Translation of reovirus messenger RNAs synthetesized in vitro into reovirus polypeptides by several mammalian cell-free extracts

Expressing Cloned Genes for Protein Production, Purification, and Analysis

Obtaining high quantities of a specific protein directly from native sources is often challenging, particularly when dealing with human proteins. To overcome this obstacle, many researchers take advantage of heterologous expression systems by cloning genes into artificial vectors designed to operate within easily cultured cells, such as Escherichia coli, Pichia pastoris (yeast), and several varieties of insect and mammalian cells. Heterologous expression systems also allow for easy modification of the protein to optimize expression, mutational analysis of specific sites within the protein and facilitate their purification with engineered affinity tags. Some degree of purification of the target protein is usually required for functional analysis. Purification to near homogeneity is essential for characterization of protein structure by X-ray crystallography or nuclear magnetic resonance (NMR) and characterization of the biochemical and biophysical properties of a protein, because contaminating proteins almost always adversely affect the results. Methods for producing and purifying proteins in several different expression platforms and using a variety of vectors are introduced here.

Cell-free synthesis of human toll-like receptor 9 (TLR9): Optimization of synthesis conditions and functional analysis

The Toll-like receptor family belongs to the group of pathogen recognition receptors which is responsible for the discrimination of self and non-self pathogen-associated molecular patterns (PAMP's). Toll-like receptors play an important role in the innate immunity and defects in protein expression or polymorphism is linked to various diseases such as Systemic Lupus Erythematosus (SLE). The elucidation of the underlying mechanism is crucial for future treatment and therapeutics of toll-like receptor linked diseases. Herein, we report the cell-free synthesis of human Toll-like receptor 9 (hTLR9) using CHO lysate and the continuous exchange cell-free (CECF) synthesis platform. The functionality of this protein was demonstrated by an ELISA binding assay using the ectodomain of TLR9 (TLR9-ECD).

Reovirus Nonstructural Protein σNS Acts as an RNA Stability Factor Promoting Viral Genome Replication

Viral nonstructural proteins, which are not packaged into virions, are essential for the replication of most viruses. Reovirus, a nonenveloped, double-stranded RNA (dsRNA) virus, encodes three nonstructural proteins that are required for viral replication and dissemination in the host. The reovirus nonstructural protein σNS is a single-stranded RNA (ssRNA)-binding protein that must be expressed in infected cells for production of viral progeny. However, the activities of σNS during individual steps of the reovirus replication cycle are poorly understood. We explored the function of σNS by disrupting its expression during infection using cells expressing a small interfering RNA (siRNA) targeting the σNS-encoding S3 gene and found that σNS is required for viral genome replication. Using complementary biochemical assays, we determined that σNS forms complexes with viral and nonviral RNAs. We also discovered, using in vitro and cell-based RNA degradation experiments, that σNS increases the RNA half-life. Cryo-electron microscopy revealed that σNS and ssRNAs organize into long, filamentous structures. Collectively, our findings indicate that σNS functions as an RNA-binding protein that increases the viral RNA half-life. These results suggest that σNS forms RNA-protein complexes in preparation for genome replication.IMPORTANCE Following infection, viruses synthesize nonstructural proteins that mediate viral replication and promote dissemination. Viruses from the family Reoviridae encode nonstructural proteins that are required for the formation of progeny viruses. Although nonstructural proteins of different viruses in the family Reoviridae diverge in primary sequence, they are functionally homologous and appear to facilitate conserved mechanisms of dsRNA virus replication. Using in vitro and cell culture approaches, we found that the mammalian reovirus nonstructural protein σNS binds and stabilizes viral RNA and is required for genome synthesis. This work contributes new knowledge about basic mechanisms of dsRNA virus replication and provides a foundation for future studies to determine how viruses in the family Reoviridae assort and replicate their genomes.

