Sequence analysis of the polymerase L gene of human respiratory syncytial virus and predicted phylogeny of nonsegmented negative-strand viruses

Potent inhibition of respiratory syncytial virus replication using a 2-5A-antisense chimera targeted to signals within the virus genomic RNA

The 2-5A system is a recognized mechanistic component of the antiviral action of interferon. Interferon-induced 2-5A synthetase generates 2-5A, which, in turn, activates the latent constitutive RNase L that degrades viral RNA. Chemical conjugation of 2-5A to an antisense oligonucleotide can target the 2-5A-dependent RNase L to the antisense-specified RNA and effect its selective destruction. Such a 2-5A-antisense chimera (NIH351) has been developed that targets a consensus sequence within the respiratory syncytial virus (RSV) genomic RNA. NIH351 was 50- to 90-fold more potent against RSV strain A2 than was ribavirin, the presently approved drug for clinical management of RSV infection. It was similarly active against a variety of RSV strains of both A and B subgroups and possessed a cell culture selectivity index comparable to ribavirin. In addition, the anti-RSV activity of NIH351 was shown to be virus-specific and a result of a true antisense effect, because a scrambled nucleotide sequence in the antisense domain of NIH351 caused a significant decrease in antiviral activity. The 2-5A system's RNase L was implicated in the mechanism of action of NIH351 because a congener with a disabled 2-5A moiety was of greatly reduced anti-RSV effectiveness. These findings represent an innovative approach to the control of RSV replication.

Bovine respiratory syncytial virus

Since the first report of BRSV in the 1970s, the understanding of this agent and its respective disease has increased dramatically. Current evidence supports a major role for this virus in bovine respiratory disease. Advances in diagnostics have increased the ability to demonstrate this virus in field outbreaks of respiratory disease. The clinical signs and pathologic features have been well described, and vaccines are available to aid in prevention and control. Still, many questions remain to be answered with respect to BRSV. It appears there may be antigenic subgroups of BRSV, but the epidemiologic significance and relevance to immunization of this remains unknown. The question of differences in virulence among isolates of this virus has yet to be addressed. From an epidemiologic standpoint, the means by which BRSV perpetuates in the cattle population has yet to be elucidated. Although progress has been made in understanding the pathogenesis and immune response to BRSV, the mechanism of disease production and immune protection is incomplete. Lastly, efficacy testing of existing vaccines need to continue, as well as the development of new vaccines and new approaches to vaccination.

The temperature-sensitive (ts) phenotype of a cold-passaged (cp) live attenuated respiratory syncytial virus vaccine candidate, designated cpts530, results from a single amino acid substitution in the L protein

cpts530, a candidate live-virus vaccine, is an attenuated strain of human respiratory syncytial virus (RSV). It was derived by subjecting a cold-passaged (cp) strain of RSV to a single round of chemical mutagenesis. cpts530 is a temperature-sensitive (ts) mutant that is attenuated in mice and chimpanzees, and its ts phenotype exhibits a high level of stability during replication in both species. In the present study, the complete nucleotide sequence of cpts530 RSV was determined. The five mutations known to be present in the parent cpRSV were retained in its cpts530 derivative, and one additional nucleotide change was identified at nucleotide (nt) 10060, which resulted in a phenylalanine-to-leucine change at amino acid 521 in the large polymerase (L) protein. To determine if this single amino acid substitution was indeed responsible for the ts phenotype of cpts530, it was introduced alone or in combination with the cp mutations into the full-length cDNA clone of the wild-type A2 RSV. Analysis of infectious viruses recovered from mutant cDNAs indicated that this single mutation specified complete restriction of plaque formation of recombinant cp530 in HEp-2 cell monolayer cultures at 40 degrees C, and the level of temperature sensitivity was not influenced by the presence of the five cpRSV mutations. These findings identify the phenylalanine-to-leucine change at amino acid 521 in the L protein as the mutation that specifies the ts phenotype of cpts530. Furthermore, these findings illustrate the feasibility of using the cDNA-based recovery system to analyze and construct defined attenuated vaccine viruses.

Analysis of the large (L) protein gene of the porcine rubulavirus LPMV: identification of possible functional domains

The complete nucleotide sequence of the porcine rubulavirus LPMV (La Piedad Michoacan virus) large (L) protein gene was determined and analysed. The L mRNA was found to span 6,786 nucleotides, containing one single large open reading frame (ORF), putatively encoding a polypeptide of 2,251 amino acids. By aligning the amino acid sequence of the LPMV L-protein with L-protein of a number of viruses belonging to the order mononegavirale, a high degree of similarity between the LPMV L-protein and other rubula virus L-proteins was demonstrated, extending through almost the whole protein. Additionally we could identify several regions as being highly conserved among all studied viruses of the order mononegavirale. The significance of these regions are discussed.

