Sequence-specific binding of the influenza virus RNA polymerase to sequences located at the 5' ends of the viral RNAs

Interaction of influenza virus polymerase with viral RNA in the 'corkscrew' conformation

The influenza virus RNA (vRNA) promoter structure is known to consist of the 5'- and 3'-terminal sequences of the RNA, within very narrow boundaries of 16 and 15 nucleotides, respectively. A complete set of single nucleotide substitutions led to the previously proposed model of a binary hooked or 'corkscrew' conformation for the vRNA promoter when it interacts with the viral polymerase. This functional structure is confirmed here with a complete set of complementary double substitutions, of both the regular A:U and G:C type and also the G:U type of base-pair exchanges. The proposed structure consists of a six base-pair RNA rod in the distal element in conjunction with two stem-loop structures of two short-range base-pairs (positions 2-9; 3-8). These support an exposed tetranucleotide loop within each branch of the proximal element, in an overall oblique organization due to a central unpaired A residue at position 10 in the 5' sequence. Long-range base-pairing between the entire 5' and 3' branches, as required for an unmodified 'panhandle' model, has been excluded for the proximal element, while it is known to represent the mode of interaction within the distal element. A large number of short-range base-pair exchanges in the proximal element constitute promoter-up mutations, which show activities several times above that of the wild-type in reporter gene assays. The unique overall conformation and rather few invariant nucleotides appear to be the core elements in vRNA recognition by polymerase and also in viral ribonucleoprotein packaging, to allow discrimination against the background of other RNA molecules in the cell.

Transient bicistronic vRNA segments for indirect selection of recombinant influenza viruses

The 5'- and 3'-terminal regions of influenza vRNA molecules are known to constitute the promoter structure upon association with viral RNA polymerase in an activated complementary conformation. An inherent requirement for their location at the very ends of the vRNA molecules always has been implied because of that natural structure, but this study demonstrates that one or both of the promoter sequences may be relocated into vRNA-internal positions and still retain their polymerase-binding function. External extensions of vRNA molecules employed include either single-stranded RNA sequences </=750 nucleotides in length or complementary, and hence potentially double-stranded sequences, or promoter duplications. 5' RACE analyses of internally promoted cRNA and mRNA molecules prove initiation to occur at exactly the 3' standard template position 1, as defined by the regular promoter structure. Thereby any template extensions are lost from the resulting RNA molecules and progeny virions. These observations have been used to construct bicistronic vRNAs with an additional 3'-promoter sequence located between the two reading frames. During propagation, these spontaneously give rise to monocistronic vRNAs upon internal initiation reactions. Accordingly designed bicistronic vRNAs can be employed for indirectly selecting any foreign gene encoded in the resulting monocistronic vRNA for incorporation into recombinant influenza viruses.

Temperature-sensitive lesions in two influenza A viruses defective for replicative transcription disrupt RNA binding by the nucleoprotein

The negative-sense segmented RNA genome of influenza virus is transcribed into capped and polyadenylated mRNAs, as well as full-length replicative intermediates (cRNAs). The mechanism that regulates the two forms of transcription remains unclear, although several lines of evidence imply a role for the viral nucleoprotein (NP). In particular, temperature-shift and biochemical analyses of the temperature-sensitive viruses A/WSN/33 ts56 and A/FPV/Rostock/34/Giessen tsG81 containing point mutations within the NP coding region have indicated specific defects in replicative transcription at the nonpermissive temperature. To identify the functional defect, we introduced the relevant mutations into the NP of influenza virus strain A/PR/8/34. Both mutants were temperature sensitive for influenza virus gene expression in transient-transfection experiments but localized and accumulated normally in transfected cells. Similarly, the mutants retained the ability to self-associate and interact with the virus polymerase complex whether synthesized at the permissive or the nonpermissive temperatures. In contrast, the mutant NPs were defective for RNA binding when expressed at the nonpermissive temperature but not when expressed at 30 degrees C. This suggests that the RNA-binding activity of NP is required for replicative transcription.

Influenza A virus RNA polymerase has the ability to stutter at the polyadenylation site of a viral RNA template during RNA replication

The viral polymerase of influenza virus, a negative-strand RNA virus, is believed to polyadenylate the mRNAs by stuttering at a stretch of five to seven uridine residues which are located close to the 5' ends of the viral RNA templates. However, a mechanism of polyadenylation based on a template-independent synthesis of the poly(A) tail has not been excluded. In this report, we present new evidence showing the inherent ability of the viral polymerase to stutter at the poly(U) stretch of a viral RNA template during RNA replication. Variants which possess 1- to 13-nucleotide-long insertions at the poly(U) stretch have been identified. These results support a stuttering mechanism for the polyadenylation of influenza virus mRNAs.

Direct evidence that the poly(A) tail of influenza A virus mRNA is synthesized by reiterative copying of a U track in the virion RNA template

The poly(A) tail of influenza virus mRNA is thought to be synthesized by reiterative copying of the U track near the 5' end of the virion RNA template. This has been widely accepted as a plausible hypothesis, but until now there has been no direct experimental evidence for it. Here, we report such direct evidence based on the fact that (i) replacing the U track with an A track directs synthesis of products with poly(U) tails, both in vitro and in vivo, and (ii) interrupting the U track abolishes polyadenylation in vitro.

