Specific binding of eukaryotic initiation factor 2 to satellite tobacco necrosis virus RNA at a 5'-terminal sequence comprising the ribosome binding site

Rewriting nature's assembly manual for a ssRNA virus

Satellite tobacco necrosis virus (STNV) is one of the smallest viruses known. Its genome encodes only its coat protein (CP) subunit, relying on the polymerase of its helper virus TNV for replication. The genome has been shown to contain a cryptic set of dispersed assembly signals in the form of stem-loops that each present a minimal CP-binding motif AXXA in the loops. The genomic fragment encompassing nucleotides 1-127 is predicted to contain five such packaging signals (PSs). We have used mutagenesis to determine the critical assembly features in this region. These include the CP-binding motif, the relative placement of PS stem-loops, their number, and their folding propensity. CP binding has an electrostatic contribution, but assembly nucleation is dominated by the recognition of the folded PSs in the RNA fragment. Mutation to remove all AXXA motifs in PSs throughout the genome yields an RNA that is unable to assemble efficiently. In contrast, when a synthetic 127-nt fragment encompassing improved PSs is swapped onto the RNA otherwise lacking CP recognition motifs, assembly is partially restored, although the virus-like particles created are incomplete, implying that PSs outside this region are required for correct assembly. Swapping this improved region into the wild-type STNV1 sequence results in a better assembly substrate than the viral RNA, producing complete capsids and outcompeting the wild-type genome in head-to-head competition. These data confirm details of the PS-mediated assembly mechanism for STNV and identify an efficient approach for production of stable virus-like particles encapsidating nonnative RNAs or other cargoes.

Revealing the density of encoded functions in a viral RNA

We present direct experimental evidence that assembly of a single-stranded RNA virus occurs via a packaging signal-mediated mechanism. We show that the sequences of coat protein recognition motifs within multiple, dispersed, putative RNA packaging signals, as well as their relative spacing within a genomic fragment, act collectively to influence the fidelity and yield of capsid self-assembly in vitro. These experiments confirm that the selective advantages for viral yield and encapsidation specificity, predicted from previous modeling of packaging signal-mediated assembly, are found in Nature. Regions of the genome that act as packaging signals also function in translational and transcriptional enhancement, as well as directly coding for the coat protein, highlighting the density of encoded functions within the viral RNA. Assembly and gene expression are therefore direct molecular competitors for different functional folds of the same RNA sequence. The strongest packaging signal in the test fragment, encodes a region of the coat protein that undergoes a conformational change upon contact with packaging signals. A similar phenomenon occurs in other RNA viruses for which packaging signals are known. These contacts hint at an even deeper density of encoded functions in viral RNA, which if confirmed, would have profound consequences for the evolution of this class of pathogens.

Eukaryotic type translation initiation factor 2: structure-functional aspects

Translation initiation factor 2 (IF2) is one of key components of the translation initiation system in living cells. In bacteria IF2 is a multidomain monomeric protein, while in eukaryotic and archaean cells e/aIF2 is heterotrimer (αβγ). Data, including our own, on eukaryotic type translation initiation factor 2 (e/aIF2) structure and functioning are presented. There are also new data on initiation factors eIF5 and eIF2B that directly interact with eIF2 and control its participation in nucleotide exchange.

A novel interaction of Cap-binding protein complexes eukaryotic initiation factor (eIF) 4F and eIF(iso)4F with a region in the 3'-untranslated region of satellite tobacco necrosis virus

Satellite tobacco necrosis virus (STNV) RNA is naturally uncapped at its 5' end and lacks polyadenylation at its 3' end. Despite lacking these two hallmarks of eukaryotic mRNAs, STNV-1 RNA is translated very efficiently. A approximately 130-nucleotide translational enhancer (TED), located 3' to the termination codon, is necessary for efficient cap-independent translation of STNV-1 RNA. The STNV-1 TED RNA fragment binds to the eukaryotic cap-binding complexes, initiation factor (eIF) 4F and eIF(iso)4F, as measured by nitrocellulose binding and fluorescence titration. STNV-1 TED is a potent inhibitor of in vitro translation when added in trans. This inhibition is reversed by the addition of eIF4F or eIF(iso)4F, and the subunits of eIF4F and eIF(iso)4F cross-link to STNV-1 TED, providing additional evidence that these factors interact directly with STNV-1 TED. Deletion mutagenesis of the STNV-1 TED indicates that a minimal region of approximately 100 nucleotides is necessary to promote cap-independent translation primarily through interaction with the cap binding subunits (eIF4E or eIF(iso)4E) of eIF4F or eIF(iso)4F.

