A protein linked to the 5' termini of both RNA components of the cowpea mosaic virus genome

Cowpea Mosaic Virus Outperforms Other Members of the Secoviridae as In Situ Vaccine for Cancer Immunotherapy

In situ vaccination for cancer immunotherapy uses intratumoral administration of small molecules, proteins, nanoparticles, or viruses that activate pathogen recognition receptors (PRRs) to reprogram the tumor microenvironment and prime systemic antitumor immunity. Cowpea mosaic virus (CPMV) is a plant virus that─while noninfectious toward mammals─activates mammalian PRRs. Application of CPMV as in situ vaccine (ISV) results in a potent and durable efficacy in tumor mouse models and canine patients; data indicate that CPMV outperforms small molecule PRR agonists and other nonrelated plant viruses and virus-like particles (VLPs). In this work, we set out to compare the potency of CPMV versus other plant viruses from the Secoviridae. We developed protocols to produce and isolate cowpea severe mosaic virus (CPSMV) and tobacco ring spot virus (TRSV) from plants. CPSMV, like CPMV, is a comovirus with genome and protein homology, while TRSV lacks homology and is from the genus nepovirus. When applied as ISV in a mouse model of dermal melanoma (using B16F10 cells and C57Bl6J mice), CPMV outperformed CPSMV and TRSV─again highlighting the unique potency of CPMV. Mechanistically, the increased potency is related to increased signaling through toll-like receptors (TLRs)─in particular, CPMV signals through TLR2, 4, and 7. Using knockout (KO) mouse models, we demonstrate here that all three plant viruses signal through the adaptor molecule MyD88─with CPSMV and TRSV predominantly activating TLR2 and 4. CPMV induced significantly more interferon β (IFNβ) compared to TRSV and CPSMV; therefore, IFNβ released upon signaling through TLR7 may be a differentiator for the observed potency of CPMV-ISV. Additionally, CPMV induced a different temporal pattern of intratumoral cytokine generation characterized by significantly increased inflammatory cytokines 4 days after the second of 2 weekly treatments, as if CPMV induced a "memory response". This higher, longer-lasting induction of cytokines may be another key differentiator that explains the unique potency of CPMV-ISV.

Inhibition of cleavage of a plant viral polyprotein by an inhibitor activity present in wheat germ and cowpea embryos

In rabbit reticulocyte lysate, the bottom component RNA of cowpea mosaic virus directs the synthesis of a 200,000-molecular-weight precursor protein (200K protein) that is cleaved during synthesis by a reticulocyte enzyme to form a 32K protein and a 170K protein. Cleavage of the 200K protein was found to be effectively inhibited by inhibitor activity in wheat germ and cowpea embryo extracts. The inhibitor was nondialyzable, precipitatable by ammonium sulfate, and partially stable at high temperatures. The activity appeared to be specific in that it caused no inhibition of the secondary cleavage reactions (cleavage of the 170K protein) at concentrations that were sufficient to cause complete inhibition of the primary cleavage reaction (cleavage of the 200K protein).

Expression of Middle-Component RNA of Cowpea Mosaic Virus: In Vitro Generation of a Precursor to Both Capsid Proteins by a Bottom-Component RNA-Encoded Protease from Infected Cells

The expression of the middle-component (M) RNA of cowpea mosaic virus was studied by means of in vitro translation. In both the wheat germ extract and the rabbit reticulocyte lysate, M RNA was translated into two overlapping polypeptides of 95 and 105 kilodaltons. Incubation of these polypeptides with 30,000 x g supernatant fractions from cowpea mesophyll protoplasts inoculated with complete virus or with separate bottom (B) components alone resulted in extensive processing, yielding polypeptides of 60, 58, 48, and 47 kilodaltons. Similar proteolytic activity was found associated with the in vitro translation products from the bottom-component RNA, demonstrating that the protease present in infected cells is encoded by B RNA. Using antisera raised against the separate capsid proteins VP23 and VP37, it was shown that the 60-kilodalton cleavage product is the precursor to both capsid proteins. Cleavage of nascent 95- and 105- kilodalton polypeptides by the in vivo protease demonstrated that this capsid protein precursor is located C terminally within both polypeptides and that the synthesis of these two overlapping polypeptides is the result of two initiation sites on middle-component RNA. In addition, a second virus-induced proteolytic activity, capable of releasing VP23 from the 95- and 105-kilodalton polypeptides, was detected in leaves of infected plants, but not in infected mesophyll protoplasts. A model for the expression of the middle-component RNA is presented.

