleoprotein (mRNP) obtained from high salt-washed rabbit reticulocyte ribosomes by treatment with EDTA was investigated using a cell-free system from mouse Krebs II ascites tumour cells. The messenger activity of the mRNP and the mRNA derived from it by mild deproteinization was compared in the presence and absence of reticulocyte initiation factors. Both forms gave identical products over a wide range of messenger concentration and there was no qualitative or quantitative difference in their efficiency as messengers. It is concluded that the proteins associated with polysomal mRNA do not alter the specificity of translation of alpha- and beta-globin messengers or the requirement for initiation factor.

Efficient translation of tobacco mosaic virus RNA and rabbit globin 9S RNA in a cell-free system from commercial wheat germ

Extracts prepared from commercially available wheat germ efficiently translate messenger RNAs from either viral or eukaryotic origin. The addition of tobacco mosaic virus RNA stimulated amino-acid incorporation more than 100-times and rabbit globin 9S RNA between 20- and 30-times. The in vitro product directed by globin 9S RNA comigrated precisely with authentic rabbit globin on sodium dodecyl sulfate-polyacrylamide gels. Furthermore, during high voltage ionophoresis two [(35)S]methionine-labeled tryptic peptides synthesized in vitro comigrated in two dimensions with alphaT5 and betaT5 tryptic peptides from authentic [(35)S]methionine-labeled rabbit globin.

Translation of rabbit hemoglobin meessenger RNA in vitro with purified and partially purified components from brain or liver of different species

Two mammalian systems have been developed for efficient in vitro translation of exogenous messenger RNA (rabbit globin mRNA). One system is completely derived from guinea pig brain, and the other is from liver of different species. Both systems consist of purified 60S and 40S ribosomal sununits, unseparated initiation factors partially purified by ammonium sulfate precipitation and DEAE-cellulose chromatography, and pH 5 enzyme fractions as sources of elongation and termination factors, aminoacyl-tRNA synthetases, and transfer RNA. Translation depends completely upon exogenous mRNA and initiation factors. Extraction of initiation factors from microsomes or ribosomes has been improved for these tissues by inclusion of Mg(++) ions in the 0.5 M KCl extraction solution. Both systems synthesize complete rabbit alpha- and beta-globin chains, but in variable ratios. The overall efficiencies of the two systems are about 60% (liver) and 40% (brain) of a comparable system with rabbit reticulocyte initiation factors.