RNA synthesis of vesicular stomatitis virus-infected cells: in vivo regulation of replication

STAT6 promotes innate immunity against BEFV and VSV by inhibiting STUB1 and NIX-mediated MAVS degradation

Signal transducers and activators of transcription 6 (STAT6), an essential member of the STAT protein family, plays vital roles in innate immunity, however, its function in regulating innate immunity through the degradation of MAVS has not been described. In this study, we found that STAT6 suppresses the replication of both bovine ephemeral fever virus (BEFV) and vesicular stomatitis virus (VSV). Further investigations revealed that STAT6 promotes the type I IFN (IFN-I) signaling pathway in the context of BEFV and VSV infection. Moreover, the knockout of STAT6 leads to the degradation of MAVS through both the ubiquitin-proteasome and autophagolysosomal pathways. Mechanistically, STAT6 results in the downregulation of E3 ubiquitin ligase STIP1 homology and Ubox-containing protein 1 (STUB1), inhibits the interaction between STUB1 and MAVS, and reduces STUB1- mediated K48-linked MAVS ubiquitination, thereby inhibiting the MAVS degradation through the ubiquitin-proteasome pathway. Furthermore, STAT6 also suppresses MAVS degradation through the autophagy receptor Bcl2 interacting protein 3 like (NIX)-mediated autophagy pathway. Taken together, our study unveils a novel mechanism by which STAT6 acts as a positive regulator of the type I IFN signaling pathway during BEFV and VSV infection, predominantly by inhibiting MAVS degradation and ultimately suppressing BEFV and VSV infection. These findings provide valuable insights into the regulation of MAVS degradation by STAT6, which may serve as a basis for the design of novel antiviral agents.

Computational multigene interactions in virus growth and infection spread

Viruses persist in nature owing to their extreme genetic heterogeneity and large population sizes, which enable them to evade host immune defenses, escape antiviral drugs, and adapt to new hosts. The persistence of viruses is challenging to study because mutations affect multiple virus genes, interactions among genes in their impacts on virus growth are seldom known, and measures of viral fitness are yet to be standardized. To address these challenges, we employed a data-driven computational model of cell infection by a virus. The infection model accounted for the kinetics of viral gene expression, functional gene-gene interactions, genome replication, and allocation of host cellular resources to produce progeny of vesicular stomatitis virus, a prototype RNA virus. We used this model to computationally probe how interactions among genes carrying up to eleven deleterious mutations affect different measures of virus fitness: single-cycle growth yields and multicycle rates of infection spread. Individual mutations were implemented by perturbing biophysical parameters associated with individual gene functions of the wild-type model. Our analysis revealed synergistic epistasis among deleterious mutations in their effects on virus yield; so adverse effects of single deleterious mutations were amplified by interaction. For the same mutations, multicycle infection spread indicated weak or negligible epistasis, where single mutations act alone in their effects on infection spread. These results were robust to simulation in high- and low-host resource environments. Our work highlights how different types and magnitudes of epistasis can arise for genetically identical virus variants, depending on the fitness measure. More broadly, gene-gene interactions can differently affect how viruses grow and spread.

Structural insights into the rhabdovirus transcription/replication complex

The rhabdoviruses have a non-segmented single stranded negative-sense RNA genome. Their multiplication in a host cell requires three viral proteins in addition to the viral RNA genome. The nucleoprotein (N) tightly encapsidates the viral RNA, and the N-RNA complex serves as the template for both transcription and replication. The viral RNA-dependent RNA polymerase is a two subunit complex that consists of a large subunit, L, and a non-catalytic cofactor, the phosphoprotein, P. P also acts as a chaperone of nascent RNA-free N by forming a N(0)-P complex that prevents N from binding to cellular RNAs and from polymerizing in the absence of RNA. Here, we discuss the recent molecular and structural studies of individual components and multi-molecular complexes that are involved in the transcription/replication complex of these viruses with regard to their implication in viral transcription and replication.