Evidence That the 32,000-Dalton Protein Encoded by Bottom-Component RNA of Cowpea Mosaic Virus is a Proteolytic Processing Enzyme

Translation of middle-component RNA of cowpea mosaic virus in vitro produced two polypeptides of 95 and 105 kilodaltons (95K and 105K, respectively) with overlapping amino acid sequences, which were specifically cleaved by a protease encoded by the bottom-component RNA. The proteolytic cleavage was studied by the addition of antibodies raised against various bottom-component RNA-encoded proteins to extracts prepared from bottom-component RNA-inoculated cowpea protoplasts. Since antiserum to the 32K polypeptide efficiently inhibited the proteolytic activity of such extracts, although antiserum to VPg or to the 170K polypeptide did not, evidence was obtained which indicates that the 32K polypeptide represents the protease involved. Fractionation of proteolytically active extract by glycerol gradient centrifugation demonstrated that 32K polypeptides do not exist as free proteins but are aggregated to the bottom-component RNA-encoded 170K, 84K, 60K, or 58K polypeptides. Maximal proteolytic activity was observed for 32K polypeptides associated with 170K polypeptides, suggesting that the activity was unstable and confined to newly synthesized molecules.

Multimeric forms of satellite of tobacco ringspot virus RNA

An approximately 350-nucleotide residue RNA replicates in association with tobacco ringspot virus (TobRV) and becomes encapsidated in TobRV coat protein. Here we show by electrophoretic analyses that this small satellite RNA, RNA S, is the most abundant and most rapidly migrating of a series of at least ten encapsidated RNAs with RNA S sequences. A largely double-stranded RNA fraction from infected tissue, when denatured, gave a similar series of up to 12 zones that contained both RNA S sequences and sequences that hybridized to RNA S. Analysis of the mobilities suggests a weight increment between each zone corresponding approximately to the size of RNA S. Thus the more slowly migrating zones appear to contain covalent multimers of RNA S or, for tissue RNA, both multimers of RNA S and multimers of the complement of RNA S sequences. Neither terminal structure of TobRV genomic RNAs was found in the satellite RNA. RNA S lacks detectable polyadenylate or oligoadenylate. Covalently linked protein was not detected in RNA S or its more slowly migrating forms, and satellite RNA biological activity, unlike that of the TobRV RNAs, was not protease sensitive. Polynucleotide kinase catalyzed the phosphorylation of satellite RNAs, indicating free 5'-hydroxyl groups.

Translation of tobacco rattle virus RNAs in vitro: four proteins from three RNAs

RNAs from the Lisse and PRN strains of tobacco rattle virus were translated in nuclease-treated reticulocyte lysates. With both strains, RNA-1 coded for two proteins of MW 170,000 and 120,000. Addition of 1-2 mM Mg2+ increased the yield of the larger product, apparently by facilitating readthrough of a leaky termination codon at the end of the region coding for the smaller protein. Experiments with yeast suppressor tRNA indicate that this is not a UAG or UAA codon. RNA-2 coded for a protein of the same size as coat protein. A product of MW 30,000 was coded by a third RNA (RNA-3) which in the case of PRN TRV had a molecular weight of 550,000 and could be separated from RNA-2 by gel electrophoresis. Translation of all three RNAs was inhibited by m7GTP. However, RNA-1 also directed a low level of m7GTP-resistant synthesis of a 30,000-dalton protein which appeared to be identical to the RNA-3 product, and it is suggested that RNA-3 corresponds to the 3' end of RNA-1. Thus the strategies of synthesis of the non-coat proteins of tobacco rattle and tobacco mosaic viruses appear to be similar.