Cotranslational incorporation of non-standard amino acids using cell-free protein synthesis

Over the last years protein engineering using non-standard amino acids has gained increasing attention. As a result, improved methods are now available, enabling the efficient and directed cotranslational incorporation of various non-standard amino acids to equip proteins with desired characteristics. In this context, the utilization of cell-free protein synthesis is particularly useful due to the direct accessibility of the translational machinery and synthesized proteins without having to maintain a vital cellular host. We review prominent methods for the incorporation of non-standard amino acids into proteins using cell-free protein synthesis. Furthermore, a list of non-standard amino acids that have been successfully incorporated into proteins in cell-free systems together with selected applications is provided.

Evidence that vesicular stomatitis virus produces double-stranded RNA that inhibits protein synthesis in a reticulocyte lysate

Cell-free protein synthesis by reticulocyte lysates was inhibited by a polyadenylated RNA fraction extracted from HeLa cells infected with vesicular stomatitis virus (VSV) but not by polyadenylated RNA from mock-infected HeLa cells. A similar inhibitor of cell-free protein synthesis was found in a polyadenylated fraction of RNA transcribed in vitro by VSV but not in untranscribed nucleocapsids. Fractionation of the VSV transcription product showed that the translation inhibitor segregated with nucleocapsids containing newly transcribed polyadenylated or non-polyadenylated RNA, as determined by oligodeoxythymidylate-cellulose chromatography. The inhibitors present in VSV-infected HeLa cells and in VSV in vitro transcripts both appeared to be double-stranded RNA, as judged by the characteristics for inhibition of reticulocyte cell-free protein synthesis described by Hunter et al. (J. Biol. Chem. 250:409-417, 1975). The double-stranded nature of the VSV RNA inhibitor was supported by the finding that the translational inhibitory effect was inactivated by melting the inhibitor in the absence of salt and by micrococcal nuclease.

Structure and function of the reovirus genome

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Discontinuous translation of silk fibroin in a reticulocyte cell-free system and in intact silk gland cells

Silk fibroin mRNA was translated in a rabbit reticulocyte cell-free system. Addition of tRNA from silk glands was essential for complete translation of the fibroin polypeptide. (Mr approximately 400,000). Synthesis of full-sized product took at least 85 min. In addition to full-size product, a large number of smaller polypeptides were observed upon analysis by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Evidence is presented that these smaller polypeptides are growing fibroin chains that transiently accumulate as discrete size classes due to discontinuities in the translation process. These discontinuities, or pauses, occur at specific sites in the fibroin mRNA template. The relative duration of the pauses can be experimentally modulated by changing the source of the supplementary tRNA added to the in vitro system. Silk glands were incubated in organ culture under conditions where essentially exclusive labeling of newly synthesized fibroins was attained. Analysis in sodium dodecyl sulfate gels showed that the labeling pattern of nascent silk fibroins is similar to the pattern observed in the reticulocyte cell-free system. This result suggests that discontinuities or pauses in polypeptide chain elongation also occur in vivo under conditions of organ culture.

Immunological and chemical identification of a neurophysin-containing protein coded by messenger RNA from bovine hypothalamus

The biosynthetic origin of the 10,000 molecular weight neurophysins, carriers of the peptide hormones oxytocin and vasopressin, has been studied by cell-free synthesis, Poly(A)-RNA was isolated from bovine hypothalamus and translated in a wheat germ system containing (35)S- or (3)H-labeled amino acids. A number of unique [(35)S]cysteine- but few [(35)S]-methionine-labeled proteins were coded by hypothalamic mRNA. A single, major, isotopically labeled protein (molecular weight 23,000-25,000) was immunoprecipitated from these translation mixtures by addition of purified antibodies against bovine neurophysin II and subsequent addition of Cowan I strain of Staphylococcus aureus. Specificity of the immunoprecipitation was demonstrated by competition with unlabeled authentic neurophysins and the absence of competition with structurally unrelated ovalbumin. Furthermore, neither nonimmune serum nor purified antibodies against ribonuclease immunoprecipitated the protein. The [(35)S]cysteine-labeled protein that was specifically immunoprecipitated was oxidized with performic acid and digested with trypsin in the presence of unlabeled, authentic bovine neurophysin II. Peptide mapping revealed that most of the major [(35)S]cysteine-labeled peptides (of the translation product) were identical to major cysteine-containing peptides of authentic neurophysin. The data show that hypothalamic mRNA directs the translation of several unique cysteine-rich proteins in an in vitro cell-free system. Furthermore, one of these proteins, which has a higher molecular weight than authentic neurophysin, is recognized by purified antibodies to bovine neurophysin II and has cysteine-containing tryptic peptides in common with those of authentic neurophysin. The data suggest that this protein is the primary translation product, pre-pro-neurophysin.

