Excess synthesis of viral mRNA 5-terminal oligonucleotides by reovirus transcriptase

Captivating Perplexities of Spinareovirinae 5' RNA Caps

RNAs with methylated cap structures are present throughout multiple domains of life. Given that cap structures play a myriad of important roles beyond translation, such as stability and immune recognition, it is not surprising that viruses have adopted RNA capping processes for their own benefit throughout co-evolution with their hosts. In fact, that RNAs are capped was first discovered in a member of the Spinareovirinae family, Cypovirus, before these findings were translated to other domains of life. This review revisits long-past knowledge and recent studies on RNA capping among members of Spinareovirinae to help elucidate the perplex processes of RNA capping and functions of RNA cap structures during Spinareovirinae infection. The review brings to light the many uncertainties that remain about the precise capping status, enzymes that facilitate specific steps of capping, and the functions of RNA caps during Spinareovirinae replication.

Characterization of rotavirus RNAs that activate innate immune signaling through the RIG-I-like receptors

In mammalian cells, the first line of defense against viral pathogens is the innate immune response, which is characterized by induction of type I interferons (IFN) and other pro-inflammatory cytokines that establish an antiviral milieu both in infected cells and in neighboring uninfected cells. Rotavirus, a double-stranded RNA virus of the Reoviridae family, is the primary etiological agent of severe diarrhea in young children worldwide. Previous studies demonstrated that rotavirus replication induces a MAVS-dependent type I IFN response that involves both RIG-I and MDA5, two cytoplasmic viral RNA sensors. This study reports the isolation and characterization of rotavirus RNAs that activate IFN signaling. Using an in vitro approach with purified rotavirus double-layer particles, nascent single-stranded RNA (ssRNA) transcripts (termed in vitro ssRNA) were found to be potent IFN inducers. In addition, large RNAs isolated from rotavirus-infected cells six hours post-infection (termed in vivo 6 hr large RNAs), also activated IFN signaling, whereas a comparable large RNA fraction isolated from cells infected for only one hour lacked this stimulatory activity. Experiments using knockout murine embryonic fibroblasts showed that RIG-I is required for and MDA5 partly contributes to innate immune signaling by both in vitro ssRNA and in vivo 6 hr large RNAs. Enzymatic studies demonstrated that in vitro ssRNA and in vivo 6 hr large RNA samples contain uncapped RNAs with exposed 5’ phosphate groups. RNAs lacking 2’-O-methylated 5’ cap structures were also detected in the in vivo 6 hr large RNA sample. Taken together, our data provide strong evidence that the rotavirus VP3 enzyme, which encodes both guanylyltransferase and methyltransferase activities, is not completely efficient at either 5’ capping or 2’-O-methylation of the 5’ cap structures of viral transcripts, and in this way produces RNA patterns that activate innate immune signaling through the RIG-I-like receptors.

Effects of viscogens on RNA transcription inside reovirus particles

The dsRNA genome of mammalian reovirus (MRV), like the dsDNA genomes of herpesviruses and many bacteriophages, is packed inside its icosahedral capsid in liquid-crystalline form, with concentrations near or more than 400 mg/ml. Viscosity in such environments must be high, but the relevance of viscosity for the macromolecular processes occurring there remains poorly characterized. Here, we describe the use of simple viscogens, glycerol and sucrose, to examine their effects on RNA transcription inside MRV core particles. Transcription inside MRV cores was strongly inhibited by these agents and to a greater extent than either predicted by theory or exhibited by a nonencapsidated transcriptase, suggesting that RNA transcription inside MRV cores is unusually sensitive to viscogen effects. The elongation phase of transcription was found to be a primary target of this inhibition. Similar results were obtained with particles of a second dsRNA virus, rhesus rotavirus, from a divergent taxonomic subfamily. Polymeric viscogens such as polyethylene glycol also inhibited RNA transcription inside MRV cores, but in a size-limited manner, suggesting that diffusion through channels in the MRV core is required for their activity. Modeling of the data suggested that the inherent intracapsid viscosity of both reo- and rotavirus is indeed high, two to three times the viscosity of water. The capacity for quantitative comparisons of intracapsid viscosity and effects of viscogens on macromolecular processes in confined spaces should be similarly informative in other systems.