Recognition of 5'-terminal TAR structure in human immunodeficiency virus-1 mRNA by eukaryotic translation initiation factor 2

TAR, a 59 nt 5'-terminal hairpin in human immuno-deficiency virus 1 (HIV-1) mRNA, binds viral Tat and several cellular proteins. We report that eukaryotic translation initiation factor 2 (eIF2) recognizes TAR. TAR and the AUG initiation codon domain, located well downstream from TAR, both contribute to the affinity of HIV-1 mRNA for eIF2. The affinity of TAR for eIF2 was insensitive to lower stem mutations that modify sequence and structure or to sequence changes throughout the remainder that leave the TAR secondary structure intact. Hence, eIF2 recognizes structure rather than sequence in TAR. The affinity for eIF2 was severely reduced by a 3 nt change that converts the single A bulge into a 7 nt internal loop. T1 footprinting showed that eIF2 protects nucleotides in the loop as well as in the strand opposite the A bulge. Thus, eIF2 recognizes the TAR loop and lower part of the sub-apical stem. Though not contiguous, these regions are brought into proximity in TAR by a bend in the helical structure induced by the UCU bulge; binding of eIF2 opens up the bulge context and apical stem. The ability to bind eIF2 suggests a function for TAR in HIV-1 mRNA translation. Indeed, the 3 nt change that reduces the affinity of TAR for eIF2 impairs the ability of reporter mRNA to compete in translation. Interaction of TAR with eIF2 thus allows HIV-1 mRNA to compete more effectively during protein synthesis.

Features of the autonomous function of the translational enhancer domain of satellite tobacco necrosis virus

The RNA of satellite tobacco necrosis virus (STNV) is a monocistronic messenger that lacks both a cap and a poly(A) tail. Translation of STNV RNA in vitro is promoted by a 120-nt translational enhancer domain (TED) in the 3'-untranslated region. TED also stimulates translation of heterologous mRNAs. In this study, we show that TED stimulates translation of a cat mRNA by increasing translation efficiency to the level of capped mRNA. This stimulatory activity is not impaired by translation through TED. TED stimulates translation efficiency from different positions within the mRNA, varying from the 5' end to 940 nt downstream of the coding region. Duplication of TED has an additive effect on translation stimulation only when located at both ends of the mRNA. On dicistronic RNAs, TED stimulates translation of both cistrons to the same extent. These data suggest that TED acts primarily by recruiting the translational machinery to the RNA.

Topography of ribosomes and initiation complexes from rat liver as revealed by atomic force microscopy

Atomic force microscopy (AFM) was used to image ribosomes and ribosomal subunits (60S, 40S and native 40S ribosomal subunits) isolated from rat liver. A variety of topographic images were obtained directly and found to be consistent with models established by other biophysical methods. In addition, the ternary complex of eIF-2 x GTP x Met-tRNA(i) and the 43S preinitiation complex have been discerned by AFM directly. Detailed information about the binding sites for eIF-1A, eIF-2, eIF-3, and Met-tRNA(i) on the 40S ribosomal subunit was derived from the AFM images. Finally, factors which may give rise to artifactual images, namely, convolution of the AFM tip on ribosomes, surface tension collapse effect and dehydration, are discussed. This work demonstrates that AFM is useful for imaging ribosomes and translational complexes and provides valuable information that can be used to complement other well-established techniques.