Reovirus-specific enzyme(s) associated with subviral particles responds in vitro to polyribocytidylate to yield double-stranded polyribocytidylate-polyriboguanylate

In reovirus-infected cells, virus-specific particles accumulate that have associated with them a polyribocytidylate [poly(C)]-dependent polymerase. This enzyme copies in vitro poly(C) to yield the double-stranded poly(C).polyriboguanylate [poly(G)]. The particles with poly(C)-dependent polymerase were heterogeneous in size, with most sedimenting from 300S to 550S. Exponential increase in these particles began at 23 h, and maximal amounts were present by 31 h, the time of onset of exponential growth of virus at 30 degrees C. Maximal amounts of particles with active transcriptase and replicase were present at 15 and 18 h after infection. Thereafter, there was a marked decrease in particles with active transcriptase and replicase until base line levels were reached at 31 h. Thus, the increase in poly(C)-responding particles occurred coincident with the decrease in particles with active transcriptase and replicase. The requirement for poly(C) as template was specific because no RNA was synthesized in vitro in response to any other homopolymer, including 2'-O-methyl-poly(C). Synthesis was optimal in the presence of Mn(2+) as the divalent cation, and no primer was necessary for synthesis. In contrast, the dinucleotide GpG markedly stimulated synthesis in the presence of 8 mM Mg(2+). The size of the poly(C).poly(G) synthesized in vitro was dependent on the size of the poly(C) used as template. This suggested that the whole template was copied into a complementary strand of similar size. The T(m) of the product was between 100 and 130 degrees C. Hydrolysis of the product labeled in [(32)P]GMP with alkali or RNase T2 yielded GMP as the only labeled mononucleotide. This does indicate that the synthesis of the poly(G) strand in vitro did not proceed by end addition to the poly(C) template, but proceeded on a separate strand.

Genome RNAs and polypeptides of reovirus serotypes 1, 2, and 3

The virus-specific double-stranded genome RNA and polypeptides present in virions and cells infected with the three mammalian reovirus serotypes have been examined by co-electrophoresis in several different polyacrylamide gel systems. The double-stranded RNA and polypeptide species previously described for type 3 Dearing were found to have corresponding species in the other serotypes examined. In each serotype several RNA and polypeptide species were found to have different electrophoretic mobilities from the corresponding RNA or polypeptide species of type 3 Dearing. The combination of electrophoretic variants among the RNAs and polypeptides of the reovirus serotypes gave electrophoretic markers in all 10 of the reovirus genes. The usefulness of these electrophoretic markers in "mapping" the reovirus genome is discussed.

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.

Control of protein synthesis by a temperature-sensitive mutant of reovirus 3. I. Temperature-sensitive function of ts261-b mutant