Stochastic kinetic modeling of vesicular stomatitis virus intracellular growth

By building kinetic models of biological networks one may advance the development of new modeling approaches while gaining insights into the biology. We focus here on building a stochastic kinetic model for the intracellular growth of vesicular stomatitis virus (VSV), a well-studied virus that encodes five genes. The essential network of VSV reactions creates challenges to stochastic simulation owing to (i) delayed reactions associated with transcription and genome replication, (ii) production of large numbers of intermediate proteins by translation, and (iii) the presence of highly reactive intermediates that rapidly fluctuate in their intracellular levels. We address these issues by developing a hybrid implementation of the model that combines a delayed stochastic simulation algorithm (DSSA) with Langevin equations to simulate the reactions that produce species in high numbers. Further, we employ a quasi-steady-state approximation (QSSA) to overcome the computational burden of small time steps caused by highly reactive species. The simulation is able to capture experimentally observed patterns of viral gene expression. Moreover, the simulation suggests that early levels of a low-abundance species, VSV L mRNA, play a key role in determining the production level of VSV genomes, transcripts, and proteins within an infected cell. Ultimately, these results suggest that stochastic gene expression contribute to the distribution of virus progeny yields from infected cells.

Model-based design of growth-attenuated viruses

Live-virus vaccines activate both humoral and cell-mediated immunity, require only a single boosting, and generally provide longer immune protection than killed or subunit vaccines. However, growth of live-virus vaccines must be attenuated to minimize their potential pathogenic effects, and mechanisms of attenuation by conventional serial-transfer viral adaptation are not well-understood. New methods of attenuation based on rational engineering of viral genomes may offer a potentially greater control if one can link defined genetic modifications to changes in virus growth. To begin to establish such links between genotype and growth phenotype, we developed a computer model for the intracellular growth of vesicular stomatitis virus (VSV), a well-studied, nonsegmented, negative-stranded RNA virus. Our model incorporated established regulatory mechanisms of VSV while integrating key wild-type infection steps: hijacking of host resources, transcription, translation, and replication, followed by assembly and release of progeny VSV particles. Generalization of the wild-type model to allow for genome rearrangements matched the experimentally observed attenuation ranking for recombinant VSV strains that altered the genome position of their nucleocapsid gene. Finally, our simulations captured previously reported experimental results showing how altering the positions of other VSV genes has the potential to attenuate the VSV growth while overexpressing the immunogenic VSV surface glycoprotein. Such models will facilitate the engineering of new live-virus vaccines by linking genomic manipulations to controlled changes in virus gene-expression and growth.

Transcription and replication of nonsegmented negative-strand RNA viruses

The nonsegmented negative-strand (NNS) RNA viruses of the order Mononegavirales include a wide variety of human, animal, and plant pathogens. The NNS RNA genomes of these viruses are templates for two distinct RNA synthetic processes: transcription to generate mRNAs and replication of the genome via production of a positive-sense antigenome that acts as template to generate progeny negative-strand genomes. The four virus families within the Mononegavirales all express the information encoded in their genomes by transcription of discrete subgenomic mRNAs. The key feature of transcriptional control in the NNS RNA viruses is entry of the virus-encoded RNA-dependent RNA polymerase at a single 3' proximal site followed by obligatory sequential transcription of the linear array of genes. Levels of gene expression are primarily regulated by position of each gene relative to the single promoter and also by cis-acting sequences located at the beginning and end of each gene and at the intergenic junctions. Obligatory sequential transcription dictates that termination of each upstream gene is required for initiation of downstream genes. Therefore, termination is a means to regulate expression of individual genes within the framework of a single transcriptional promoter. By engineering either whole virus genomes or subgenomic replicon derivatives, elements important for signaling transcript initiation, 5' end modification, 3' end polyadenylation, and transcription termination have been identified. Although the diverse families of NNS RNA virus use different sequences to control these processes, transcriptional termination is a common theme in controlling gene expression and overall transcriptional regulation is key in controlling the outcome of viral infection. The latest models for control of replication and transcription are discussed.