Rapid evolution of H5N1 influenza viruses in chickens in Hong Kong

The H5N1 avian influenza virus that killed 6 of 18 persons infected in Hong Kong in 1997 was transmitted directly from poultry to humans. Viral isolates from this outbreak may provide molecular clues to zoonotic transfer. Here we demonstrate that the H5N1 viruses circulating in poultry comprised two distinguishable phylogenetic lineages in all genes that were in very rapid evolution. When introduced into new hosts, influenza viruses usually undergo rapid alteration of their surface glycoproteins, especially in the hemagglutinin (HA). Surprisingly, these H5N1 isolates had a large proportion of amino acid changes in all gene products except in the HA. These viruses maybe reassortants each of whose HA gene is well adapted to domestic poultry while the rest of the genome arises from a different source. The consensus amino acid sequences of "internal" virion proteins reveal amino acids previously found in human strains. These human-specific amino acids may be important factors in zoonotic transmission.

A hairpin loop at the 5' end of influenza A virus virion RNA is required for synthesis of poly(A)+ mRNA in vitro

We present evidence, based on extensive mutagenesis, that a hairpin loop at the 5' end of influenza A virus virion RNA (vRNA) is required for the synthesis of polyadenylated mRNA from model vRNA templates in vitro. The hairpin loop, which we term the vRNA 5' hook, contains a stem of 2 bp formed by the second and third residues pairing with the ninth and eighth residues, respectively, and a 4-nucleotide loop composed of the intervening residues 4 to 7. Disruption of the base pairs of the vRNA 5' hook by introducing point mutations prevented polyadenylation, except in two mutants where a G-U base pair reformed. The polyadenylation activity of point mutants could be rescued by constructing double mutants with reformed base pairs in the stem of the vRNA 5' hook. These results suggest that base pairing rather than a particular nucleotide sequence was critical. We also show that mutation of the analogous region in the 3' arm of vRNA did not interfere with the synthesis of polyadenylated mRNA, suggesting that a hook structure in the 3' arm is not required for transcription of polyadenylated mRNA in vitro.

Mutational analysis of influenza A virus nucleoprotein: identification of mutations that affect RNA replication

The influenza A virus nucleoprotein (NP) is a multifunctional polypeptide which plays a pivotal role in virus replication. To get information on the domains and specific residues involved in the different NP activities, we describe here the preparation and characterization of 20 influenza A virus mutant NPs. The mutations, mostly single-amino-acid substitutions, were introduced in a cDNA copy of the A/Victoria/3/75 NP gene and, in most cases, affected residues located in regions that were highly conserved across the NPs of influenza A, B, and C viruses. The mutant NPs were characterized (i) in vivo (cell culture) by analyzing their intracellular localization and their functionality in replication, transcription, and expression of model RNA templates; and (ii) in vitro by analyzing their RNA-binding and sedimentation properties. The results obtained allowed us to identify both a mutant protein that accumulated in the cytoplasm and mutations that altered the functionality and/or the oligomerization state of the NP polypeptide. Among the mutations that reduced the NP capability to express chloramphenicol acetyltransferase protein from a model viral RNA (vRNA) template, some displayed a temperature-sensitive phenotype. Interestingly, four mutant NPs, which showed a reduced functionality in synthesizing cRNA molecules from a vRNA template, were fully competent to reconstitute complementary ribonucleoproteins (cRNPs) capable of synthesizing vRNAs, which in turn yielded mRNA molecules. Based on the phenotype of these mutants and on previously published observations, it is proposed that these mutant NPs have a reduced capability to interact with the polymerase complex and that this NP-polymerase interaction is responsible for making vRNPs switch from mRNA to cRNA synthesis.

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.

Characterization of influenza virus PB1 protein binding to viral RNA: two separate regions of the protein contribute to the interaction domain

The interaction of the PB1 subunit of the influenza virus polymerase with the viral RNA (vRNA) template has been studied in vitro. The experimental approach included the in vitro binding of labeled model vRNA to PB1 protein immobilized as an immunoprecipitate, as well as Northwestern analyses. The binding to model vRNA was specific, and an apparent Kd of about 2 x 10(-8) M was determined. Although interaction with the isolated 3' arm of the panhandle was detectable, interaction with the 5' arm was prominent and the binding was optimal with a panhandle analog structure (5'+3' probe). When presented with a panhandle analog mixed probe, PB1 was able to retain the 3' arm as efficiently as the 5' arm. The sequences of the PB1 protein involved in vRNA binding were identified by in vitro interaction tests with PB1 deletion mutants. Two separate regions of the PB1 protein sequence proved positive for binding: the N-terminal 83 amino acids and the C-proximal sequences located downstream of position 493. All mutants able to interact with model vRNA were capable of binding the 5' arm more efficiently than the 3' arm of the panhandle. Taken together, these results suggest that two separate regions of the PB1 protein constitute a vRNA binding site that interacts preferentially with the 5' arm of the panhandle structure.