The ability of a temperature-sensitive (ts) mutant of reovirus, ts261-b, to synthesize virus-specific RNAs and proteins during infection at the nonpermissive temperature (37 degrees C) was investigated. The relative amounts of the mutant virus-specific single-stranded (ss) RNA's and double-stranded (ds) RNA's synthesized in cells at 37 degrees C were 20 to 25% as much as those synthesized in the wild-type virus-infected cells. The 10 segments of the mutant ds RNAs and the three size classes of the ss RNAs were synthesized in the usual proportions. The methylation of the mutant viral mRNA's (ss RNAs) was not blocked at 37 degrees C in infected cells. A striking temperature-sensitive restricted function of the ts261-b mutant was expressed in the synthesis of the viral proteins. This study, which uses an in vitro protein-synthesizing system reconstituted with an endogenous polysomal fraction and a postribosomal supernatant from reovirus-infected cells, has demonstrated that the endogenous polysomes obtained from ts261-b mutant-infected cells at 37 degrees C are not active in the synthesis of the viral polypeptides of known molecular weights, and the amounts of the mutant viral polypeptides synthesized in vitro by these polysomes are 5 to 9% of those synthesized by the corresponding fraction from wild-type-infected cells. The impaired protein-synthesizing capacity of the mutant virus-specific polysomes can be restored during maintenance of the infected cells at 30 degrees C after shift-down from 37 degrees C. The in vitro synthesis of viral polypeptides of known size by the active endogenous polysomes derived from cells infected at the permissive temperature is accelerated by the addition of the postribosomal supernatant obtained from cells infected at the permissive temperature. The postribosomal supernatant from mutant-infected cells at 37 degrees C did not have a stimulatory effect, but rather, it inhibited in vitro viral protein synthesis.

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.

Reovirus-specific polypeptides: analysis using discontinuous gel electrophoresis

The electrophoretic analysis of reovirus-specific polypeptides in infected cells using a discontinuous gel system has allowed the resolution of additional viral-specific polypeptides, including one large-sized gamma3 and two (or possibly three) medium-sized (mu3, mu4, mu5(?)) species. The proteins designated mu0, sigma1, and sigma2 based on electrophoretic mobility in gel systems containing phosphate-urea correspond to mu4, sigma2, and sigma1, respectively, when analyzed in systems containing Tris-glycine. It is likely that protein modifications (phosphorylation and glycosylation) are responsible for at least some of these differences.

Control of peptide chain initiation in uninfected and virus infected cells by membrane mediated events

Initiation of protein synthesis in tissue culture cells is rapidly inhibited or blocked by addition of either DMSO, ethanol, TPCK, cytochalasin B, or sucrose to the growth medium. In contrast, these agents do not interfere with the initiation of protein synthesis in cell-free extracts to a comparable extent. These results support the hypothesis that protein synthesis in tissue culture cells can be influenced by membrane mediated events. Translation of viral mRNA in RNA virus infected cells is resistant to a number of these inhibitors of peptide chain initiation and proceeds under conditions where translation of host mRNA is almost completely suppressed. It appears that viral mRNA possesses a greater ability than host mRNA to form mRNA-ribosome initiation complexes when the overall rate of peptide chain initiation is reduced. This observation has led to a number of predictions concerning the strategy of virus directed suppression of host mRNA translation. Under optimal growth conditions protein synthesis appears to be regulated mainly, but not exclusively, by the amount of the mRNA available for translation. However, when cellular growth and/or the overall rate of peptide chain initiation is restricted, control of protein synthesis at the translational level becomes decisive with the translation of each mRNA species proceeding with its own characteristic efficiency most probably as a result of inherent differential affinities of individual mRNA species for ribosomes.

Complete translation of poliovirus RNA in a eukaryotic cell-free system

Poliovirus RNA stimulates imcorporation of 35S from both [35S]methionine and formyl-[35S]methionyl-tRNAfMet in cell-free systems derived from HeLa cells or from poliovirus-infected HeLa cells. The largest product formed under the direction of the viral RNA is the same size as the polyprotein thought to represent translation of the entire RNA. Synthesis of this polyprotein and other large products was stimulated greatly by increasing the salt concentration during the reaction from the optimum for initiation (90 mM) to the optimum for elongation (155 mM). Only one initiation peptide could be identified, and a tryptic digest of the product contained mainly peptides that cochromatographed with peptides from authentic viral proteins. The RNA from a deletion mutant of poliovirus initiated protein synthesis at the same site used by standard RNA and programmed synthesis of an appropriately deleted set of polypeptides. The results strongly support the model of translation of poliovirus RNA from a single initiation site into a continuous polyprotein that is cleaved to form the functional proteins. It is suggested that uninfected HeLa cell extracts can carry out the cleavages of nascent polyprotein.