Nucleotidylylation of the VPg protein of a human norovirus by its proteinase-polymerase precursor protein

Caliciviruses have a positive strand RNA genome covalently-linked at the 5'-end to a small protein, VPg. This study examined the biochemical modification of VPg by the ProPol form of the polymerase of human norovirus strain MD145 (GII.4). Recombinant norovirus VPg was shown to be nucleotidylylated in the presence of Mn2+ by MD145 ProPol. Phosphodiesterase I treatment of the nucleotidylylated VPg released the incorporated UMP, which was consistent with linkage of RNA to VPg via a phosphodiester bond. Mutagenesis analysis of VPg identified Tyrosine 27 as the target amino acid for this linkage, and suggested that VPg conformation was important for the reaction. Nucleotidylylation was inefficient in the presence of Mg2+; however the addition of full- and subgenomic-length MD145 RNA transcripts led to a marked enhancement of the nucleotidylylation efficiency in the presence of this divalent cation. Furthermore, evidence was found for the presence of an RNA element near the 3'-end of the polyadenylated genome that enhanced the efficiency of nucleotidylylation in the presence of Mg2+.

The RNA2 5' leader of Blackcurrant reversion virus mediates efficient in vivo translation through an internal ribosomal entry site mechanism

The 5' and 3' non-translated regions (NTRs) of mRNAs of eukaryotes and their viruses often contain translational enhancers, including internal ribosomal entry sites (IRESs) comprised in the 5' leaders of many uncapped viral mRNAs. Blackcurrant reversion virus (BRV) has a genome composed of two uncapped, polyadenylated RNAs with relatively short 5' NTRs, almost devoid of secondary structure. In this work, a role of the RNA2 5' NTR in translation was studied by using mono- and dicistronic Photinus pyralis and Renilla reniformis luciferase reporter mRNAs in protoplasts of Nicotiana benthamiana. The RNA2 5' leader was found to confer efficient in vivo translation compared with the control 5' NTR, and each half of the BRV leader was essential for stimulatory function. Such efficient translational enhancement was mediated, at least in part, through an IRES mechanism. Multiple RNA2 5' NTR regions, complementary to a fragment of plant 18S rRNA demonstrated previously to be accessible for intermolecular mRNA-rRNA interactions and conserved between eukaryotes, were shown to be important for efficient translation. Similar mRNA-rRNA base-pairing potential was also predicted for the 5' leaders of other nepoviruses.

Role of the RNA2 3' non-translated region of Blackcurrant reversion nepovirus in translational regulation

The 3' non-translated regions (NTRs) of mRNAs of eukaryotes and their viruses often contain translational enhancers (TEs). Blackcurrant reversion nepovirus (BRV) has a genome composed of two uncapped polyadenylated RNAs with very long 3' NTRs, nucleotide sequences of which are very conserved between different BRV isolates. In this work, we studied a role of the RNA2 3' NTR in translation, using mutagenesis of the firefly luciferase reporter mRNA, in protoplasts of Nicotiana benthamiana. The RNA2 3' NTR was found to contain a cap-independent TE (3' CITE), which must base pair with the 5' NTR to facilitate translation. The BRV 3' CITE and poly(A) tail provided a major contribution to translational efficiency, with less input from other 3' NTR parts. The BRV 3' CITE does not share similarity in nucleotide sequence and secondary structure with other viruses and thus represents a new class of 3' CITE.

Identification of the 5' terminal structure of influenza virus genome RNA by a newly developed enzymatic method

A combination of T4 polynucleotide kinase, Escherichia coli alkaline phosphatase, yeast Saccharomyces cerevisiae capping enzyme consisting of alpha (RNA guanylyltransferase) and beta (RNA 5'-triphosphatase) subunits. and its alpha subunit without RNA 5'-phosphatase activity was used to establish a simple enzymatic method for determination of RNA species with 5'-hydroxyl, 5'-monophosphate, 5'-diphosphate or 5'-triphosphate termini. Using this method, we found that viral genome RNA (vRNA) segments of both A-type and C-type influenza viruses carry tri- or diphosphates at their 5' termini. The conclusion was based on the observations that: (i) 5' phosphorylation of vRNAs by T4 polynucleotide kinase takes place only after phosphatase treatment; and (ii) capping of vRNAs can be observed with both the intact yeast capping enzyme and its alpha subunit alone devoid of RNA 5'-triphosphatase activity; but (iii) the level of capping is higher for the alphabeta holoenzyme than the alpha subunit though the relative level varies depending on RNA preparations. The results support the de novo initiation for the RNA replication although transcription of influenza vRNAs is initiated by host cell capped RNAs as primers.