Spatial perturbations within an RNA promoter specifically recognized by a viral RNA-dependent RNA polymerase (RdRp) reveal that RdRp can adjust its promoter binding sites

RNA synthesis during viral replication requires specific recognition of RNA promoters by the viral RNA-dependent RNA polymerase (RdRp). Four nucleotides (-17, -14, -13, and -11) within the brome mosaic virus (BMV) subgenomic core promoter are required for RNA synthesis by the BMV RdRp (R. W. Siegel et al., Proc. Natl. Acad. Sci. USA 94:11238-11243, 1997). The spatial requirements for these four nucleotides and the initiation (+1) cytidylate were examined in RNAs containing nucleotide insertions and deletions within the BMV subgenomic core promoter. Spatial perturbations between nucleotides -17 and -11 resulted in decreased RNA synthesis in vitro. However, synthesis was still dependent on the key nucleotides identified in the wild-type core promoter and the initiation cytidylate. In contrast, changes between nucleotides -11 and +1 had a less severe effect on RNA synthesis but resulted in RNA products initiated at alternative locations in addition to the +1 cytidylate. The results suggest a degree of flexibility in the recognition of the subgenomic promoter by the BMV RdRp and are compared with functional regions in other DNA and RNA promoters.

Sequences 5' of the conserved tRNA-like promoter modulate the initiation of minus-strand synthesis by the brome mosaic virus RNA-dependent RNA polymerase

Each of the brome mosaic virus (BMV) genomic RNAs contains a conserved tRNA-like structure that is sufficient to direct minus-strand RNA synthesis in vitro. The tRNA-like promoters, tB1 and tB3, direct approximately equal amounts of synthesis in vitro. However, 5' sequences were found to affect the amount of minus-strand synthesis, suggesting that sequences beyond the tRNA-like structure are important in moderating minus-strand synthesis. Consistent with this, sequences upstream the tRNA-like structure are able to partially suppress mutations at or near the initiation site. This activity is observed in the 5' sequences of both BMV and CMV (cucumber mosaic virus) templates. However, a chimeric RNA containing the CMV tRNA-like promoter fused to the 5' sequences of BMV was not able to suppress mutations at the initiation site, suggesting that homologous 5' and 3' sequences are required to affect initiation. The ability to suppress mutations at the initiation site was correlated with a slight increase in the ability of the BMV RNA-dependent RNA polymerase to interact with the RNA.

RNA-dependent activation of primer RNA production by influenza virus polymerase: different regions of the same protein subunit constitute the two required RNA-binding sites

The capped RNA primers required for the initiation of influenza virus mRNA synthesis are produced by the viral polymerase itself, which consists of three proteins PB1, PB2 and PA. Production of primers is activated only when the 5'- and 3'-terminal sequences of virion RNA (vRNA) bind sequentially to the polymerase, indicating that vRNA molecules function not only as templates for mRNA synthesis but also as essential cofactors which activate catalytic functions. Using thio U-substituted RNA and UV crosslinking, we demonstrate that the 5' and 3' sequences of vRNA bind to different amino acid sequences in the same protein subunit, the PB1 protein. Mutagenesis experiments proved that these two amino acid sequences constitute the functional RNA-binding sites. The 5' sequence of vRNA binds to an amino acid sequence centered around two arginine residues at positions 571 and 572, causing an allosteric alteration which activates two new functions of the polymerase complex. In addition to the PB2 protein subunit acquiring the ability to bind 5'-capped ends of RNAs, the PB1 protein itself acquires the ability to bind the 3' sequence of vRNA, via a ribonucleoprotein 1 (RNP1)-like motif, amino acids 249-256, which contains two phenylalanine residues required for binding. Binding to this site induces a second allosteric alteration which results in the activation of the endonuclease that produces the capped RNA primers needed for mRNA synthesis. Hence, the PB1 protein plays a central role in the catalytic activity of the viral polymerase, not only in the catalysis of RNA-chain elongation but also in the activation of the enzyme activities that produce capped RNA primers.

The RNA polymerase of influenza virus, bound to the 5' end of virion RNA, acts in cis to polyadenylate mRNA

We previously demonstrated, by limited mutagenesis, that conserved sequence elements within the 5' end of influenza virus virion RNA (vRNA) are required for the polyadenylation of mRNA in vitro. To further characterize the nucleotide residues at the 5' end of vRNA which might be involved in polyadenylation, a complete set of short and long model vRNA-like templates with mutations at nucleotides 1' to 13' (prime notation denotes numbering from the 5' end) of vRNA were synthesized and transcribed in vitro. The products were assayed for mRNA production with both reverse transcription-PCR and [alpha-32P]ATP incorporation assays. Results from these independent assays showed that vRNA templates with point mutations at positions 2', 3', 7' to 9', and 11' to 13' synthesized polyadenylated transcripts inefficiently compared with those with mutations at positions 1', 4' to 6', and 10'. Positions 2', 3', 7' to 9', and 11' are known to be involved in RNA polymerase binding. Furthermore, residues at positions 11' to 13' are known to be involved in base pairing between the 3' and 5' ends of vRNA. These findings demonstrate that the RNA polymerase has to bind to the 5' end of the template vRNA, which must then interact with the 3' end of the same template for polyadenylation to occur. These results support a model in which a cis-acting RNA polymerase is required for the polyadenylation of influenza virus.