Synthesis of reovirus-specific polypeptides in cells pretreated with cycloheximide

When L cells are infected with reovirus in the presence of cycloheximide neither virus-specific polypeptides nor viral double-stranded RNA are synthesized. There is some synthesis of viral single-stranded RNA, transcribed mainly from segments L1, M3, S3, and S4 of the 10 viral genomic segments, and in previous work this has been termed the early mRNA pattern. In an attempt to determine whether these early transcripts are functional mRNA's, the transcripts were allowed to accumulate for a period of 17.5 h at 31 C in cycloheximide-treated cells. The cycloheximide was removed and the cells were exposed for various periods to radioactive amino acids to label any virus-specific polypeptides that might be synthesized. An immunoprecipitation technique was used to separate the viral polypeptides from cellular extracts and this precipitate was then analyzed on sodium dodecyl sulfate-polyacrylamide gels. Within 30 min of cycloheximide removal, four major polypeptides (lambda2, mu0, sigma2a, and sigma3) and two minor polypeptides (lambda1 and mu2) were found. In infected cells without cycloheximide eight viral polypeptides (lambda1, lambda2, mu0, mu2, sigma1, sigma2, sigma2a, sigma3) were found at 17.5 h after infection and the same pattern was found between 3 to 4 h after removal of cycloheximide which had been present for 17.5 h after infection. The latter result shows that the cycloheximide inhibition is reversible and that the cells readily recovered and synthesized the normal complement of viral polypeptides. In one set of experiments cordycepin was added to infected cells immediately after the removal of cycloheximide at 17.5 h to inhibit the synthesis of new viral transcripts. During the succeeding 4 h in the presence of cordycepin, the pattern of protein synthesis was the same as that obtained during the 30 min after cycloheximide removal. It is concluded that the polypeptides formed right after removal of cycloheximide are the translation products of transcripts accumulated during cycloheximide treatment and, therefore, that these transcripts are functional viral mRNA's.

Messenger activity of RNA transcribed in vitro by DNA-RNA polymerase associated to vaccinia virus cores

The coding properties of RNA transcribed in vitro by purified vaccinia cores have been investigated using Krebs ascites tumor cells, L cells, and reticulocyte lysates. Six to 10 proteins synthesized in vitro are separated on polyacrylamide gels by electrophoresis in the presence of sodium dodecyl sulfate. Their molecular weights vary from 10,000 to 44,000. The electrophoretic behavior of these proteins is similar to that of early proteins isolated from infected L cells. The tryptic peptide analysis of one of these proteins indicates similarity in amino acid sequences. These results show fidelity of both in vitro transcription and molecular weight above 44,000 are synthesized in vitro does not seem due to a competition between 12S mRNA synthesized in excess and RNA of a higher sedimentation coefficient present in a lower amount.

5'-Terminal 7-methylguanosine in eukaryotic mRNA is required for translation

Unmethylated reovirus and VSV mRNAs are specifically methylated to form 5'-terminal structures of the type, m-7-G(5')ppp(5')N by protein synthesising extracts prepared from wheat germ and mouse L cells. Reticulocyte mRNA also contains 5'-terminal m-7-G. MRNAs having 5'-terminal m-7-G stimulate protein synthesis in vitro. Removal of m-7-G by beta-elimination abolishes translation of the mRNAs.

Translation of individual species of vesicular stomatitis viral mRNA

Vesicular stomatitis virus mRNAs from three of the four bands fractionated by polyacrylamide gel electrophoresis in 99% formamide have been eluted from gels and translated in the Krebs II ascites cell-free system. Band 2 mRNA (0.7 times 10-6 daltons) directed the synthesis of the protein moiety of the glycoprotein (G), and band 3 (0.55 times 10-6 daltons) coded for the nucleocapsid (N) protein. Band 4 mRNA (o.28 times 10-6 daltons) directed the synthesis of the NS and matrix (M) proteins. The authenticity of viral proteins synthesized in vitro was shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by analysis of (35-S)metionine-labeled tryptic peptides. These results are consistent with the complexity analysis and coding capacities for the vesicular stomatitis virus mRNA species presented in the accompanying paper.