Inhibition of respiratory syncytial virus replication by antisense oligodeoxyribonucleotides

Oligodeoxyribonucleotides targeted against respiratory syncytial virus (RSV) genomic RNA inhibited RSV replication in cell culture by an apparent antisense mechanism. HEp-2 cells were infected with RSV strain A2 and incubated in the presence of oligonucleotides. Virus replication was measured by enzyme-linked immunosorbent assay (ELISA), virus yield assay, or production of specific RSV mRNAs. Using ELISA, 50% effective concentration (EC50) values were about 0.5-1 microM for an antisense oligonucleotide targeted to the start of the NS2 gene. All oligonucleotides inhibited virus antigen production as measured by ELISA. In all assays, this antisense oligonucleotide was more potent than: (1) a control oligonucleotide containing the reverse sequence; (2) oligonucleotides targeted at RSV mRNA; (3) a random sequence oligonucleotide; and (4) ribavirin. Reverse transcriptase polymerase chain reaction (PT-PCR) showed sequence specific depletion of the genomic RNA target following treatment of cells with the antisense oligonucleotide. Specific cleavage of the genomic target RNA has been detected at the antisense oligonucleotide binding site, suggesting that cellular Rnase H participates in the reaction. These results indicate that antisense oligonucleotides targeted against RSV genomic RNA can effectively inhibit RSV replication and may have therapeutic value.

The complete genome structure and phylogenetic relationship of infectious hematopoietic necrosis virus

Infectious hematopoietic necrosis virus (IHNV), a member of the family Rhabdoviridae, causes a severe disease with high mortality in salmonid fish. The nucleotide sequence (11,131 bases) of the entire genome was determined for the pathogenic WRAC strain of IHNV from southern Idaho. This allowed detailed analysis of all 6 genes, the deduced amino acid sequences of their encoded proteins, and important control motifs including leader, trailer and gene junction regions. Sequence analysis revealed that the 6 virus genes are located along the genome in the 3' to 5' order: nucleocapsid (N), polymerase-associated phosphoprotein (P or M1), matrix protein (M or M2), surface glycoprotein (G), a unique non-virion protein (NV) and virus polymerase (L). The IHNV genome RNA was found to have highly complementary termini (15 of 16 nucleotides). The gene junction regions display the highly conserved sequence UCURUC(U)7RCCGUG(N)4CACR (in the vRNA sense), which includes the typical rhabdovirus transcription termination/polyadenylation signal and a novel putative transcription initiation signal. Phylogenetic analysis of M, G and L protein sequences allowed insights into the evolutionary and taxonomic relationship of rhabdoviruses of fish relative to those of insects or mammals, and a broader sense of the relationship of non-segmented negative-strand RNA viruses. Based on these data, a new genus, piscivirus, is proposed which will initially contain IHNV, viral hemorrhagic septicemia virus and Hirame rhabdovirus.

RNA replication by respiratory syncytial virus (RSV) is directed by the N, P, and L proteins; transcription also occurs under these conditions but requires RSV superinfection for efficient synthesis of full-length mRNA

Previously, a cDNA was constructed so that transcription by T7 RNA polymerase yielded a approximately 1-kb negative-sense analog of genomic RNA of human respiratory syncytial virus (RSV) containing the gene for chloramphenicol acetyltransferase (CAT) under the control of putative RSV transcription motifs and flanked by the RSV genomic termini. When transfected into RSV-infected cells, this minigenome was "rescued," as evidenced by high levels of CAT expression and the production of transmissible particles which propagated and expressed high levels of CAT expression during serial passage (P.L. Collins, M. A. Mink, and D. S. Stec, Proc. Natl. Acad. Sci. USA, 88:9663-9667, 1991). Here, this cDNA, together with a second one designed to yield an exact-copy positive-sense RSV-CAT RNA antigenome, were each modified to contain a self-cleaving hammerhead ribozyme for the generation of a nearly exact 3' end. Each cDNA was transfected into cells infected with a vaccinia virus recombinant expressing T7 RNA polymerase, together with plasmids encoding the RSV N, P, and L proteins, each under the control of a T7 promoter. When the plasmid-supplied template was the mini-antigenome, the minigenome was produced. When the plasmid-supplied template was the minigenome, the products were mini-antigenome, subgenomic polyadenylated mRNA and progeny minigenome. Identification of progeny minigenome made from the plasmid-supplied minigenome template indicates that the full RSV RNA replication cycle occurred. RNA synthesis required all three RSV proteins, N, P, and L, and was ablated completely by the substitution of Asn for Asp at position 989 in the L protein. Thus, the N, P, and L proteins were sufficient for the synthesis of correct minigenome and antigenome, but this was not the case for subgenomic mRNA, indicating that the requirements for RNA replication and transcription are not identical. Complementation with N, P, and L alone yielded an mRNA pattern containing a large fraction of molecules of incomplete, heterogeneous size. In contrast, complementation with RSV (supplying all of the RSV gene products) yielded a single discrete mRNA band. Superinfection with RSV of cells staging N/P/L-based RNA synthesis yielded the single discrete mRNA species. Some additional factor supplied by RSV superinfection appeared to be involved in transcription, the most obvious possibility being one or more additional RSV gene products.