Attenuation of influenza A virus mRNA levels by promoter mutations

We have engineered influenza A/WSN/33 viruses which have viral RNA (vRNA) segments with altered base pairs in the conserved double-stranded region of their vRNA promoters. The mutations were introduced into the segment coding for the neuraminidase (NA) by using a reverse genetics system. Two of the rescued viruses which share a C-G-->A-U double mutation at positions 11 and 12' at the 3' and 5' ends of the NA-specific vRNA, respectively, showed approximately a 10-fold reduction of NA levels. The mutations did not dramatically affect the NA-specific vRNA levels found in virions or the NA-specific vRNA and cRNA levels in infected cells. In contrast, there was a significant decrease in the steady-state levels of NA-specific mRNAs in infected cells. Transcription studies in vitro with ribonucleoprotein complexes isolated from the two transfectant viruses indicated that transcription initiation of the NA-specific segment was not affected. However, the majority of NA-specific transcripts lacked poly(A) tails, suggesting that mutations in the double-stranded region of the influenza virus vRNA promoter can attenuate polyadenylation of mRNA molecules. This is the first time that a promoter mutation in an engineered influenza virus has shown a differential effect on influenza virus RNA transcription and replication.

Cellular factors in the transcription and replication of viral RNA genomes: a parallel to DNA-dependent RNA transcription

Viral RNA replication and transcription involves not only viral RNA-dependent RNA polymerases, but also cellular proteins, the majority of which are subverted from the RNA-processing or translation machineries of host cells. These factors interact with viral RNA or polymerases to form transcription or replication ribonucleoprotein complexes and may provide template specificity for RNA-dependent RNA synthesis, suggesting a close parallel to the mechanism of DNA-dependent RNA synthesis. The types of cellular proteins involved and their modes of action are reviewed.

Polyadenylation of influenza virus mRNA transcribed in vitro from model virion RNA templates: requirement for 5' conserved sequences

Here we report the development of two independent assays which demonstrate for the first time that exogenous model RNA templates based on influenza virus virion RNA (vRNA) are transcribed in vitro to produce polyadenylated mRNA. We investigated the activities of mutated templates with known polymerase binding properties to test our model that polyadenylation occurs when a polymerase complex, which is bound to conserved 5' sequences of vRNA, prevents read-through of the U track at which polyadenylation subsequently occurs by reiterative copying. Mutated templates with perturbed polymerase binding sites (i.e., a deletion mutant lacking the first 4 5' residues and a U-->A point mutant at the third residue) initiated transcription in the in vitro assay but failed to produce polyadenylated transcripts, whereas an A-->U point mutant at the fourth residue, which retained polymerase binding properties similar to those of the wild type, produced polyadenylated transcripts. Our results show that nucleotides within the conserved 5' sequence are required for polyadenylation and support the hypothesis that polymerase binding to 5' sequences of the template is required for mRNA synthesis.

A functional antigenomic promoter for the paramyxovirus simian virus 5 requires proper spacing between an essential internal segment and the 3' terminus

A previous analysis of naturally occurring defective interfering (DI) RNA genomes of the prototypic paramyxovirus simian virus 5 (SV5) indicated that 113 bases at the 3' terminus of the antigenome were sufficient to direct RNA encapsidation and replication. A nucleotide sequence alignment of the antigenomic 3'-terminal 113 bases of members of the Rubulavirus genus of the Paramyxoviridae family identified two regions of sequence identity: bases 1 to 19 at the 3' terminus (conserved region I [CRI]) and a more distal region consisting of antigenome bases 73 to 90 (CRII) that was contained within the 3' coding region of the L protein gene. To determine whether these regions of the antigenome were essential for SV5 RNA replication, a reverse genetics system was used to analyze the replication of copyback DI RNA analogs that contained a foreign gene (GL, encoding green fluorescence protein) flanked by 113 5'-terminal bases and various amounts of SV5 3'-terminal antigenomic sequences. Results from a deletion analysis showed that efficient encapsidation and replication of SV5-GL DI RNA analogs occurred when the 90 3'-terminal bases of the SV5 antigenomic RNA were retained, but replication was reduced approximately 5- to 14-fold in the case of truncated antigenomes that lacked the 3'-end CRII sequences. A chimeric copyback DI RNA containing the 3'-terminal 98 bases including the CRI and CRII sequences from the human parainfluenza virus type 2 (HPIV2) antigenome in place of the corresponding SV5 sequences was efficiently replicated by SV5 cDNA-derived components. However, replication was reduced approximately 20-fold for a truncated SV5-HPIV2 chimeric RNA that lacked the HPIV2 CRII sequences between antigenome bases 72 and 90. Progressive deletions of 6 to 18 bases in the region located between the SV5 antigenomic CRI and CRII segments (3'-end nucleotides 21 to 38) resulted in a approximately 25-fold decrease in SV5-GL RNA synthesis. Surprisingly, replication was restored to wild-type levels when these length alterations between CRI and CRII were corrected by replacing the deleted bases with nonviral sequences. Together, these data suggest that a functional SV5 antigenomic promoter requires proper spacing between an essential internal region and the 3' terminus. A model is presented for the structure of the 3' end of the SV5 antigenome which proposes that positioning of CRI and CRII along the same face of the helical nucleocapsid is an essential feature of a functional antigenomic promoter.