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.

Non-specific stimulation of cell-free protein synthesis by a dialyzable factor isolated from reticulocyte initiation factors ("iRNA")

The specificity of a dialyzable component, isolated from rabbit reticulocyte initiation factors, in stimulating protein synthesis was examined. It appeard that his factor (hereafter designated as "iRNA") was able to restore the activity of initiation factor preparations that were inactivated by dialysis. The "iRNA" from reticulocytes stimulated polypeptide synthesis directed by the homologous globin mRNA as well as heterologous lens crystallin mRNAs. No selectivity in its stimulating action with regard to the type of mRNA studied could be observed. The source of ribosomes or supernatant enzymes did not influence the effect of "iRNA". However, in an ascites lysate that was dependent on the addition of initiation factors, "iRNA" increased polypeptide formation only marginally, suggesting that in this lysate a similar factor was already present in an active form. It is concluded that "iRNA" may regulate protein synthesis, but without exhibiting specificity towards mRNAs, at least those tested so far.

The identification of collagen messenger RNA

RNA isolated from calvaria of 16- to 18- day-old chick embryos, assayed in rabbit reticulocyte lysates, programs the synthesis of a collagenase-sensitive protein with the molecular weight of collagen pro-alpha-chains. When RNA labeled with [(3)H]uridine for 2 hr and chased for 1 or 2 hr was electrophoresed on aqueous polyacrylamide gels, most of the radioactivity not in 28S or 18S rRNA migrated with an apparent molecular weight of about 1,800,000. After oligo(dT)-cellulose chromotography and analysis in 99% formamide gels, this nonribosomal, rapidly labeled calvaria RNA species migrates at 28S-30S and thus has a molecular weight of at least 1,600,000. Both the ability to program the synthesis of collagenase-sensitive protein in reticulocyte lysates and the presence of a single prominent rapidly labeled 30S peak in acrylamide gels strongly support the deduction that there is only one major mRNA species in calvaria and that this species is collagen messenger RNA.

Methylated messenger RNA synthesis in vitro by purified reovirus

Purified human reovirus contains RNA methylase activity in addition to an RNA polymerase. Virions incubated under appropriate conditions in the presence of S-adenosyl-L-methionine synthesize mRNA that is specifically methylated in the 5'-terminal guanosine. Alkaline digestion of the methylated RNA released a 5'-terminal dinucleotide, ppG'pCp, indicating that the guanosine contains 2'-O-methylribose. The possible roles of methylation in viral and cellular mRNA function is discussed.

Poly(A) polymerase activity in reovirus

An enzymatic activity which synthesized oligo(A) in vitro was found in highly purified reovirus. The poly(A) polymerase activity was dependent on Mn(2+) and utilized only ATP, whereas the virion-associated RNA polymerase required all four ribonucleoside triphosphates and Mg(2+). Oligo(A) synthesis was demonstrated with complete virions and infectious subviral particles derived from virus by limited chymotrypsin digestion but not with cores, a product of extensive chymotrypsin digestion of virus. The enzymatic product and the oligo(A) from purified virions were isolated by binding to oligo(dT)-cellulose columns. Most of the in vitro product was similar in size and structure to the oligo(A) from purified virions by the criteria of gel electrophoresis, DEAE-cellulose chromatography, end-group analysis, and sensitivity to RNase. The evidence suggests that oligo(A) synthesis is mediated by the poly(A) polymerase during a late step in viral morphogenesis and may result from an alternative activity of the virion-associated transcriptase.