Transcriptional control of the RNA-dependent RNA polymerase of vesicular stomatitis virus

The nonsegmented negative strand (NNS) RNA viruses include some of the mosr problematic human, animal and plant pathogens extant: for example, rabies virus, Ebola virus, respiratory syncytial virus, the parainfluenza viruses, measles and infectious hemapoietic necrosis virus. The key feature of transcriptional control in the NNS RNA viruses is polymerase entry at a single 3' proximal site followed by obligatory sequential transcription of the linear array of genes. The levels of gene expression are primarily regulated by their position on the genome. The promoter proximal gene is transcribed in greatest abundance and each successive downstream gene is synthesized in progressively lower amounts due to attenuation of transcription at each successive gene junction. In addition, NNS RNA virus gene expression is regulated by cis-acting sequences that reside at the beginning and end of each gene and the intergenic junctions. Using vesicular stomatitis virus (VSV), the prototypic NNS, many of these control elements have been identified.The signals for transcription initiation and 5' end modification and for 3' end polyadenylation and termination have been elucidated. The sequences that determine the ability of the polymerase to slip on the template to generate polyadenylate have been identified and polyadenylation has been shown to be template dependent and integral to the termination process. Transcriptional termination is a key element in control of gene expression of the negative strand RNA viruses and a means by which expression of individual genes may be silenced or regulated within the framework of a single transcriptional promoter. In addition, the fundamental question of the site of entry of the polymerase during transcription has been reexamined and our understanding of the process altered and updated. The ability to engineer changes into infectious viruses has confirmed the action of these elements and as a consequence, it has been shown that transcriptional control is key to controlling the outcome of a viral infection. Finally, the principles of transcriptional regulation have been utilized to develop a new paradigm for systematic attenuation of virulence to develop live attenuated viral vaccines.

Regulation of RNA synthesis by the genomic termini of vesicular stomatitis virus: identification of distinct sequences essential for transcription but not replication

The RNA-dependent RNA polymerase of vesicular stomatitis virus (VSV), a nonsegmented negative-strand RNA virus, directs two discrete RNA synthetic processes, transcription and replication. Available evidence suggests that the two short extragenic regions at the genomic termini, the 3' leader (Le) and the complement of the 5' trailer (TrC), contain essential signals for these processes. We examined the roles in transcription and replication of sequences in Le and TrC by monitoring the effects of alterations to the termini of subgenomic replicons, or infectious viruses, on these RNA synthetic processes. Distinct elements in Le were found to be required for transcription that were not required for replication. The promoter for mRNA transcription was shown to include specific sequence elements within Le at positions 19 to 29 and 34 to 46, a separate element at nucleotides 47 to 50, the nontranscribed leader-N gene junction. The sequence requirements for transcription within the Le region could not be supplied by sequences found at the equivalent positions in TrC. In contrast, sequences from either Le or TrC functioned well to signal replication, indicating that within the confines of the VSV termini, the sequence requirements for replication were less stringent. Deletions engineered at the termini showed that the terminal 15 nucleotides of either Le or TrC allowed a minimal level of replication. Within these confines, levels of replication were affected by both the extent of complementarity between the genomic termini and the involvement of the template in transcription. In agreement with our previous observations, increasing the extent of complementarity between the natural termini increased levels of replication, and this effect was most operative at the extreme genome ends. In addition, abolishing the use of Le as a promoter for transcription enhanced replication. These analyses (i) identified signals at the termini required for transcription and replication and (ii) showed that Le functions as a less efficient promoter for replication than TrC at least in part because of its essential role in transcription. Consequently, these observations help explain the asymmetry of VSV replication which results in the synthesis of more negative- than positive-sense replication products in infected cells.