Cell culture adaptation of Puumala hantavirus changes the infectivity for its natural reservoir, Clethrionomys glareolus, and leads to accumulation of mutants with altered genomic RNA S segment

This paper reports the establishment of a model for hantavirus host adaptation. Wild-type (wt) (bank vole-passaged) and Vero E6 cell-cultured variants of Puumala virus strain Kazan were analyzed for their virologic and genetic properties. The wt variant was well adapted for reproduction in bank voles but not in cell culture, while the Vero E6 strains replicated to much higher efficiency in cell culture but did not reproducibly infect bank voles. Comparison of the consensus sequences of the respective viral genomes revealed no differences in the coding region of the S gene. However, the noncoding regions of the S gene were found to be different at positions 26 and 1577. In one additional and independent adaptation experiment, all analyzed cDNA clones from the Vero E6-adapted variant were found to carry substitutions at position 1580 of the S segment, just 3 nucleotides downstream of the mutation observed in the first adaptation. No differences were found in the consensus sequences of the entire M segments from the wt and the Vero E6-adapted variants. The results indicated different impacts of the S and the M genomic segments for the adaptation process and selective advantages for the variants that carried altered noncoding sequences of the S segment. We conclude that the isolation in cell culture resulted in a phenotypically and genotypically altered hantavirus.

Sequence-specific recognition of a subgenomic RNA promoter by a viral RNA polymerase

RNA templates of 33 nucleotides containing the brome mosaic virus (BMV) core subgenomic promoter were used to determine the promoter elements recognized by the BMV RNA-dependent RNA polymerase (RdRp) to initiate RNA synthesis. Nucleotides at positions -17, -14, -13, and -11 relative to the subgenomic initiation site must be maintained for interaction with the RdRp. Changes to every other nucleotide at these four positions allow predictions for the base-specific functional groups required for RdRp recognition. RdRp contact of the nucleotide at position -17 was suggested with a template competition assay. Comparison of the BMV subgenomic promoter to those from other plant and animal alphaviruses shows a remarkable degree of conservation of the nucleotides required for BMV subgenomic RNA synthesis. We show that the RdRp of the plant-infecting BMV is capable of accurately, albeit inefficiently, initiating RNA synthesis from the subgenomic promoter of the animal-infecting Semliki Forest virus. The sequence-specific recognition of RNA by the BMV RdRp is analogous to the recognition of DNA promoters by DNA-dependent RNA polymerases.

Sequence analysis of the Puumala hantavirus Sotkamo strain L segment

Sequence of the Puumala virus (PUU) Sotkamo strain L segment is provided, completing the total genome of this prototype PUU virus strain. The L segment is 6530 nucleotides long and it can encode 2156 amino-acids-long L protein, RNA-dependent RNA polymerase. The strain Sotkamo, originally isolated in Finland, showed for the L genome segment nucleotide (84.6%) and amino acid (97.3%) homology to a previously sequenced PUU Russian isolate, strain Bashkiria/CG1820 (B1820) and the L genome segment appeared to be at least as conserved as the S segment. Phylogenetic analysis based on the S, M and L segment sequences proposes that the three viral genes have a similar evolutionary history with no evidence for genome segment reassortment. Precise sequencing of the L segment termini demonstrated that the Puumala strains differ from the conserved sequences of the other hantaviruses at two positions.

Differential effect of modified capped RNA substrates on influenza virus transcription

The RNA-dependent RNA polymerase of influenza virus transcribes messenger RNA through a unique cap scavenging mechanism. Viral enzyme binds to the cap structure of host mRNA, cleaves the molecule 9-15 bases downstream of the cap, and uses the short capped oligonucleotide as a primer for mRNA synthesis. Previously, we have shown that the viral polymerase can efficiently bind capped RNAs shorter than 9 nucleotides in length, but the viral enzyme can not utilize these RNAs as primers. For this reason, these short capped oligonucleotides are potent inhibitors of influenza virus transcription. In these studies, it is now shown that short capped oligomers inhibit capped-RNA dependent transcription at the initial step of cap binding. In contrast, low concentrations of these short capped RNAs can actually stimulate viral transcription primed with high concentrations of the dinucleotide ApG. Another capped RNA derivative containing phosphorothioate oligonucleotides was also investigated as a potential polymerase inhibitor. This longer capped RNA was able to bind to the polymerase, but could not be cleaved to primer length by the enzyme associated endonuclease. Thus, the capped phosphorothioate RNA inhibited cap-primed transcription at the step of cap binding. However, in contrast to the short capped oligonucleotide, it also inhibited ApG primed viral transcription.