Translation in plants--rules and exceptions

Translation processes in plants are very similar to those in other eukaryotic organisms and can in general be explained with the scanning model. Particularly among plant viruses, unconventional mRNAs are frequent, which use modulated translation processes for their expression: leaky scanning, translational stop codon readthrough or frameshifting, and transactivation by virus-encoded proteins are used to translate polycistronic mRNAs; leader and trailer sequences confer (cap-independent) efficient ribosome binding, usually in an end-dependent mechanism, but true internal ribosome entry may occur as well; in a ribosome shunt, sequences within an RNA can be bypassed by scanning ribosomes. Translation in plant cells is regulated under conditions of stress and during development, but the underlying molecular mechanisms have not yet been determined. Only a small number of plant mRNAs, whose structure suggests that they might require some unusual translation mechanisms, have been described.

The plant translational apparatus

Protein synthesis in both eukaryotic and prokaryotic cells is a complex process requiring a large number of macromolecules: initiation factors, elongation factors, termination factors, ribosomes, mRNA, amino-acylsynthetases and tRNAs. This review focuses on our current knowledge of protein synthesis in higher plants.

Evidence for involvement of trans-acting factors in selection of the AUG start codon during eukaryotic translational initiation

The molecular mechanism with which an appropriate AUG codon is selected as the start site for translational initiation by eukaryotic ribosomes is not known. By using a cell-free translation system, small RNA molecules containing single AUG codons, surrounded by various nucleotide sequences, were tested for their abilities to interfere with the translation of a reporter mRNA. RNAs containing the AUG in an ACCAUGG context (Kozak consensus sequence) were able to inhibit translation of the reporter mRNA. In contrast, RNAs containing the AUG in a less favorable context for start site selection (for example, CAGAUGG) had no effect on the translation of the reporter mRNA. The effect mediated by the ACCAUGC-containing RNAs was not due to sequestration of ribosomal subunits or to particular structural features in these RNAs. To identify potential trans-acting factors that might be preferentially bound by ACCAUGG-containing RNAs, ACCAUGG- and CAGAUGC-containing RNAs with a single 4-thiouridine residue at the AUG were incubated with partially fractionated extracts, and AUG-binding proteins were identified after irradiation of the complexes with UV light and subsequent analysis by gel electrophoresis. The analysis (of such complexes in competition experiments revealed that proteins, approximately 50 and 100 kDa in size, were found to bind directly at the AUG codon embedded in the ACCAUGG motif. One of these proteins has been identified as the La autoantigen. These findings indicate that trans-acting factors may play a role in AUG start site selection during translational initiation.

Initiation of protein synthesis in eukaryotes

The study of the regulation of initiation of protein synthesis has recently gained momentum because of the established relationship between translation initiation, cell growth and tumorigenesis. Therefore much effort is devoted to the role of protein kinases which are activated in signal transduction cascades and which are responsible for the phosphorylation of a number of initiation factors. These specific factors are mainly involved in the binding of messenger RNA to the 40S ribosome, a process that makes the unwinding of the 5' untranslated region necessary. It appears that the phosphorylation of these factors increases their ability for cap recognition and helicase activity. The enhanced phosphorylation of the messenger binding factors results not only in an overall stimulation of translation, but especially weak messengers are positively discriminated. The above mechanisms mainly deal with qualitative control of translation, i.e., messenger selection, but phosphorylation also plays a role in quantitative regulation of protein synthesis. The generation of active eIF-2, the initiation factor that binds the Met-tRNA(i) and GTP, is dependent on a factor involved in the GDP-GTP exchange. Phosphorylation of eIF-2 results in sequestration of the exchange factor and a slowing down of the rate of initiation.

The 3' untranslated region of satellite tobacco necrosis virus RNA stimulates translation in vitro

The RNA of satellite tobacco necrosis virus (STNV) is a monocistronic messenger that lacks both a 5' cap structure and a 3' poly(A) tail. We show that in a cell-free translation system derived from wheat germ, STNV RNA lacking the 600-nucleotide trailer is translated an order of magnitude less efficiently than full-size RNA. Deletion analyses positioned the translational enhancer domain (TED) within a conserved hairpin structure immediately downstream from the coat protein cistron. TED enhances translation when fused to a heterologous mRNA, but the level of enhancement depends on the nature of the 5' untranslated sequence and is maximal in combination with the STNV leader. The STNV leader and TED have two regions of complementarity. One of the complementary regions in TED resembles picornavirus box A, which is involved in cap-independent translation but which is located upstream of the coding region.