Formation of Sindbis virus capsid protein in mammalian cell-free extracts programmed with viral messenger RNA

Extracts from Krebs II ascites cells and rabbit reticulocytes effectively synthesize viral proteins with Sindbis viral mRNA isolated from Sindbis-infected BHK cells. The major product is identical to Sindbis capsid protein on the basis of its electrophoretic mobility in sodium dodecyl sulfate-acrylamide gels and two-dimensional tryptic-peptide fingerprints. Various amounts of several additional discrete polypeptides are formed, depending on the components of the cell-free extracts. One of these polypeptides may be a prematurely terminated part of the viral-capsid protein, while another is larger in molecular weight than capsid protein but contains the capsid tryptic peptides. Several of the proteins formed in vitro also are detected in extracts of Sindbis-infected BHK cells labeled with [(35)S]methionine. The three proteins found in Sindbis virions are postulated to originate by proteolytic cleavage from a larger molecular weight polypeptide precursor that is translated from a polycistronic mRNA presumed to contain a single site for initiation of protein synthesis. The two in vitro systems appear to translate this polycistronic viral mRNA to yield specific viral capsid although no evidence was found for post-translational proteolysis. Other mechanisms for production of the capsid protein in the cell-free extracts are considered, and some of these may function in the viral-infected cell where unusually large amounts of viral capsid proteins are frequently detected.

Translation of mRNA from simian virus 40-infected cells into simian virus 40 capsid protein by cell-free extracts

Messenger RNA was isolated from simian virus 40 (SV40)-infected and mock-infected cells by chromatography on poly(U) sepharose. When added to cell-free extracts from Chinese hamster ovary cells or rabbit reticulocytes, RNA from the infected cells, but not from mock-infected cells, stimulated synthesis of the major SV40 capsid protein. Identification of this species was done by sodium dodecyl sulfate gel electrophoresis, peptide mapping, and immunoprecipitation. The in vitro synthesized capsid protein was slightly different from virion assembled capsid protein, as shown by separation upon chromatography on hydroxylapatite and by minor differences in the peptide map.

Translation of vesicular stomatitis messenger RNA by extracts from mammalian and plant cells

RNA was isolated from polyribosomes of vesicular stomatitis virus (VSV)-infected cells and tested for its ability to direct protein synthesis in extracts of animal and plant cells. In cell-free, non-preincubated extracts of rabbit reticulocytes, the 28S VSV RNA stimulated synthesis of a protein the size of the vesicular stomatitis virus L protein whereas the 13 to 15S RNA directed synthesis of the VSV M, N, NS, and possibly G proteins. In wheat germ extracts, 13 to 15S RNA also directed synthesis of the N, NS, M, and possibly G proteins. Analysis of extracts labeled with formyl [(35)S]methionine showed that the 28S RNA directed the initiation of synthesis of one protein, whereas the 13 to 15S RNA directed initiation of at least four proteins. It is concluded that the 28S RNA encodes only the L protein, whereas the 13 to 15S RNA is a mixture of species, presumably monocistronic, which code for the four other known vesicular stomatitis virus proteins.

Translation of virus mRNA: synthesis of bacteriophage Q beta proteins in a cell-free extract from wheat embryo

The RNA from bacteriophage Qbeta can be translated by cell-free extracts from wheat embryos. This translation, by 80S ribosomes, occurs at a low magnesium ion concentration. Three products are synthesized which coelectrophorese with Qbeta proteins synthesized in Escherichia coli extracts. The smallest of these has been identified as coat protein. Although the polycistronic bacteriophage message is translated with fidelity, the efficiency is much less than when the monocistronic brome mosaic virus coat protein message is translated.

Cell-free synthesis of rat growth hormone

Growth hormone has been synthesized in a cell-free system derived from Krebs II ascites cells, under the direction of RNA prepared from rat pituitary tumor (GC) cells. Growth hormone synthesized in the cell-free system was identified by precipitation with antiserum against growth hormone developed in baboon, followed by electrophoretic analysis of the dissolved precipitate on sodium dodecyl sulfate-polyacrylamide gels. RNA from both the membrane and the post-membrane fractions of the cytoplasm of GC cells stimulated protein synthesis in the cell-free system, but only RNA from the membrane fraction was found to direct the synthesis of growth hormone.