Stable protein-RNA interaction involves the terminal domains of bluetongue virus mRNA, but not the terminally conserved sequences

The interaction of bluetongue virus (BTV) proteins with viral RNA was investigated in vitro by means of a biochemical approach. By subjecting cytoplasmic extracts from virus-infected baby hamster kidney cells and in vitro synthesized radiolabeled RNA to ultraviolet cross-linking assays, we demonstrated that, of all the BTV proteins, NS2 becomes most intimately associated with the labeled viral RNA. Competition binding studies indicated that NS2 has the greatest affinity for the 3' region of the viral transcripts. By analyzing the binding efficiency of NS2 to mutant RNA transcripts which lacked the fully conserved 5'- and/or 3'-terminal hexanucleotides, we have established that these sequences are not necessary for optimal binding. The specificity of the NS2-RNA interaction was investigated by competition experiments with unlabeled BTV-specific homologous and heterologous competitor RNAs as well as with viral double-stranded RNA (dsRNA). Although apparent differences in the ability of NS2 to bind to the different RNA transcripts were observed, it did not bind to the dsRNA.

The influenza A virus PB2 polymerase subunit is required for the replication of viral RNA

The transcription and replication of influenza virus RNA (vRNA) were reconstituted in vivo. The experimental approach involved the transfection of plasmids encoding the viral subunits of the polymerase and the nucleoprotein into cells infected with a vaccinia virus recombinant virus expressing the T7 RNA polymerase. As templates, one of two model RNAs was transfected: vNSZ or cNSZ RNA. The RNAs were 240 nucleotides in length, contained the terminal sequences of the NS viral segment, and were of negative or positive polarity, respectively. The accumulation of cRNA and mRNA in cells transfected with vNSZ RNA and the accumulation of vRNA and mRNA in cells transfected with cNSZ RNA were determined by RNase protection assays with labeled vNSZ-L or cNSZ-L probes. The patterns of protected bands obtained indicated that both cRNA replication intermediate and mRNA accumulated when the system was reconstituted with vNSZ RNA. Likewise, both vRNA and mRNA accumulated after reconstitution with cNSZ RNA. The reconstitution of incomplete systems in which any of the subunits of the polymerase or the model RNA were omitted was completely negative for the accumulation of cRNA or vRNA, indicating that the presence of the PB2 subunit in the polymerase is required for replication of vRNA.

Secondary structure of the panhandle RNA of influenza virus A studied by NMR spectroscopy

The double-stranded panhandle structure of the influenza virus RNA is important for replication, transcription and packaging into the virion of the virion RNA. The solution structure of a 34 nt RNA which contains the conserved panhandle sequences has been investigated by one- and two-dimensional NMR spectroscopy. The partially complementary 5'- and 3'-ends of the RNA form a double helical structure which is, on average, close to A-form. The stem contains bulges at nucleotides A10, A12 and C26. In between these bulges, C11 and G25 form a Watson-Crick base pair. The structural features of the panhandle provide a framework for the explanation of mutational analysis and for a better understanding of RNA-polymerase interactions.

Influenza virus polymerase basic protein 1 interacts with influenza virus polymerase basic protein 2 at multiple sites

Three polymerase proteins of influenza type A virus interact with each other to form the active polymerase complex. Polymerase basic protein 1 (PB1) can interact with PB2 in the presence or absence of polymerase acidic protein. In this study, we investigated the domains of PB1 involved in complex formation with PB2 in vivo, using coexpression and coimmunoprecipitation of the PB1-PB2 complex with monospecific antibodies. Results show that PB1 possesses at least two regions which can interact independently and form stable complexes with PB2. Both of these regions are located at the NH2 terminus of PB1; the COOH-terminal half of PB1 is not involved in interacting with PB2. Deletion analysis further demonstrated that the interacting regions of PB1 encompass amino acids (aa) 48 to 145 and aa 251 to 321. Linker insertions throughout the PB1 sequences did not affect complex formation with PB2. Deletion and linker-insertion mutants of PB1 were tested for polymerase activity in vivo. For this analysis, we developed a simplified assay for viral polymerase activity that uses a reporter chloramphenicol acetyltransferase gene containing the 5' and 3' ends of influenza viral promoter and nontranslating regions (minus sense) of the NS gene joined to a hepatitis delta virus ribozyme at its 3' end. This assay demonstrated that all deletion mutants of PB1 exhibited either background or greatly reduced polymerase activity irrespective of the ability to interact with PB2 and that all linker-insertion mutants except one at the extreme COOH end (L-746) of PB1 were also negative for viral polymerase activity. These results show that compared with complex formation of PB1 with PB2, the polymerase activity of PB1 was extremely sensitive to structural perturbation.

Mutations in the nonconserved noncoding sequences of the influenza A virus segments affect viral vRNA formation

Influenza A virus replication and packaging is mediated by cis-acting signals, which are located at the 3' and the 5' end of the viral segments. The terminal residues can be divided into conserved and nonconserved residues. We have constructed a mutant influenza A/WSN/33 virus, which contains multiple mutations in the nonconserved residues of the neuraminidase (NA) segment. This virus shows a segment-specific reduction of the genomic RNA content in the infected cell and in the progeny virus. Further mutants and revertant viruses revealed that it was not possible to define specific residues, which were responsible for the reduction of the NA-specific RNA. Thus, it appears that an efficient vRNA formation is dependent on the synergistic effect of the terminal sequences.