Functional cDNA clones of the human respiratory syncytial (RS) virus N, P, and L proteins support replication of RS virus genomic RNA analogs and define minimal trans-acting requirements for RNA replication

The RNA-dependent RNA polymerase of human respiratory syncytial (RS) virus was expressed in a functional form from a cDNA clone. Coexpression of the viral polymerase (L) protein, phosphoprotein (P), and nucleocapsid (N) protein allowed us to develop a system for expression and recovery of replicable RS virus RNA entirely from cDNA clones. cDNA clones of the N, P, and L genes were constructed in pGEM-based expression plasmids and shown to direct expression of the appropriate polypeptides. Two types of RS virus genomic RNA analogs were expressed from an intracellular transcription plasmid that directed the synthesis of RNAs with defined 5' and 3' ends. One analog included the authentic 5' and 3' termini of the genome, and the second contained the authentic 5' terminus and its complement at the 3' terminus as found in copyback defective interfering RNAs of other negative-strand RNA viruses. Both types of genomic analogs were encapsidated and replicated in cells expressing the RS virus N, P, and L proteins. Omission of any of the three viral proteins abrogated replication, thereby defining the N, P, and L proteins as the minimal trans-acting proteins required for RNA replication. This system has the advantages that expression occurs at a level sufficient to allow direct biochemical analysis of the products of RNA replication and that neither the use of reporter genes nor wild-type RS helper virus is required. These features allow analysis of both cis- and trans-acting factors involved in the control of replication of RS virus RNA.

The complete nucleotide sequence of two cold-adapted, temperature-sensitive attenuated mutant vaccine viruses (cp12 and cp45) derived from the JS strain of human parainfluenza virus type 3 (PIV3)

Two cold-passaged mutant vaccine viruses (cp12 and cp45) derived from the JS wild-type (wt) strain of human parainfluenza virus type 3 (PIV3) have been sequenced. These mutant viruses display the cold-adapted (ca), temperature-sensitive (ts), and attenuation (att) phenotypes. Sequence data indicate that both cp12 and cp45 sustained nucleotide substitutions during cold passage and subsequent cloning. Fifteen nucleotide changes were present in cp12 and 18 in cp45. Of these changes, some were present in the sequence of the prototype wt strain (Wash/47885/57) or were non-coding changes present in the open reading frames (ORFs). These were considered unlikely to be of significance in contributing to phenotypic differences between the mutants and the JS wt. There were nine remaining changes in cp12 and eight in cp45 that would most likely contribute to their phenotypes. For cp12, two were non-coding changes in regulatory regions, one in the 3' genome leader and one in the NP gene transcription start signal. The remaining seven changes resulted in amino acid substitutions in NP, F, HN, and L. For cp45, two mutations were in a non-coding regulatory region, the 3' genome leader. The remaining six changes resulted in amino acid substitutions in F, HN, and L. Only one amino acid substitution was conserved between cp12 and cp45 (a valine to alanine change at position 384 of the HN gene). These results should prove useful in the future in understanding the genetic basis of attenuation of the cold-passaged PIV3 candidate vaccine viruses.

Sequence similarity between Borna disease virus p40 and a duplicated domain within the paramyxovirus and rhabdovirus polymerase proteins

We report the sequence of a Borna disease virus clone (pBDV-40) that encodes a 40-kDa protein (p40) found in the nuclei of infected cells. Comparative sequence analysis indicates that p40 is distantly similar to two different regions in the L-polymerase proteins encoded by paramyxoviruses and rhabdoviruses. The p40 sequence similarity indicates a previously undetected duplication in these viral polymerases. Phylogenetic reconstruction suggests that the gene that encodes p40 last shared a common ancestor with these viral polymerase genes prior to the duplication event. These findings support the hypothesis that Borna disease virus is a negative-strand RNA virus and suggest that p40 is involved in transcription and/or replication. The discovery of a duplication within the polymerase proteins of paramyxoviruses and rhabdoviruses has profound implications for the mapping of enzymatic activities within these multifunctional proteins.