Probing the transcription mechanisms of reovirus cores with molecules that alter RNA duplex stability

The mammalian reovirus (MRV) genome comprises 10 double-stranded RNA (dsRNA) segments, packaged along with transcriptase complexes inside each core particle. Effects of four small molecules on transcription by MRV cores were studied for this report, chosen for their known capacities to alter RNA duplex stability. Spermidine and spermine, which enhance duplex stability, inhibited transcription, whereas dimethyl sulfoxide and trimethylglycine, which attenuate duplex stability, stimulated transcription. Different mechanisms were identified for inhibition or activation by these molecules. With spermidine, one round of transcription occurred normally, but subsequent rounds were inhibited. Thus, inhibition occurred at the transition between the end of elongation in one round and initiation in the next round of transcription. Dimethyl sulfoxide or trimethylglycine, on the other hand, had no effect on transcription by a constitutively active fraction of cores in each preparation but activated transcription in another fraction that was otherwise silent for the production of elongated transcripts. Activation of this other fraction occurred at the transition between transcript initiation and elongation, i.e., at promoter escape. These results suggest that the relative stability of RNA duplexes is most important for certain steps in the particle-associated transcription cycles of dsRNA viruses and that small molecules are useful tools for probing these and probably other steps.

Role of initiating nucleoside triphosphate concentrations in the regulation of influenza virus replication and transcription

The mechanisms regulating the synthesis of mRNA, cRNA, and viral genomic RNA (vRNA) by the influenza A virus RNA-dependent RNA polymerase are not fully understood. Previous studies in our laboratory have shown that virion-derived viral ribonucleoprotein complexes synthesize both mRNA and cRNA in vitro and early in the infection cycle in vivo. Our continued studies showed that de novo synthesis of cRNA in vitro is more sensitive to the concentrations of ATP, CTP, and GTP than capped-primer-dependent synthesis of mRNA. Using rescued recombinant influenza A/WSN/33 viruses, we now demonstrate that the 3'-terminal sequence of the vRNA promoter dictates the requirement for a high nucleoside triphosphate (NTP) concentration during de novo-initiated replication to cRNA, whereas this is not the case for the extension of capped primers during transcription to mRNA. In contrast to some other viral polymerases, for which only the initiating NTP is required at high concentrations, influenza virus polymerase requires high concentrations of the first three NTPs. In addition, we show that base pair mutations in the vRNA promoter can lead to nontemplated dead-end mutations during replication to cRNA in vivo. Based on our observations, we propose a new model for the de novo initiation of influenza virus replication.

Initiation of viral RNA-dependent RNA polymerization

This review summarizes the combined insights from recent structural and functional studies of viral RNA-dependent RNA polymerases (RdRPs) with the primary focus on the mechanisms of initiation of RNA synthesis. Replication of RNA viruses has traditionally been approached using a combination of biochemical and genetic methods. Recently, high-resolution structures of six viral RdRPs have been determined. For three RdRPs, enzyme complexes with metal ions, single-stranded RNA and/or nucleoside triphosphates have also been solved. These advances have expanded our understanding of the molecular mechanisms of viral RNA synthesis and facilitated further RdRP studies by informed site-directed mutagenesis. What transpires is that the basic polymerase right hand shape provides the correct geometrical arrangement of substrate molecules and metal ions at the active site for the nucleotidyl transfer catalysis, while distinct structural elements have evolved in the different systems to ensure efficient initiation of RNA synthesis. These elements feed the template, NTPs and ions into the catalytic cavity, correctly position the template 3′ terminus, transfer the products out of the catalytic site and orchestrate the transition from initiation to elongation.

Loss of activities for mRNA synthesis accompanies loss of lambda2 spikes from reovirus cores: an effect of lambda2 on lambda1 shell structure

The 144-kDa lambda2 protein, a component of the transcriptionally active reovirus core particle, catalyzes the last three enzymatic activities for formation of the 5' cap 1 structure on the viral plus-strand transcripts. Limited evidence suggests it may also play a role in transcription per se. Particle-associated lambda2 forms pentameric turrets ("spikes") around the fivefold axes of the icosahedral core. To address the requirements for lambda2 in core functions other than the known functions in RNA capping, particles depleted of lambda2 were generated from cores in vitro by a series of treatments involving heat, protease, and ionic detergent. The resulting particles contained less than 5% of pretreatment levels of lambda2 but showed negligible loss of the other four core proteins or the 10 double-stranded RNA genome segments. Transmission cryo-electron microscopy (cryo-TEM) and scanning cryo-electron microscopy demonstrated loss of the lambda2 spikes from these otherwise intact particles. In functional analyses, the "spikeless cores" showed greatly reduced activities not only for RNA capping but also for transcription and nucleoside triphosphate hydrolysis, suggesting enzymatic or structural roles for lambda2 in all these activities. Comparison of the core and spikeless core structures obtained by cryo-TEM and three-dimensional image reconstruction revealed changes in the lambda1 core shell that accompany lambda2 loss, most notably the elimination of small pores that span the shell near the icosahedral fivefold axes. Changes in the shell may explain the reductions in transcriptase-related activities by spikeless cores.