Influenza A virus RNA-dependent RNA polymerase: analysis of RNA synthesis in vitro

Influenza A virus RNA-dependent RNA polymerase, purified from virion ribonucleoprotein particles and from which endogenous genomic RNA (vRNA) has been depleted by treatment with micrococcal nuclease, was used to study transcription initiation, elongation, and termination in vitro. Templates that contained either minus- or plus-sense influenza virus nucleoprotein minigenes with conserved 5' and 3' termini and the uridylate tract were constructed. The dinucleotide ApG and alfalfa mosaic virus RNA4 (AlMV4) were used as primers. ApG primed the synthesis of full-length positive-strand or cRNA products and shorter transcripts, depending upon the molar ratio between the nucleoprotein and the vRNA template. Sequence analysis of the ends of these transcripts demonstrated that the 5' termini of both transcripts and the 3' terminus of the full-length product were complementary to the 3' and 5' termini of the vRNA template, respectively, whereas the 3' terminus of the incomplete product corresponded to a sequence located 40 bases downstream from the 5' terminus of the template and was about 20 nucleotides downstream from the uridylate tract, which is the putative signal for polyadenylation. Binding of the cap structure of AlMV4 by the polymerase activated RNA synthesis by ligation-elongation of small genomic RNA fragments which were likely derived from a genome segment protected by the polymerase from micrococcal nuclease digestion. The sequence of these fragments mapped to a region 14 to 28 nucleotides upstream of the 3' terminus of the viral genome. Polymerase subunit involvement in transcription initiation with ApG or AlMV4 was characterized by studying the effect of purified polyclonal antisubunit immunoglobulins of the G class (IgGs) in transcription assays. These results showed that anti-PB2 IgG inhibited transcription initiation in both ApG- and AlMV4-primed reactions, whereas anti-PB1 antibodies also blocked transcription initiated with AlMV4. The differences observed in product size, product sequence, and differential inhibition by antisubunit IgGs are discussed. These observations would support the notion that the influenza virus RNA-dependent RNA polymerase undergoes a conformational change after the binding of the cap structure of host cell heterogeneous nuclear RNA by PB2, which then usually leads to endonucleolytic cleavage of the capped primer 13 nucleotides downstream from the cap.

Mutational analysis of influenza B virus RNA transcription in vitro

The roles of the 3'- and 5'-terminal nucleotides and the panhandle structure of influenza B virus virion RNA were analyzed in vitro by transcription of model RNA templates with influenza B virus RNA polymerase. The results suggest that the stability of the panhandle and breathing of the extreme ends of the panhandle are important factors for efficient transcription. Influenza B virus polymerase appears to be more tolerant of mutations in the panhandle structure than influenza A virus polymerase. This is consistent with the greater degree of heterogeneity observed naturally in the 3'-terminal nucleotides of the virion RNA of influenza B virus than in influenza A virus.

Complete genetic characterization and analysis of isolation of Sin Nombre virus

This study reports completion of the genetic characterization of the entire genome of Sin Nombre (SN) virus (NMH10) detected in autopsy tissues from a patient who died of hantavirus pulmonary syndrome (HPS). The large (L) genome segment was found to be 6,562 nucleotides in length and encoded a putative L polymerase that was 2,153 amino acids in length. No evidence of segment reassortment with other well-characterized hantaviruses was obtained. The sequence of the entire S, M, and L genome segments of SN virus (strain NMR11) isolated from a mouse (trapped in the residence of the patient infected with SN virus [NMH10]) by passage two times in Peromyscus maniculatus and then by five passages in E6 Vero cells was determined and compared with that of the virus detected in autopsy tissues. Only 16 nucleotide differences were detected between the virus genomes, and none of these resulted in virus protein amino acid substitutions. Determination of the exact 5'- and 3'-terminal sequences of all genome segments of SN virus and representatives of other serologic groups in the Hantavirus genus, family Bunyaviridae, showed the existence of conserved nucleotide domains that may be involved in important regulatory mechanisms, such as RNA encapsidation, polymerase binding, and control of transcription and replication.

Genetic characterization of a human isolate of Puumala hantavirus from France

PUU90-13 is a strain of Puumala (PUU) virus (family Bunyaviridae: genus Hantavirus) isolated from a human in northeastern France (Rollin et al., 1995). This report describes the full-length sequences of the small (S) and medium (M) genomic RNAs of PUU90-13. The terminal sequences of both the S and M genomic RNAs were found to be conserved and imperfectly complementary. The S RNA of PUU90-13 is 1847 nt in length and contains the nucleocapsid (N) protein gene and a potential overlapping open reading frame (ORF-2) previously described in other hantaviruses. Statistical analysis of the third base substitution frequency in the N ORFs of PUU90-13 and other PUU viruses suggests that the ORF-2 is functional. The M RNA is 3681 nt in length and encodes the glycoprotein precursor. Both genomic segments share the highest degree of nucleotide and amino acid sequence identity with PUUBerkel, a PUU virus from Germany. Phylogenetic analyses of sequences from both segments indicate that PUU90-13 occupies a distinct Western European PUU virus lineage that it shares with PUUBerkel. Both PUU90-13 and PUUBerkel lack a potential N-linked glycosylation site found on the G2 glycoprotein of other PUU viruses.