Improvements of the infectivity of in vitro transcripts from cloned cowpea mosaic virus cDNA: impact of terminal nucleotide sequences

Full-length DNA copies of both B- and M-RNA of cowpea mosaic virus (CPMV) were constructed downstream from a T7 promoter. By removal of nucleotides from the promoter sequence, B- and M-RNA-like transcripts with varying numbers of additional nonviral sequences at the 5' end were obtained upon transcription with T7 RNA polymerase. The infectivity of the transcripts in cowpea protoplasts was greatly affected by only a few extra nonviral nucleotides at the 5' end. The addition of about 400 nonviral nucleotides at the 3' end did not have any effect. Using the most infectious transcripts, in 40% of the cowpea protoplasts replication and expression of B-RNA like transcripts were observed and in 10% of the protoplasts both B- and M-RNA-like transcripts multiplied. Moreover, cowpea plants could also be infected with these transcripts. Sequence analysis showed that the 5' terminus of the M-RNA transcripts and the 3' terminus of the B-RNA transcripts were completely restored during replication in plants, including a poly(A) tail of variable length. Swapping experiments have been used to identify an influential point mutation in the coding region for the viral polymerase of a noninfectious B transcript. This experiment demonstrates the potential of the optimized infection system for future analysis of virus-encoded functions.

The complete nucleotide sequence and genome organization of red clover necrotic mosaic virus RNA-1

The complete nucleotide sequence of red clover necrotic mosaic virus (RCNMV) RNA-1 has been determined. RNA-1 is 3889 nucleotides in length with a 5' terminal m7GpppA cap. The RNA contains three large open reading frames (ORFs): the 5' proximal ORF, encoding a 27-kDa polypeptide; the internal ORF, coding for a 57-kDa polypeptide; and the 3' terminal ORF, encoding the 37-kDa capsid protein. The sequence results confirm in vitro translation of 27-, 50-, and 37-kDa products but do not account for the observed 90-kDa product. A translational frameshift event from the 27- to the 57-kDa ORFs is proposed to explain the synthesis of the observed 90-kDa in vitro product. The putative translational frameshift region is structurally similar to several retrovirus frameshift regions and the putative barley yellow dwarf virus (BYDV) frameshift regions. Extensive amino acid homology was observed in the 57-kDa downstream ORF with the downstream domains of the carnation mottle virus (CarMV), turnip crinkle virus (TCV), maize chlorotic mottle virus (MCMV) readthrough, and BYDV fusion proteins. The 57-kDa ORF contained the conserved "GDD" motif. A significant alignment between the capsid proteins of RCNMV, CarMV, and TCV was also observed. Given the extensive amino acid sequence similarity of RCNMV, CarMV, and TCV polymerase and capsid proteins, we speculate that they are closely related, evolutionarily.

High-level synthesis of cowpea mosaic virus RNA polymerase and protease in Escherichia coli

An expression system for the production of polymerase proteins of cowpea mosaic virus (CPMV) in Escherichia coli cells is described. High-level synthesis of proteins containing protease and polymerase moieties (110-kDa protein) and polymerase alone (87-kDa protein) were obtained from cells containing different plasmid constructions. Precursor and processed forms of CPMV proteins were detected by immunoblotting with antisera directed against 170-kDa precursor polyprotein and 24-kDa viral protease. Crude lysates and supernatant fractions of the lysates from E. coli cells harboring the various plasmid constructions were analysed for poly(A)-oligo(U) polymerase activity and found to be negative for CPMV activity under conditions where similar expression systems for the production of poliovirus RNA polymerase activity were positive. Thus, conditions for CPMV RNA replication may indeed be different from those for poliovirus even though the genomic organization of these viruses is similar.