The 3' untranslated region of satellite tobacco necrosis virus RNA stimulates translation in vitro

The RNA of satellite tobacco necrosis virus (STNV) is a monocistronic messenger that lacks both a 5' cap structure and a 3' poly(A) tail. We show that in a cell-free translation system derived from wheat germ, STNV RNA lacking the 600-nucleotide trailer is translated an order of magnitude less efficiently than full-size RNA. Deletion analyses positioned the translational enhancer domain (TED) within a conserved hairpin structure immediately downstream from the coat protein cistron. TED enhances translation when fused to a heterologous mRNA, but the level of enhancement depends on the nature of the 5' untranslated sequence and is maximal in combination with the STNV leader. The STNV leader and TED have two regions of complementarity. One of the complementary regions in TED resembles picornavirus box A, which is involved in cap-independent translation but which is located upstream of the coding region.

Mechanism and regulation of eukaryotic protein synthesis

This review presents a description of the numerous eukaryotic protein synthesis factors and their apparent sequential utilization in the processes of initiation, elongation, and termination. Additionally, the rare use of reinitiation and internal initiation is discussed, although little is known biochemically about these processes. Subsequently, control of translation is addressed in two different settings. The first is the global control of translation, which is effected by protein phosphorylation. The second is a series of specific mRNAs for which there is a direct and unique regulation of the synthesis of the gene product under study. Other examples of translational control are cited but not discussed, because the general mechanism for the regulation is unknown. Finally, as is often seen in an active area of investigation, there are several observations that cannot be readily accommodated by the general model presented in the first part of the review. Alternate explanations and various lines of experimentation are proposed to resolve these apparent contradictions.

In vitro mutational analysis of cis-acting RNA translational elements within the poliovirus type 2 5' untranslated region

Initiation of translation on poliovirus RNA occurs by internal binding of ribosomes to a region within the 5' untranslated region (UTR) of the mRNA. This region has been previously roughly mapped between nucleotides 140 and 631 of the 5' UTR and termed the ribosome landing pad. To identify cis-acting elements in the 5' UTR of poliovirus type 2 (Lansing strain) RNA that confer cap-independent internal initiation, we determined the in vitro translational efficiencies of a series of deletion and point mutations within the 5' UTR of the mRNA. The results demonstrate that the 3' border of the core poliovirus ribosome landing pad is located between nucleotides 556 and 585, whereas a region extending between nucleotides 585 and 612 confers enhanced translation. We studied two cis-acting elements within this region of the 5' UTR: a pyrimidine stretch which is critical for translation and an AUG (number 7 from the 5' end) that is located approximately 20 nucleotides downstream from the pyrimidine stretch and augments translation. We also show that the stem-loop structure which contains this AUG is not required for translation.

The 5'-untranslated region of picornaviral genomes

Picornaviruses are small naked icosahedral viruses with a single-stranded RNA genome of positive polarity. According to current taxonomy, the family includes four genera: Enterouirus (polioviruses, coxsackieviruses, echoviruses, and other enteroviruses), Rhinovirus, Curdiouirus [encephalomyocarditis virus (EMCV), mengovirus, Theiler's murine encephalomyelitis virus (TMEV)], and Aphthouirus [foot-and-mouth disease viruses (FMDV)]. There are also some, as yet, unclassified picornaviruses [e.g., hepatitis A virus (HAW] that should certainly be assessed as a separate genus. Studies on the molecular biology of picornaviruses might be divided into two periods: those before and after the first sequencing of the poliovirus genome. The 5'-untranslated region (5-UTR) of the viral genome was one of the unexpected problems. This segment proved to be immensely long: about 750 nucleotides or ∼10% of the genome length. There were also other unusual features (e.g., multiple AUG triplets preceding the single open reading frame (ORF) that encodes the viral polyprotein). This chapter shows that the picornaviral 5-UTRs are not only involved in such essential events as the synthesis of viral proteins and RNAs that could be expected to some extent, although some of the underlying mechanisms appeared to be quite a surprise, but also may determine diverse biological phenotypes from the plaque size or thermosensitivity of reproduction to attenuation of neurovirulence. Furthermore, a close inspection of the 5-UTR structure unravels certain hidden facets of the evolution of the picornaviral genome. Finally, the conclusions drawn from the experiments with the picornaviral5-UTRs provide important clues for understanding the functional capabilities of the eukaryotic ribosomes.