The choice of alternative 5' splice sites in influenza virus M1 mRNA is regulated by the viral polymerase complex

The influenza virus M1 mRNA has two alternative 5' splice sites: a distal 5' splice site producing mRNA3 that has the coding potential for 9 amino acids and a proximal 5' splice site producing M2 mRNA encoding the essential M2 ion-channel protein. Only mRNA3 was made in uninfected cells transfected with DNA expressing M1 mRNA. Similarly, using nuclear extracts from uninfected cells, in vitro splicing of M1 mRNA yielded only mRNA3. Only when the mRNA3 5' splice site was inactivated by mutation was M2 mRNA made in uninfected cells and in uninfected cell extracts. In influenza virus-infected cells, M2 mRNA was made, but only after a delay, suggesting that newly synthesized viral gene product(s) were needed to activate the M2 5' splice site. We present strong evidence that these gene products are the complex of the three polymerase proteins, the same complex that functions in the transcription and replication of the viral genome. Gel shift experiments showed that the viral polymerase complex bound to the 5' end of the viral M1 mRNA in a sequence-specific and cap-dependent manner. During in vitro splicing catalyzed by uninfected cell extracts, the binding of the viral polymerase complex blocked the mRNA3 5' splice site, resulting in the switch to the M2 mRNA 5' splice site and the production of M2 mRNA.

Differential activation of the influenza virus polymerase via template RNA binding

Primary transcripts synthesized by the influenza virus polymerase contain the capped 5' ends of eukaryotic mRNAs. These sequences are derived from host mRNA and scavenged by the viral polymerase as a prerequisite to transcription. The first step in this reaction is the specific binding of the viral polymerase to the cap structure of the host RNA. The role that template RNA plays in this RNA binding reaction was examined in quantitative capped mRNA binding and endonuclease assays. Capped RNA binding was shown to be a template-dependent property of the influenza virus polymerase. Addition of only the 5' end of viral RNA stimulates capped mRNA binding by the viral polymerase, but endonuclease activity requires the addition of the 3' end. The addition of template RNA corresponding to the positive-sense complementary RNA replicative intermediate was also able to stimulate capped mRNA binding but was not able to efficiently activate the viral endonuclease. Thus, regulation of endonuclease activity by the influenza virus polymerase can be dependent on template RNA binding.

Characterization of the RNA-fork model of virion RNA in the initiation of transcription in influenza A virus

It has been shown that both 3' and 5' conserved termini of influenza A virus virion RNA are involved in the initiation of transcription. An RNA-fork model has been proposed, according to which there is a crucial double-stranded region formed by complementary bases at positions 10 to 12 of the 3' terminus and bases at positions 11' to 13' of the 5' terminus, which are extended by 2 or 3 segment-specific base pairs. The two termini at positions 1 to 9 and 1' to 10' in the 3' and 5' termini, respectively, are in a single-stranded conformation. Here we further characterize this model, focusing on the individual roles of the proposed duplex region and the proposed two single-stranded ends. Residues within the conserved 5' terminus that are involved in the initiation of transcription were determined. Single, double, and triple mutations in the proposed duplex region provided further evidence that, for the initiation of transcription in vitro, the duplex RNA is more important than the actual sequence of these residues, although some restrictions in sequence were apparent. On the other hand, there was evidence that base pairing is not required at residues 1 to 7. We propose that the 5' terminus of virion RNA should be treated as an integral part of the virion RNA promoter and discuss a possible mechanism for the recognition of the virion RNA promoter by the influenza A virus RNA polymerase complex.

Identification of an RNA binding region within the N-terminal third of the influenza A virus nucleoprotein

The influenza A virus nucleoprotein (NP) has been examined with regard to its RNA-binding characteristics. NP, purified from virions and devoid of RNA, bound synthetic RNAs in vitro and interacted with the ribonucleotide homopolymers poly(A), poly(G), poly(U), and poly(C) in a salt-dependent manner, showing higher binding affinity for polypyrimidine homopolymers. To map the NP regions involved in RNA binding, a series of deleted forms of the NP were prepared, and these truncated polypeptides were tested for their ability to bind poly(U) and poly(C) homopolymers linked to agarose beads. Proteins containing deletions at the N terminus of the NP molecule showed reduced RNA-binding activity, indicating that this part of the protein was required to bind RNA. To identify the NP region or regions which directly interact with RNA, proteins having the maltose-binding protein fused with various NP fragments were obtained and tested for binding to radioactively labeled RNAs in three different assays: (i) nitrocellulose filter binding assays, (ii) gel shift assays, and (iii) UV light-induced cross-linking experiments. A maltose-binding protein fusion containing the N-terminal 180 amino acids of NP behaved as an RNA-binding protein in the three assays, demonstrating that the N terminus of NP can directly interact with RNA. This NP region could be further subdivided into two smaller regions (amino acids 1 to 77 and 79 to 180) that also retained RNA-binding activity.