The nucleotide sequence and gene organization of red clover necrotic mosaic virus RNA-2

Red clover necrotic mosaic virus, a member of the dianthovirus group, is characterized by a genome composed of two nonhomologous single-stranded RNAs of approximately 4.0 (RNA-1) and 1.4 kb (RNA-2). The complete nucleotide sequence of the RNA-2 has been determined. RNA-2 is 1448 nucleotides in length with a 5' terminal m7G cap and no 3' terminal poly-A tail or 5' terminal VPg. An open reading frame beginning at the first initiation codon at nucleotide 80 and ending at nucleotide 1030 has been identified which can encode a polypeptide of 35 kDa. RNA-2 directs the synthesis of a 35 kDa polypeptide in vitro. SP6 and T7 transcripts from full length RNA-2 cDNA clones directed the synthesis of a polypeptide with the same electrophoretic mobility as the polypeptide directed from authentic RNA-2. Clones with various 3' terminal deletions both outside and within the 35 kDa open reading frame were transcribed and translated in vitro to define the limits of the 35 kDa open reading frame. A second, small open reading frame capable of encoding a polypeptide of 4.9 kDa was also indicated from the sequence; however, there was no evidence for a protein product of that size. RNA-2 is presumed to be monocistronic and encode a cell-to-cell movement function. A small but significant amino acid sequence homology was observed with the brome mosaic virus RNA-3a polypeptide.

Infectious RNA transcripts derived from full-length DNA copies of the genomic RNAs of cowpea mosaic virus

A set of full-length DNA copies of both M and B RNA of cowpea mosaic virus (CPMV) was cloned downstream of a phage T7 promoter. Upon in vitro transcription using T7 RNA polymerase, M and B RNA-like transcripts were obtained from these DNA copies with only two additional nucleotides at the 5' end and five extra nucleotides at the 3' end in comparison to natural viral RNA. In cowpea protoplasts the transcripts of several cDNA clones of B RNA were able to replicate leading to detectable synthesis of viral RNA and proteins. Transcripts of M cDNA clones inoculated together with these B RNA transcripts were also expressed, although the number of protoplasts in which both transcripts were expressed was very low. Preliminary infectivity tests with mutagenized RNA transcripts indicate essential roles of the B RNA-encoded 24K and 32K polypeptides in viral RNA replication.

Two viral proteins involved in the proteolytic processing of the cowpea mosaic virus polyproteins

A series of specific deletion mutants derived from a full-length cDNA clone of cowpea mosaic virus (CPMV) B RNA was constructed with the aim to study the role of viral proteins in the proteolytic processing of the primary translation products. For the same purpose cDNA clones were constructed having sequences derived from both M and B RNA of CPMV. In vitro transcripts prepared from these clones with T7 RNA polymerase, were efficiently translated in rabbit reticulocyte lysates. The translation products obtained were processed in the lysate by specific proteolytic cleavages into smaller products, which made it possible to study subsequently the effect of the various mutations on this process. The results obtained indicate that the B RNA-encoded 24K polypeptide represents a protease responsible for all cleavages in the polyproteins produced by both CPMV B and M RNA. For efficient cleavage of the glutamine-methionine site in the M RNA encoded polyprotein the presence of a second B RNA encoded protein, the 32K polypeptide, is essential, although the 32K polypeptide itself does not have proteolytic activity. A number of cleavage-site mutants were constructed in which the coding sequence for the glutamine-glycine cleavage site between the two capsid proteins was changed. Subsequent in vitro transcription and translation of these cleavage site mutants show that a correct dipeptide sequence is a prerequisite for efficient cleavage but that the folding of the polypeptide chain also plays an important role in the formation of a cleavage site.

Proteolytic activity of the cowpea mosaic virus encoded 24K protein synthesized in Escherichia coli

The function of the 24-kilodalton (24K) protein encoded by cowpea mosaic virus (CPMV) has been studied by constructing a bacterial expression plasmid that contained a cloned chimeric segment consisting of partial DNA copies of CPMV M-RNA (including sequences coding for both capsid proteins) and B-RNA (including sequences coding for the 24K protein). Viral sequences were transcribed from the phage T7 promoter phi 10 of plasmid pT7-6 using T7-RNA polymerase expressed from plasmid pGP1-2 present in the same cells. Upon inducing the synthesis of T7-RNA polymerase several new polypeptides that contained CPMV-specific sequences were expressed, as demonstrated by immunoprecipitation and immunoblotting. Furthermore a proteolytic activity was detected in induced cells which cleaved the viral protein sequences specifically at two glutamine-glycine sites. One of the cleavage products represented capsid protein VP23. The proteolytic activity was absent when an 87-bp deletion was introduced in the coding region for the 24K protein, indicating that this protein represented the protease involved in the proteolytic processing at those specific sites.