A cellular protein that binds to the 5'-noncoding region of poliovirus RNA: implications for internal translation initiation

Initiation of translation on poliovirus mRNA occurs by internal binding of ribosomes to a region within the 5'-noncoding portion of the mRNA. The mechanistic details and trans-acting factors involved in this event are not understood fully. We used a mobility-shift electrophoresis assay to identify a specific RNA-protein complex, which can form between an RNA fragment that contains nucleotides 559-624 of the poliovirus 5' UTR (untranslated region) and a component or components of a HeLa cell extract. Complex formation was reduced greatly in a reticulocyte lysate or a wheat-germ extract. A 52-kD polypeptide (p52) has been identified as part of the protein-RNA complex by use of an UV cross-linking assay. This polypeptide apparently is not a known translation initiation or elongation factor. The possible involvement of p52 in translation initiation of poliovirus protein synthesis is discussed.

Natural mRNA is required for directing Met-tRNA(f) binding to 40S ribosomal subunits in animal cells: involvement of Co-eIF-2A in natural mRNA-directed initiation complex formation

Two protein factors, eIF-2 as well as a high molecular weight protein complex from reticulocyte ribosomal high-salt wash which we term Co-eIF-2, promote Met-tRNA(f) binding to 40S ribosomes. This binding is dependent on the presence of an AUG codon or natural mRNAs [Roy et al. (1984) Biochem. Biophys. Res. Commun. 122, 1418-1425]. Co-eIF-2 contains two component activities, Co-eIF-2A and Co-eIF-2C. Previously, we have purified an 80-kDa polypeptide containing Co-eIF-2A activity and showed that this polypeptide is a component of Co-eIF-2 and is responsible for Co-eIF-2A activity in Co-eIF-2 [Chakravarty et al. (1985) J. Biol. Chem. 260, 6945-6949]. We now report purification of a protein complex (subunits of Mr 180K, 110K, 65K, 63K, 53K, 50K, 43K, and 40K) containing Co-eIF-2C activity and devoid of Co-eIF-2A activity. In SDS-PAGE, the purified Co-eIF-2C preparation and an eIF-3 preparation (purified in Dr. A. Wahba's laboratory) separated into seven similar major polypeptides (Mr 110K, 65K, 63K, 53K, 50K, 43K, and 40K). The 50-kDa polypeptide in Co-eIF-2C was immunoreactive with a monoclonal antibody against eIF-4A (50 kDa). We have studied the roles of purified Co-eIF-2A and Co-eIF-2C activities in ternary and Met-tRNA(f).40S ribosome complex formation. The results are as follows: (1) At low and presumably physiological factor concentration (30 nM), eIF-2 did not form detectable levels of ternary complex. Moreover, such complex formation was totally dependent on the presence of Co-eIF-2A and/or Co-eIF-2C.(ABSTRACT TRUNCATED AT 250 WORDS)

The nature of the interaction of eukaryotic initiation factor 2 with double-stranded RNA