Overlapping signals for transcription and replication at the 3' terminus of the vesicular stomatitis virus genome

Transcription and replication signals within the negative-sense genomic RNA of vesicular stomatitis virus (VSV) are located at the 3' terminus. To identify these signals, we have used a transcription- and replication-competent minigenome of VSV to generate a series of deletions spanning the first 47 nucleotides at the 3' terminus of the VSV genome corresponding to the leader gene. Analysis of these mutants for their ability to replicate showed that deletion of sequences within the first 24 nucleotides abrogated or greatly reduced the level of replication. Deletion of downstream sequences from nucleotides 25 to 47 reduced the level of replication only to 55 to 70% of that of the parental template. When transcription activity of these templates was measured, the first 24 nucleotides were also found to be required for transcription, since deletion of these sequences blocked or significantly reduced transcription. Downstream sequences from nucleotides 25 to 47 were necessary for optimal levels of transcription. Furthermore, replacement of sequences within the 25 to 47 nucleotides with random heterologous nonviral sequences generated mutant templates that replicated well (65 to 70% of the wild-type levels) but were transcribed poorly (10 to 15% of the wild-type levels). These results suggest that the minimal promoter for transcription and replication could be as small as the first 19 nucleotides and is contained within the 3'-terminal 24 nucleotides of the VSV genome. The sequences from nucleotides 25 to 47 may play a more important role in optimal transcription than in replication. Our results also show that deletion of sequences within the leader gene does not influence the site of transcription reinitiation of the downstream gene.

Replication signals in the genome of vesicular stomatitis virus and its defective interfering particles: identification of a sequence element that enhances DI RNA replication

We have analyzed the role of terminal sequences of a defective interfering (DI) particle RNA of vesicular stomatitis virus (VSV) in replication. A series of internal deletion mutants of DI cDNA was generated to obtain DI genomic RNAs that differed from one another by the presence of different lengths of 3'-terminal and/or 5'-terminal sequences. Analyses of the mutant. RNAs for their ability to replicate in cells transfected with the corresponding plasmids suggested that distinct regions at the termini of DI RNA are important for RNA replication. Region I, encompassing nucleotides 1-24, is absolutely required for replication since DI RNA genomes lacking any part of this region failed to replicate. Region II, spanning nucleotides 25-45, is not essential for replication but it functions as an enhancer of replication in that the presence of these specific sequences confers high efficiency of replication to the template. Deleting these specific sequences from both termini of DI RNA but maintaining the length of terminal complementarity as seen in wild-type DI RNA resulted in a template that replicated poorly (about 20-fold less efficiently). Furthermore, insertion or substitution of these sequences into the 3'-terminus of a VSV minigenome resulted in a template that replicated more efficiently (at least 4-fold to as high as 15-fold) than the parental minigenome. These results strongly support the conclusion that the presence of specific sequences rather than the extent of complementarity at the termini of DI RNA is a major determinant of the efficiency of replication. The presence of the specific sequences at the 3'-terminus of both genomic and antigenomic DI RNAs may explain in part the replicative dominance of DI RNA over the full-length VSV genome which contains these sequences only at the 3'-terminus of the antigenome.

The vesicular stomatitis virus L protein possesses the mRNA methyltransferase activities

We have previously shown that the vesicular stomatitis virus (VSV) host range mutant, hr 1, is completely defective for the mRNA methyltransferase activities, but can synthesize full-length, unmethylated mRNAs in vitro [S. M. Horikami and S. A. Moyer (1982). Proc. Natl. Acad. Sci. USA 79, 7694-7698] and in vivo [S. M. Horikami, F. De Ferra, and S. A. Moyer (1984). Virology 138, 1-15]. Here we have used the hr 1 mutant to identify the viral protein which possesses the methyltransferase activities. The wild-type VSV L and NS proteins, subunits of the viral RNA polymerase, were separately purified and added to high salt dissociated mutant hr 1 nucleocapsids for in vitro transcription reactions. The results show that the purified wild-type L protein, but not the NS protein, restores methylation and thus possesses the viral mRNA methyltransferase activities.