Identification and characterization of a protein covalently bound to DNA of minute virus of mice

We identified a protein which is covalently linked to a fraction of the DNA synthesized in cells infected with minute virus of mice. This protein is specifically bound to the 5' terminus of the extended terminal conformers of the minute virus of mice replicative-form DNA species and of a variable fraction of single-stranded viral DNA. The chemical stability of the protein-DNA linkage is characteristic of a phosphodiester bond between a tyrosine residue in the protein and the 5' end of the DNA. The terminal protein (TP) bound on all DNA forms has a relative molecular weight of 60,000; it is also seen free in extracts from infected cells. Immunologic comparison of the TP with the other known viral proteins suggests that the TP is not related to the capsid proteins or NS-1.

An absolute requirement for the 5' cap structure for mRNA translation in sea urchin eggs

Translation of a variety of RNAs was studied in a cell-free translation system derived from sea urchin eggs. While RNAs such as globin or tobacco mosaic virus are efficiently translated, viral RNAs which do not contain the 5' cap structure, such as cow pea mosaic virus (CPMV) and poliovirus, are not translated. Mixing experiments with reticulocyte lysates indicated that the lack of translation of uncapped viral RNAs is not due to the presence of a potent inhibitor or the absence of an activating agent. RNA competition experiments between capped and uncapped RNAs indicated that uncapped RNAs do not interact with the sea urchin egg initiation machinery. Proteolytic removal of the 5' viral protein did not allow the translation of CPMV RNA. However, chemical decapping of vesicular stomatitis virus mRNA completely inhibited the translation of this mRNA in the sea urchin cell-free system. We conclude that the sea urchin egg lacks the initiation pathway used to initiate uncapped mRNAs in mammalian cells and thus has an absolute requirement for the 5' cap structure for initiation. In addition we discuss the implications of these findings for the control of protein synthesis after fertilization of the sea urchin egg.

Relationship between synthesis and cleavage of poliovirus-specific proteins

Poliovirus proteinase was studied in vitro in lysates from poliovirus-infected HeLa cells. Preincubation of these lysates caused (i) a reduction in poliovirus proteinase activity and (ii) a partial dependence on exogenous mRNA for optimal translation. Proteins translated from endogenous poliovirus RNA in preincubated extracts from virus-infected HeLa cells are poorly cleaved. This cleavage deficiency is alleviated by adding fresh poliovirus RNA to the translation system, thus, allowing re-initiation to occur. This suggests that the poliovirus proteinase is highly unstable.

Orientation of the Cleavage Map of the 200-Kilodalton Polypeptide Encoded by the Bottom-Component RNA of Cowpea Mosaic Virus

The genomic organization of the bottom-component RNA of cowpea mosaic virus was studied. In vivo, this RNA encodes at least eight different polypeptides of 170, 110, 87, 84, 60, 58, 32, and 4 kilodaltons (K), the last polypeptide representing the genome-bound protein VPg. In rabbit reticulocyte lysates, bottom-component RNA is translated into a 200K polypeptide which is then processed to give the 32 and 170K polypeptides also found in vivo. By pulse-labeling the 200K primary translation product, we now show that the 32 and 170K polypeptides are derived from the NH 2 -terminal and COOH-terminal parts of this polypeptide, respectively. Comparison of the proteolytic peptide patterns of 170K polypeptides synthesized in vitro and pulse-labeled at either the NH 2 -terminal or the COOH-terminal end with the patterns of the 170 and 110K polypeptides found in vivo demonstrates that the order within the 200K primary translation product of cowpea mosaic virus bottom-component RNA is as follows: NH 2 -32K polypeptide-58K polypeptide-VPg-24K polypeptide-87K polypeptide-COOH.

The H protein isolated from tobacco mosaic virus reassociates with virions reconstituted in vitro