In addition to binding messenger RNA molecules at specific sequences, eukaryotic initiation factor 2 (eIF-2) also binds to double-stranded RNA (dsRNA). The dsRNA is a powerful inhibitor of initiation of eukaryotic translation, causing the inactivation of eIF-2, but in the presence of certain mRNA templates, dsRNA fails to establish inhibition. Such mRNA templates bind to eIF-2 with higher affinity than does dsRNA, while globin mRNA, a template sensitive to inhibition, binds with lower affinity. Here, the nature of the interaction between dsRNA and eIF-2 was studied by examining both the binding of eIF-2 to Penicillium chrysogenum dsRNA molecules carrying 32P label at their 5' ends, and the ability of eIF-2 to protect such label against pancreatic ribonuclease digestion. The results reveal binding sites for eIF-2 at the 5' ends, as well as throughout internal regions of the dsRNA molecule. At least 15 molecules of eIF-2 can be accommodated on a 3000-base molecule of P. chrysogenum dsRNA. eIF-2 protects a 105-base-pair 5'-terminal fragment in dsRNA against digestion, but exhibits no noticeable preference for the 5' ends. By contrast, eIF-2 fails to protect label at the 5' ends of denatured dsRNA molecules, even though it binds to them at internal sites more avidly than to native dsRNA. Binding of eIF-2 to dsRNA is not restricted to specific sequences: eIF-2 binds with equal affinity to the synthetic dsRNA sequence, poly(rI . rC). The data support the interpretation that eIF-2 recognizes the A conformation in dsRNA rather than sequence. Apparently binding of eIF-2 at sites spaced 200 base pairs apart prevents relaxation of the intervening length of the double helix, thereby stabilizing the dsRNA molecule against ribonuclease attack. These results show that, even though dsRNA and mRNA compete in their binding to eIF-2, the structural features recognized by eIF-2 in these RNA species are distinct.

Inhibition of mRNA binding to ribosomes by localized activation of dsRNA-dependent protein kinase

The initiation of protein synthesis can be regulated in mammalian cells by protein kinases which phosphorylate the alpha subunit of initiation factor eIF-2. This phosphorylation results in a block in the recycling of eIF-2 and in the inhibition of messenger RNA binding to 80S initiation complexes. After eIF-2 alpha is phosphorylated, the mRNA becomes associated with 48S complexes consisting of a 40S ribosomal subunit, eIF-2 (alpha P), GDP and Met-tRNAf. One of the eIF-2 alpha kinases is activated by low concentrations of double-stranded RNA (dsRNA). This kinase (PKds) is present at a basal level in all mammalian cells investigated and its synthesis is induced in cells treated with interferon. The PKds may be involved in the inhibition of translation of viral mRNA in interferon-treated cells infected with RNA viruses, as it is activated by viral replicative complexes. It is not known, however, if the activated PKds preferentially inhibits the translation of viral mRNA when cellular protein synthesis proceeds at a normal rate in infected cells. We now report that mRNA covalently linked to dsRNA is preferentially inhibited from binding to 80S complexes by a localized activation of PKds. This suggests that in interferon-treated cells the binding of some nascent viral mRNAs to functional initiation complexes may be preferentially inhibited by a similar mechanism.

Isolation of a heme-controlled inhibitor of translation that blocks the interaction between messenger rna and eukaryotic initiation factor 2

A heme-controlled inhibitor of translation was isolated from the S-100 of rabbit reticulocytes by a novel procedure including chromatography on double-stranded ribonucleic acid (dsRNA)-cellulose. The inhibitor thus purified is extremely active and functionally resembles previously studied heme-controlled inhibitor preparations in terms of kinetics and extent of inhibition of translation, relief of inhibition by eukaryotic initiation factor 2 (eIF-2), relief of inhibition by 2-aminopurine, and preferential inhibition of alpha-over beta-globin synthesis. The action of this inhibitor on translation is resistant to treatment with bacterial alkaline phosphatase, micrococcal nuclease, or trypsin and to incubation at 95 degrees C, pH 2 or pH 12. The inhibitor not only is retained on DEAE-cellulose, phosphocellulose, and dsRNA-cellulose but also exhibits a high affinity for the dye Cibacron Blue, properties that suggest that it may be a protein. Unlike previously described heme-controlled inhibitor preparations, or preparations that did not pass over dsRNA-cellulose, the inhibitor recovered upon dsRNA-cellulose chromatography does not exhibit eIF-2 kinase activity. The inhibitor does not block ternary complex formation between eIF-2, methionyl-tRNAfMet, and GTP but inhibits the ability of eIF-2 to form a complex with labeled globin mRNA. In the presence of inhibitor, the formation of mRNA/eIF-2 complexes can be restored effectively by an excess of eIF-2 but not by an excess of mRNA. The inhibitor thus appears to block the interaction between eIF-2 and mRNA not by competing with eIF-2 for a binding site on mRNA but, instead, by acting on eIF-2 itself.(ABSTRACT TRUNCATED AT 250 WORDS)