Identification and characterization of a transcription pause site in rotavirus

In rotavirus, transcription of the 11 double-stranded RNA genome segments occurs within the structurally intact subviral particle, and nascent transcripts are released through channels penetrating the two capsid layers at the icosahedral vertices. To gain insight into the early molecular events in transcription, we used high-resolution polyacrylamide gel electrophoresis to investigate the length distribution of transcription products at various times following initiation. We observed that, in the subviral particle under normal conditions, transcript initiation and capping are followed by a momentary pause in elongation after the addition of 6 to 7 nucleotides. In the absence of the capping reaction cofactor S-adenosylmethionine, conditions under which the rate of nucleotide incorporation is reduced, we observe a significant decrease in the ratio of paused to full-length transcripts. We propose that this pause site may represent the point at which specific molecular events take place to facilitate processive elongation. Furthermore, our results indicate that the presence of specific ligands on the viral surface, such as VP7 in the mature virion, inhibits polymerase function. From the perspective of the viral replication cycle, this inhibition may serve to ensure that transcription occurs with greatest efficiency only after the virus has entered the cytoplasm and assumed the form of a double-layered particle.

Transcriptional activities of reovirus RNA polymerase in recoated cores. Initiation and elongation are regulated by separate mechanisms

The particle-associated reovirus polymerase synthesizes mRNA within only certain viral particle types. Reovirus cores, subviral particles lacking outer capsid proteins mu1, sigma3, and sigma1, produce mRNA and abortive transcripts. Reovirus virions, which contain complete outer capsids, cannot produce mRNA and produce few abortive transcripts. Recoated cores are virion-like particles generated by the addition of recombinant outer capsid proteins to cores. We used recoated cores to analyze transcriptional regulation by reovirus outer capsid proteins. Partially recoated particles, containing less than virion amounts of mu1 and sigma3, synthesized mRNA at levels inversely proportional to outer capsid protein levels. Fully recoated cores exhibited undetectable mRNA synthesis levels, as did virions. However, recoated cores produced high levels of abortive transcripts. Recoated core abortive transcripts remained particle-associated and appeared to inhibit further abortive transcript production. Proteolysis of recoated cores removing mu1 and sigma3 released accumulated abortive transcripts and relieved inhibition of mRNA and abortive transcript synthesis. These results suggest transcriptional elongation, but not initiation, is blocked by virion-like amounts of mu1 and sigma3. Particle-associated abortive transcripts may down-regulate transcriptional initiation. Minor outer capsid protein sigma1 had no demonstrable effect on transcriptional activities. Transcriptional regulation may ensure progeny virions do not compete with transcribing particles for ribonucleoside triphosphates.

Viral and cellular mRNA capping: past and prospects

This chapter focuses on the history of the discovery of cap and an update of research on viral and cellular-messenger RNA (mRNA) capping. Cap structures of the type m⁷ GpppN(m)pN(m)p are present at the 5′ ends of nearly all eukaryotic cellular and viral mRNAs. A cap is added to cellular mRNA precursors and to the transcripts of viruses that replicate in the nucleus during the initial phases of transcription and before other processing events, including internal N⁶A methylation, 3′-poly (A) addition, and exon splicing. Despite the variations on the methylation theme, the important biological consequences of a cap structure appear to correlate with the N⁷-methyl on the 5′-terminal G and the two pyrophosphoryl bonds that connect m⁷G in a 5′–5′ configuration to the first nucleotide of mRNA. In addition to elucidating the biochemical mechanisms of capping and the downstream effects of this 5′- modification on gene expression, the advent of gene cloning has made available an ever-increasing amount of information on the proteins responsible for producing caps and the functional effects of other cap-related interactions. Genetic approaches have demonstrated the lethal consequences of cap failure in yeasts, and complementation studies have shown the evolutionary functional conservation of capping from unicellular to metazoan organisms.