Virions of two strains of tobacco mosaic virus (U1 and Cc) have associated with them a small amount of a minor protein called H protein (A. Asselin and M. Zaitlin, 1978, Virology 91, 173-181), now known to be related to the viral coat protein (C.W. Collmer, V.M. Vogt, and M. Zaitlin, 1983, Virology 126, 429-448.). In the present study, a quantification technique involving disruption of virions followed by direct analysis of the component parts on SDS polyacrylamide gels was used to confirm an average of one molecule of H protein per virion for U1 TMV. H protein was separated from coat protein and purified by electrofocusing in a flatbed of granulated gel under stringent dissociating conditions. When assayed in the presence of urea, H protein has a pI of approximately 5.4, coat protein has a pI of approximately 4.9. Proteinase K-treated TMV RNA and H-protein-free TMV coat protein were reconstituted in vitro with or without H protein and the resulting virions were analyzed. A small amount of H protein reassociated with virions reconstituted in vitro (less than 10% of the amount found in native virions) and became resistant to degradation by trypsin, but such virions were no different from virions reconstituted without H protein in terms of yield of reconstituted particles or infectivity. In mixed reconstitution experiments with RNA and coat protein from strains U1 and Cc in all four possible combinations and with U1 H protein, the H protein always associated with the U1 coat protein. This demonstrated U1-H protein affinity for a specific coat protein rather than a specific RNA. It is unlikely that H protein functions in the early stages of viral infection, although the possibility of its having some other role in the life cycle of TMV remains.

Translational stability of plant viral RNAs microinjected into living cells. Influence of a 3'-poly(A) segment

Three different alternative structural features have been shown to be present at the 3' terminus of plant viral RNAs: (a) a poly(A) track, (b) a tRNA-like structure, (c) no special structural or sequence characteristic. We have compared the translational stability after injection into frog oocytes of a representative of each type: (a) the small genomic RNA (M-RNA) of cowpea mosaic virus (CPMV), (b) the subgenomic mRNA for coat protein (RNA 4) of brome mosaic virus (BMV), (c) the subgenomic mRNA for coat protein (RNA 4) of alfalfa mosaic virus (AIMV). It has been shown that CPMV M-RNA exhibits the highest translational stability. However, the stability of AIMV RNA 4 is remarkably high and moreover significantly higher than that of BMV RNA 4. We demonstrate that, for all three viral RNA species considered, the presence of a poly(A) segment at the 3' end of the molecules improves the translational stability. From a comparative investigation in which AIMV RNA 4 was also injected into HeLa cells, it is concluded that the stability of a given non-adenylylated mRNA depends on the nature of the cytoplastic environment.

Cowpea Mosaic Virus-Encoded Protease Does Not Recognize Primary Translation Products of M RNAs from Other Comoviruses

The protease encoded by the large (B) RNA segment of cowpea mosaic virus was tested for its ability to recognize the in vitro translation products of the small (M) RNA segment from the comoviruses squash mosaic virus, red clover mottle virus, and cowpea severe mosaic virus (CPsMV, strains Dg and Ark), and from the nepovirus tomato black ring virus. Like M RNA from cowpea mosaic virus, the M RNAs from squash mosaic virus, red clover mottle virus, CPsMV-Dg, and CPsMV-Ark were all translated into two large polypeptides with apparent molecular weights which were different for each virus and even for the two CPsMV strains. Neither the in vitro products from squash mosaic virus, red clover mottle virus, and CPsMV M RNAs nor the in vitro product from tomato black ring virus RNA-2 were processed by the cowpea mosaic virus-encoded protease, indicating that the activity of this enzyme is highly specific.

The location of the first AUG codons in cowpea mosaic virus RNAs

We have made use of the known sequence of the 5' ends of both CPMV RNAs to synthesise an oligodeoxynucleotide which can prime second-strand DNA synthesis on full-length cDNA copies of both RNAs. By priming synthesis in the presence of dideoxynucleoside triphosphates, we have determined the positions of the first AUG codons in each RNA. These occur at positions 115 and 207 on M and B RNA respectively. By using a cloned double-stranded DNA fragment derived from near the 5' end of M RNA as a primer additional sequence from the 5' terminal region of M RNA has been obtained.

Expression of the Bottom-Component RNA of Cowpea Mosaic Virus: Evidence that the 60-Kilodalton VPg Precursor Is Cleaved into Single VPg and a 58-Kilodalton Polypeptide

In cowpea protoplasts infected with cowpea mosaic virus, a bottom-component (B) RNA-encoded 60-kilodalton (60K) polypeptide is synthesized, which is membrane-bound and represents the direct precursor to the genome-bound protein VPg. The relationship between this VPg precursor and other B-RNA-encoded polypeptides was studied. Digestion of the B-RNA-encoded 170K and 84K polypeptides with Staphylococcus aureus protease V8 and subsequent analysis of the generated peptides with antiserum against VPg showed that a VPg sequence resides internally in these polypeptides. Furthermore, a new B-RNA-encoded polypeptide was detected, with a size of 58K, which differed from the 60K polypeptide only in the lack of VPg sequences. A model is presented in which the 60K VPg precursor is generated from the 200K primary translation product from B RNA and further processed to a 58K polypeptide and single VPg.