Viral RNA polymerases

Recent progress in molecular biological techniques revealed that genomes of animal viruses are complex in structure, for example, with respect to the chemical nature (DNA or RNA), strandedness (double or single), genetic sense (positive or negative), circularity (circle or linear), and so on. In agreement with this complexity in the genome structure, the modes of transcription and replication are various among virus families. The purpose of this article is to review and bring up to date the literature on viral RNA polymerases involved in transcription of animal DNA viruses and in both transcription and replication of RNA viruses. This review shows that the viral RNA polymerases are complex in both structure and function, being composed of multiple subunits and carrying multiple functions. The functions exposed seem to be controlled through structural interconversion.

Vesicular stomatitis virus in Drosophila melanogaster cells: regulation of viral transcription and replication

Vesicular stomatitis virus RNA synthesis was investigated during the establishment of persistent infection in Drosophila melanogaster cells. The transcription rate declined as early as 5 h after infection and was strongly inhibited after 7 h, leading to a decrease in viral mRNA levels and in viral protein synthesis rates. Full-length plus-strand antigenomes and minus-strand genomes were detected after a 3-h lag time and accumulated until 15 h after infection. Short encapsidated plus-strand molecules were also generated corresponding to the 5' end of viral defective antigenomes. Assembly and release of virions were not restricted, but their infectivity was extremely reduced. In persistently infected cells, an equilibrium was reached where the level of intracellular genomes maintained was constant and maximal even after the rate of all viral syntheses had decreased. These results are discussed with regard to the establishment of persistent infection.

Characterization of the infections of permissive and nonpermissive cells by host range mutants of vesicular stomatitis virus defective in RNA methylation

Two host range mutants of VSV, hr 1 and hr 8, which, unlike the wild-type virus, have a mRNA methylation defect and direct the in vitro synthesis of full-length capped but unmethylated viral mRNAs have been described previously (S.M. Horikami and S.A. Moyer, 1982, Proc. Natl, Acad. Sci. USA 79, 7694-7698). It is shown that the in vivo nonpermissive infection of HEp-2 cells by either of these two mutants is characterized by the reduced synthesis of full-length mRNAs at levels characteristic of primary transcription and the total lack of synthesis of genome-length RNA. The VSV mRNAs synthesized by either mutant in HEp-2 cells are not translated either in vivo or in vitro in mRNA-dependent rabbit reticulocyte lysates. Subsequent isolation and analysis of the mRNAs from infected HEp-2 cells has shown that the 5' termini of the messages contain a cap structure which is guanylylated, but unmethylated (GpppA), a finding that might account for the lack of translatability. Hence these mutants are unable to properly methylate mRNAs whether they are synthesized in vitro or in vivo within nonpermissively infected cells. It is also shown that unlike hr 1, the undermethylation of mRNA synthesized by hr 8 is partially reversible by the addition of high levels of AdoMet in vitro. It is interesting to note, therefore, that permissive baby hamster kidney (BHK) cells have a 10-fold higher level of endogenous AdoMet than the nonpermissive HEp-2 cells. Unlike singly infected cells, the coinfection of HEp-2 cells with either hr mutant and a poxvirus yields a permissive infection for these two host range mutants. Analysis of the VSV mRNAs produced in vivo under the conditions of rescue reveals the presence of fully methylated caps (7mGppp(m)Am), suggesting that poxvirus may rescue the mutants by converting the VSV mRNAs to a translationally active form due to methylation by the cytoplasmic poxvirus mRNA methyltransferase enzymes. Both mutants are, however, able to grow normally in permissive BHK cells. An analysis of the translationally active mRNAs from infected permissive cells shows the presence primarily of a 5'-monomethylated cap, 7mGpppA. Finally, we have examined the nonpermissive infections of two other host range mutants of VSV (hr 5 and hr 7). Unlike mutants hr 1 and hr 8 described above, these two mutants synthesize mRNA in HEp-2 cells which is translated both in vivo and in vitro.