Mechanism of genome transcription in segmented dsRNA viruses

Genome transcription is a critical stage in the life cycle of a virus, as this is the process by which the viral genetic information is presented to the host cell protein synthesis machinery for the production of the viral proteins needed for genome replication and progeny virion assembly. Viruses with dsRNA genomes face a particular challenge in that host cells do not produce proteins which can transcribe from a dsRNA template. Therefore, dsRNA viruses contain all of the necessary enzymatic machinery to synthesize complete mRNA transcripts within the core without the need for disassembly. Indeed one of the more striking observations about genome transcription in dsRNA viruses is that this process occurs efficiently only when the transcriptionally competent particle is fully intact. This observation suggests that all of the components of the TCP, including the viral genome, the transcription enzymes, and the viral capsid, function together to produce and release mRNA transcripts and that each component has a specific and critical role to play in promoting the efficiency of this process. This review has examined the process of genome transcription in dsRNA viruses from the perspective of rotavirus as a model system. However, despite numerous architectural and organizational differences among the families of dsRNA viruses, numerous studies suggest that the basic mechanism of mRNA production may be similar in most, if not all, viruses having dsRNA genomes. Important functional similarities include (1) the presence of a capsid-bound RNA-dependent RNA polymerase, which produces single-stranded mRNA transcripts from the dsRNA genome and regenerates the dsRNA genome from single-stranded RNA templates; (2) in viruses infecting eukaryotic hosts, the presence of all the enzymatic activities needed to generate the 5' cap required by the eukaryotic translation machinery; (3) the high degree of structural order present in the packaged genome, suggesting the requirement for organization in the viral core; (4) the role of the innermost capsid protein as a scaffold on which the core components of the transcription apparatus are assembled; and (5) the release of nascent mRNA transcripts through channels at the icosahedral vertices. The process of genome transcription in dsRNA viruses will become better understood as structural studies progress to higher resolution and as more viruses become amenable to study using site-directed mutagenesis coupled with viral reconstitution to generate recombinant particles having precise functional and structural changes. Future studies will dissect important intermolecular interactions required for efficient mRNA synthesis and will shed further light on the reasons for which the viral core must be structurally intact in order for transcription to occur efficiently. Structural studies of the capping enzymes at atomic resolution will reveal how multiple enzyme activities reside within a single polypeptide and how they act in concert to synthesize the 5' cap on the end of each mature transcript. Perhaps most interestingly, high resolution structural studies of actively transcribing virions will provide insight into the conformational changes that occur within the core during mRNA synthesis. Together, these studies will clarify the function of this complex macromolecular machine and will also shed additional light on the basic principles of virus architecture and assembly, as well as provide avenues for the design of antiviral therapies.

Comparative structural analysis of transcriptionally competent and incompetent rotavirus-antibody complexes

During genome transcription in rotavirus, as with many segmented double-stranded RNA viruses, mRNA is transcribed within the intact subviral particle and translocated through specific channels in the capsid. To understand how the conformation of the capsid affects the efficiency of transcriptional events in the viral core, we carried out a series of comparative structural and biochemical studies to characterize four different structural forms of the virus exhibiting differing transcriptional behavior. Two of these were virus-antibody complexes having contrasting transcriptional capabilities, and two were variant structural forms of the virus that exist during the life cycle and also exhibit contrasting transcriptional behavior. Three-dimensional structural studies using electron cryomicroscopy showed that the binding of one Fab (8H2/G5) does not affect the conformation of the capsid, and the efficiency of mRNA production is similar to that of the native subviral particle. The other Fab (2A11/E9) introduces conformational changes in the capsid similar to those seen in the transcriptionally incompetent mature particle. In both of the transcriptionally incompetent particle types, mRNA synthesis was arrested after limited elongation with the resulting oligonucleotide transcripts remaining trapped inside the particles. Our results indicate that the continuous translocation of nascent mRNA through the capsid is critical for efficient transcript elongation and that the blockage of translocation causes premature termination of transcription.

A novel 3'-end repair mechanism in an RNA virus

Many positive-stranded RNA viruses contain short, single-stranded 3' ends that are vulnerable to degradation by host cellular RNases. Therefore, the existence of a 3'-end repair mechanism (analogous to cellular telomerases) must be required and/or advantageous for RNA viruses. Accordingly, we provide evidence suggesting that deletions of up to 6 nt from the 3' end of satellite (sat-) RNA C (a small parasitic RNA associated with turnip crinkle carmovirus) are repaired to the wild-type sequence in vivo and in vitro. The novel 3'-end repair mechanism involves the production of 4-8 nt oligoribonucleotides by abortive synthesis by the viral replicase using the 3' end of the viral genomic RNA as template. Based on our in vitro results, we postulate that the oligoribonucleotides are able to prime synthesis of wild-type negative-strand sat-RNA C in a reaction that does not require base pairing of the oligoribonucleotides to the mutant, positive-strand RNA template. The discovery of a 3'-end repair mechanism opens up new strategies for interfering with viral infections.