Antibodies Against the Genome-Linked Protein VPg of Cowpea Mosaic Virus Recognize a 60,000-Dalton Precursor Polypeptide

We have prepared a rabbit antiserum specifically directed against the genome-linked protein (VPg) of cowpea mosaic virus by injecting an hydrolysate of purified virion RNA. Using this antiserum as a probe in combination with “Western” (protein) blots of subcellular fractions of cowpea mosaic virus-infected cowpea ( Vigna unguiculata ) cells, we have detected a bottom component RNA-encoded, 60,000-dalton polypeptide which is membrane bound and presumably represents the immediate precursor of VPg.

Two forms of VPg on poliovirus RNAs

The protein (VPg) linked to the 5' termini of poliovirus RNAs resolved into two species when subjected to non-equilibrium electrofocusing. The differently charged forms of VPg were not due to protein phosphorylation nor to variability of the number of phosphate residues associated with the nucleotide moiety remaining after RNase digestion of the nucleoprotein. Single-stranded viral RNA isolated from mature virions contained predominantly the more basic form of VPg, whereas unpackaged single-stranded RNa remaining in cells at the end of the virus replication cycle contained predominantly the more acidic form of VPg. Replicative-form (RF) molecules also contained both species of VPg, with the more acidic form representing the major species. Both plus and minus RNA strands in RF had similar VPg compositions; however, there appeared to be a strongly selective loss of VPg from only the minus strand in RF, particularly at late times postinfection.

Expression of Bottom Component RNA of Cowpea Mosaic Virus in Cowpea Protoplasts

Upon inoculation of cowpea protoplasts with the bottom component of cowpea mosaic virus, at least six virus-induced proteins (with sizes of 170, 110, 87, 84, 60, and 32 kilodaltons) are synthesized, but not the capsid proteins (37 and 23 kilodaltons). These bottom-component-induced proteins were studied with respect to their genetic origin and mode of synthesis. The analyses were based on their electrophoretic peptide patterns resulting from partial digestion with Staphylococcus aureus protease V8. Comparison of the peptide patterns of the virus-induced proteins with those of the cowpea mosaic virus RNA-coded polypeptides produced in rabbit reticulocyte lysate showed that the 170- and 32-kilodalton polypeptides, which are the first viral products in cowpea mosaic virus-infected cells, were actually coded by the bottom component RNA of the virus. The 110-, 87-, and 84-kilodalton polypeptides, and possibly the 60-kilodalton polypeptide, appeared to have amino acid sequences in common with the 170-kilodalton polypeptide, demonstrating that they were virus coded as well. The results indicated that cowpea mosaic virus bottom component RNA was translated in vivo into a single 200-kilodalton polyprotein from which probably all bottom-component-specific proteins arose by three successive cleavages.

Protein p3 is linked to the DNA of phage phi 29 through a phosphoester bond between serine and 5'-dAMP

To investigate the role of protein p3 in bacteriophage phi 29 initiation of replication, we have studied the nature of the covalent linkage between protein p3 and phi 29 DNA. The protein-DNA compound was digested with micrococcal nuclease and pronase resulting in a nucleotidyl-peptide that was further digested by alkaline phosphatase and snake venom phosphodiesterase yielding 5'-dAMP. The DNA-protein linkage is sensitive to alkali. Treatment of the nucleotidyl-peptide with 0.1 M NaOH at 37 degrees C for 3 hr after phosphatase digestion released 5'-dAMP. Hydrolysis of the nucleotidyl-peptide with 5.8 M HCl at 110 degrees C for 90 min yielded O-phosphoserine. These results, together with the sensitivity of the DNA-protein linkage to snake venom phosphodiesterase and its resistance to hydroxylamine, indicate that protein p3 is covalently linked to phi 29 DNA through a phosphoester bond between L-serine and 5'-dAMP, namely a O,5'-deoxyadenylyl-L-serine bond.