Nucleotide sequence and host La protein interactions of rabies virus leader RNA

Rabies virus leader RNA was detected in infected BHK-21 cell extracts by hybridization to end-labeled genomic RNA. Similar to the leader RNA of vesicular stomatitis virus, the leader RNA of rabies virus was also found to be associated with the La protein by specific immunoprecipitation with antisera from lupus patients. The 3' end of the genomic RNA of rabies virus was sequenced, and the size and termination site of leader RNA were determined. In addition, extension of the sequence into the nucleocapsid gene of rabies virus showed an open reading frame for at least 37 amino acid residues. Sequence relationships between rabies virus and vesicular stomatitis virus leader genes and the possible involvement of the La protein in rhabdovirus biology are discussed.

A host protein (La) binds to a unique species of minus-sense leader RNA during replication of vesicular stomatitis virus

Baby hamster kidney cells infected with the minus-strand RNA virus vesicular stomatitis virus (VSV) were found to contain three small viral leader RNA species of the minus sense. The longest minus-strand leader RNA was 54 nucleotides long and was complexed with the host cell La protein that was immunoprecipitated by antisera from patients with systemic lupus erythematosus. The La protein is normally found associated with RNA polymerase III transcripts in their unprocessed form. Shorter minus-strand leader RNA species of 45-48 nucleotides were more abundant but were not associated with the La protein. Unlike the plus-strand leader RNA of VSV, the minus-strand leader RNAs were not detected in the nucleus in any form. The minus-strand leader RNAs accumulated gradually throughout the infection and could not be found in association with the viral nucleocapsid protein. The sequence required for La protein binding on the 54-nucleotide-long minus-strand leader is similar to that at the 3' end of the La protein binding-plus-strand leader RNA and, thus, we propose a role for the La protein in the replication of VSV.

Synthesis in vitro of full length genomic RNA and assembly of the nucleocapsid of vesicular stomatitis virus in a coupled transcription-translation system

Synthesis of a small amount of 42S RNA in addition to the VSV specific mRNA species was observed in a coupled transcription-translation system containing ribonucleoprotein particles from L cell infected with vesicular stomatitis virus and nuclease-treated ribosomal extract obtained from uninfected HeLa cells. Analysis on a CsCl density gradient showed that the synthesized 42S RNA was associated with newly synthesized by protein as a nucleoprotein of bouyant density of 1.3 g/ml. The 42S RNA and the N protein present in the nucleoprotein were resistant to nuclease and protease, respectively. About 35% of the remaining 65% had a complementary polarity. The evidence presented here demonstrates that both the full length genomic and the complementary RNA are associated with N protein in the in vitro replication process. A template role for the complementary 42S RNA for replication of the genomic RNA is also suggested.

Novel phenotype of RNA synthesis expressed by vesicular stomatitis virus isolated from persistent infection

Vesicular stomatitis virus (VSV) stocks isolated from two persistently infected mouse L-cell lines (designated VSV-PI stocks) express an altered phenotype of RNA synthesis. This phenotype is different from the RNA synthesis phenotype expressed by the viruses used to initiate the persistently infected lines, wild-type VSV and VSV ts-0-23 (a group III, ts-, RNA+ mutant). At 34 and 37 degrees C in L cells productively infected with VSV-PI stocks derived from the two cell lines, transcription of virus mRNA was significantly reduced, whereas replication of the 40S genomic RNA species was enhanced compared with wild-type VSV or ts-0-23. At 34 and 37 degrees C, both VSV-PI stocks replicated with equal or greater efficiency than wild-type VSV; 37 degrees C was the temperature at which the persistently infected cultures were maintained. At 40 degrees C, both VSV-PI stocks were temperature sensitive, and clonal VSV-PI isolates from both cell lines belong to complementation group I (RNA-). Standard ts- mutants (derived by mutagenesis of wild-type VSV) belonging to RNA- complementation groups I, II, and IV do not express the VSV-PI RNA synthesis phenotype at the permissive temperature, making this phenotype distinctive to persistent infection. Since the two VSV-PI populations from persistently infected cell lines initiated with different viruses both evolved this unique phenotype of RNA synthesis, the expression of this phenotype may play an important role in the maintenance of persistence.