Heterogeneous initiation due to slippage at the bacteriophage 82 late gene promoter in vitro

RNAs synthesized in vitro by purified Escherichia coli RNA polymerase from a bacteriophage 82 promoter are heterogeneous at the 5' end. We show that this heterogeneity results from variable addition of extra adenine residues, allowed by slippage of the initial oligonucleotide pppAAA-OH against its DNA template sequence TTT. Slippage backward by one base allows another A to be added, giving pppAAAA-OH, and this cycle can continue more than 20 times before it is ended by incorporation of UMP encoded by the fourth template base A. Slippage is abolished by mutation of the TTT template sequence to TGT and is sensitive to the concentrations of UTP and ATP in the reaction mixture. Analysis of deletions, substitutions, and point mutants implies that the slippage reaction requires only the existence of TTT at the initiation site of the template strand, although changes in neighboring nucleotides slightly affect its efficiency.

Studies on the mechanism of the antiviral activity of ribavirin against reovirus

We have examined the mechanism by which ribavirin inhibits the multiplication of reovirus. At a concentration of 12.5 microM (3 micrograms/ml) ribavirin inhibits viral multiplication, ssRNA formation, dsRNA formation, and protein synthesis by about 90%; when much higher concentrations are used for brief periods of time, the primary target of ribavirin is seen to be viral ssRNA synthesis. When the effect of ribavirin triphosphate (RTP) was tested on the in vitro transcription by cores of the dsRNA genome segments into plus-stranded RNA, elongation, that is, the formation of intact mRNA molecules, was found to be inhibited to the greatest extent; initiation was at least 2.5 times less sensitive, and cap formation and methylation were almost unaffected. The inhibition of elongation and initiation was not competitive with respect to any of the four nucleoside triphosphates. Remarkably, the transcription of plus strands into minus strands by immature reovirus particles (the replicase reaction) was insensitive to RTP. A model is proposed that envisages RTP binding to a site close to the catalytic site of the transcriptase. This binding is postulated to inhibit the helicase function of the transcriptase and lower its affinity for template RNA so that the likelihood of premature termination is greatly increased. The helicase activity is not, of course, necessary for the transcription of plus strands into minus strands, which would account for the differential sensitivity of the transcriptase and the replicase to RTP.

Interaction of RNA polymerase with lacUV5 promoter DNA during mRNA initiation and elongation. Footprinting, methylation, and rifampicin-sensitivity changes accompanying transcription initiation

We have used enzymatic and chemical probes to follow the movement of Escherichia coli RNA polymerase along lacUV5 promoter DNA during transcription initiation. The RNA polymerase does not escape from the promoter but remains tightly bound during the synthesis of the initial bases of the transcript. This initial phase of RNA synthesis involves the reiterative synthesis and release of RNA chains up to ten bases long via the RNA polymerase cycling reaction and the enzyme remains sensitive to rifampicin inhibition. When longer chains are made, promoter-specific binding is disrupted and the enzyme forms a rifampicin-resistant elongation complex with downstream DNA sequences. This elongation complex covers less than half as much DNA and lacks the DNase I-hypersensitive sites and the base-specific contacts that characterize promoter-bound RNA polymerase. These results lead us to suggest that lacUV5 mRNA synthesis is primed by a promoter-bound enzyme complex that synthesizes the initial nine or ten bases in the mRNA chain. Subsequently, when a chain of ten bases, or slightly longer, is made, contacts with promoter DNA are irreversibly disrupted, sigma subunit is lost, and a "true" elongation complex is formed.

Transcriptional regulation of three double-stranded RNA segments of bacteriophage phi 6 in vitro

Three double-stranded RNA segments of bacteriophage phi 6 (L, M, and S) were transcribed in vitro by a virion-associated RNA polymerase. Regulation of L transcription was distinct from regulation of M and S transcription. Transcription of the L segment, which codes for early proteins, required manganous ion and high concentrations of all four ribonucleoside triphosphates and was inhibited by polyamines such as spermine. Transcription of the M and S segments, which code for late proteins, required manganous or magnesium ion and relatively low concentrations of all ribonucleoside triphosphates except GTP and was enhanced by polyamines. Optimal conditions for L transcription were more stringent than those for M and S transcription. These two apparently different patterns produced in in vitro transcription presumably reflect the two distinct in vivo transcription patterns; i.e., (i) similar amounts of three single-stranded RNA species were transcribed from the three corresponding segments of double-stranded RNA (early pattern) and (ii) a much larger amount of single-stranded RNA species was transcribed from M and S segments than from the L segment (late pattern). The early transcription pattern may be changed into the late pattern by a change of environment, such as substrate concentration. This suggests that the different enzymatic properties under the different environmental conditions of the virion-associated transcriptase are responsible for the transcriptional regulation throughout the infection cycle of bacteriophage phi 6.