Translational competition by mRNA species encoding albumin, ferritin, haemopexin and globin

Messenger RNA from rat liver was translated in a micrococcal-nuclease-treated reticulocyte lysate supplemented with liver tRNA. Synthesis of the liver proteins haemopexin, ferritin and albumin was analyzed by quantitative immunoprecipitation. The relative translation yield of these proteins changed as a function of the amount of mRNA present during protein synthesis, revealing the existence of translational competition between individual species of mRNA from the liver. The results show that the mRNA species encoding haemopexin, ferritin and albumin possess distinctly different abilities to compete for one or more critical components in translation, with competitive strength increasing in this order. Although on a weight basis total liver mRNA is apparently as effective a template for protein synthesis as is globin mRNA, the latter displays a greater resistance to inhibition of its translation by KCl. In analogy with the translation properties of alpha-globin and beta-globin mRNA [Di Segni, G., Rosen, H. and Kaempfer, R. (1979) Biochemistry, 18, 2847-2854], this finding suggests that globin mRNA possesses greater competitive strength than does total liver mRNA. Increasing amounts of globin mRNA competitively inhibit the translation of albumin and ferritin mRNA present in total liver mRNA. The competition is relieved by the addition of eukaryotic initiation factor eIF-2. Translation of ferritin mRNA responds more vigorously to relief by eIF-2 than does translation of albumin mRNA, a finding consistent with the observation that albumin mRNA competes more effectively than ferritin mRNA in translation. The results support the assumption that albumin mRNA possesses a greater affinity for eIF-2 than does ferritin mRNA.

Genomic RNA of mengovirus. VI. Translation of its two cistrons in lysates of interferon-treated cells

The addition of low levels (40 ng/ml) of the synthetic double-stranded polyribonucleotide poly I:C to lysates of interferon-treated L-cells resulted in a strong inhibition (70 to 75%) of the in vitro translation of mengovirus RNA. Under these conditions, the rates of incorporation of [35S]methionine or formyl-[35S]methionine were depressed to a comparable extent. The sequences of mengovirus RNA recognized by ribosomes of interferon-treated cells at initiation of translation were compared with those present in initiation complexes formed by ribosomes of untreated controls. Fingerprint analysis revealed that the same sequences of mengovirus RNA were protected against nuclease attack by the 80S and the 40S initiation complexes formed in vitro in lysates of control or interferon-treated L-cells. Mengovirus RNA-coded proteins were labeled at their N-terminal end with formyl-[35S]methionine and digested to completion with trypsin. The resulting fragments were separated by high-voltage paper electrophoresis. Two different formyl-[35S]methionine-labeled N termini were resolved. Further analyses supported the notion that the two radioactive peaks originated in the initiation of translation at two different sites. This pattern did not change when mengovirus RNA was translated in lysates of interferon-treated cells.

Possible involvement of messenger RNA-associated proteins in protein synthesis

Two distinct forms of globin messenger RNA were isolated from mouse spleen cells infected with Friend erythroleukemia virus: polyribosomal messenger ribonucleoprotein particles (15S mRNP), and their corresponding protein-free mRNAs obtained by chemical deproteinization. The translation efficiencies of both messenger forms were assayed in a Krebs II ascites cell-free system. Selective removal of RNA-binding proteins from the ascites cell lysate did not affect globin synthesis when the mRNA was supplied as 15S mRNP; deproteinized mRNA however was not translated. Only in the presence of two fractions of RNA-binding proteins was the protein-free mRNA translated. Some of the RNA-binding proteins have the same molecular weights and isoelectric points as the principal proteins of 15S mRNP.