Rapid and transient localization of the leader RNA of vesicular stomatitis virus in the nuclei of infected cells

The leader RNA transcript from the 3' end of the genome of vesicular stomatitis virus (VSV) has been detected in both the nucleus and cytoplasm of infected baby hamster kidney (BHK) cells. In the cytoplasm, leader RNA accumulated gradually throughout the infection to about 200 molecules per cell at 6 hr after infection. In the nucleus, however, there was a sharp and rapid increase in the concentration of leader RNA to approximately equal to 300 molecules per cell at about 2 hr after infection that decreased rapidly by 3 hr. This report presents evidence for nuclear localization of transcription products of a (-)-strand RNA virus other than influenza and supports the hypothesis that the leader RNA plays a role in the shutoff of host cell transcription.

In vitro reassembly of vesicular stomatitis virus skeletons

Vesicular stomatitis virus (VSV) has been disrupted with nonionic detergent plus 0.5 M NaCl under conditions which result in solubilization of the viral glycoprotein (G), matrix protein (M), and lipids, leaving the nucleocapsid in a highly extended state. Dialysis of these suspensions to remove NaCl was found to result in reassociation of nucleocapsids with M protein. Reassociated structures were highly condensed and similar in appearance to "native" VSV skeletons produced by extraction of virions with detergent at low ionic strength. For instance, electron microscopic analysis revealed that, like "native" skeletons, "reassembled" skeletons were cylindrical in shape, with diameters in the range of 51.0 to 55.0 nm and cross-striations spaced approximately 6.0 nm apart along the length of the structure. Like native skeletons, reassembled skeletons were found by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to contain the viral N and M proteins, but they lacked the glycoprotein entirely. Both native and reassembled skeletons were found to be capable of in vitro RNA-dependent RNA synthesis (transcription). In vivo skeleton assembly required the presence of M protein and nucleocapsids. No skeleton-like structures were formed by dialysis of nucleocapsids in the absence of M protein or of M protein in the absence of nucleocapsids. These results provide strong support for the view that the VSV M protein plays a functional role in condensing the viral nucleocapsid in vitro and raise the possibility that it may play a similar role in vivo.

Replicative RNA synthesis and nucleocapsid assembly in vesicular stomatitis virus-infected permeable cells

A permeable-cell system has been developed to study the replication of vesicular stomatitis virus. When vesicular stomatitis virus-infected BHK cells were permeabilized by lysolecithin treatment, they incorporated nucleoside triphosphates into RNA and amino acids into proteins at nearly normal rates. The viral mRNA's synthesized appeared normal in polarity, size distribution, and polyadenylation, and all five viral proteins were synthesized. Replication of the viral genome proceeded, and full-length RNA strands were synthesized in amounts and polarities resembling those found in intact cells. These full-length RNAs associated with viral N proteins to form RNase-resistant nucleocapsids of normal buoyant density. Permeable cells appear to represent ideal hosts for studying vesicular stomatitis virus replication since they closely mimic in vivo conditions while retaining much of the experimental flexibility of current in vitro systems.

Effect of defective interfering particles on plus- and minus- strand leader RNAs in vesicular stomatitis virus-infected cells

Vesicular stomatitis virus-infected cells contain short RNA transcripts, called leader RNAs, which are coded by the exact 3' end of both the minus-strand and plus-strand nucleocapsid templates. The molar amounts of both the plus-strand leader RNA (which is templated from the minus-strand genome) and the minus-strand leader RNA (which is templated from the plus-strand antigenome) were determined both in standard-virus- and mixed-virus-infected cells by using end-labeled genome probes. The results demonstrate that the presence of defective interfering particles in the infecting virus stock decreases the amount of plus-strand leader RNA but increases the amount of minus-strand leader RNA found in the infected cells. In addition, considerably more minus-strand leader RNA per mole of nucleocapsid template is synthesized in mixed-virus-infected cells than plus-strand leader RNA per mole of nucleocapsid template in both standard-virus- and mixed-virus-infected cells.