Humanized animal viruses with special reference to the primate adaptation of morbillivirus

This review article discusses the evolution of human viruses with special reference to paramyxoviruses. This family of viruses causes epidemics representing the dissemination of infection from one acutely infected host to the next. Since there is no repository for human paramyxoviruses in animals or in the form of persistent infections in man, the history of epidemics afflicting human civilization is short, presumably not exceeding 4000-5000 years. Evolutionary relationships can be deduced for comparison of nucleotide sequences of genes or even complete genomes. The present paramyxovirus genus will probably in the future be divided into two separate genera. In the genus morbillivirus, two pairs of more closely related virus types can be distinguished: canine and phocid viruses, and rinder-pest and measles viruses, respectively. It is speculated that recombination events may have occurred in the evolution of the morbillivirus archetype.

Transcription of human respiratory syncytial virus genome RNA in vitro: requirement of cellular factor(s)

Extracts made from human respiratory syncytial virus (RSV)-infected Hep-2 cells synthesized mRNAs encoded by all known viral genes. In contrast, RSV ribonucleoproteins purified from infected cells failed to transcribe in vitro; transcription was restored by addition of a cytoplasmic extract of uninfected Hep-2 cells, demonstrating that a cellular factor(s) has a role in RSV gene expression. Quantitation of the individual gene mRNAs transcribed in vitro revealed polarity of transcription of the genome.

Structure of the L (polymerase) protein gene of sonchus yellow net virus

The complete nucleotide sequence of the L protein gene of sonchus yellow net virus (SYNV), a plant rhabdovirus, was determined by dideoxynucleotide sequencing of cloned cDNAs derived from the negative-strand genomic RNA. The L protein gene is composed of 6401 nucleotides (nt) located between positions 7158 and 13558 relative to the 3' end of the genomic RNA. Sequence analysis suggests that the complementary mRNA contains a 44 nt untranslated 5' leader sequence preceding an open reading frame of 6348 nucleotides that is capable of encoding a polypeptide of 2116 amino acids with a deduced molecular weight of 241,569 Da. The L protein is positively charged, has a high proportion of the amino acids Leu and Ile, and contains putative polymerase and RNA binding domains. Extended alignment of the SYNV L protein amino acid sequence with those of other nonsegmented negative-strand RNA virus polymerases reveals conservation of sequences within 12 blocks that appear sequentially along the protein. A cluster dendrogram derived from the L protein alignments indicates that SYNV is more closely related to animal rhabdoviruses than to the paramyxoviruses and that the animal rhabdoviruses have diverged less from each other than from SYNV.

Rapid sequencing of the Sendai virus 6.8 kb large (L) gene through primer walking with an automated DNA sequencer

The determination of the complete DNA sequence of the large (L) polymerase gene of Sendai virus strain Fushimi was used to explore the potential and feasibility of primer walking with fluorescent dye-labelled dideoxynucleotide terminators on an automated ABI DNA sequencer. The rapid identification of the complete sequence demonstrated that this approach is a time- and cost-saving alternative to classical sequencing techniques. Analysis of the data revealed that the L gene of Sendai virus strain Fushimi consists of exactly 6800 nucleotides and that the deduced amino acid sequence identifies a single open reading frame encoding a protein of 252.876 kDa. In contrast to Sendai virus strain Enders, the L mRNA of strain Fushimi is monocistronic. The comparison of the deduced amino acid sequences of the L genes of three different Sendai virus strains confirmed the existence of conserved as well as variable regions in the L protein and revealed a high grade of conservation in the carboxyterminal third. Furthermore, functional amino acid sequence motifs, like elements of RNA-dependent RNA polymerases and ATP-binding sites as postulated previously, were identified.

Gene junction sequences of bovine respiratory syncytial virus

The nucleotide sequences of seven gene junctions (N-P, P-M, M-SH, SH-G, G-F, F-M2 and M2-L) of bovine respiratory syncytial virus (BRSV) strain A51908 were determined by dideoxynucleotide sequencing of cDNAs from polytranscript mRNAs and from genomic RNA. By comparison with the consensus sequences derived from human respiratory syncytial virus (HRSV) mRNAs, gene-start and gene-end sequences were found in all BRSV mRNAs. There was a perfect match between the BRSV and HRSV in all gene-start sequences, except for the sequence of the SH gene which contained one nucleotide difference compared to HRSV A2; and the gene-start sequence of the L gene, which was one nucleotide shorter than the corresponding sequence of HRSV. Analysis of the intergenic regions showed a high degree of divergence in the nucleotide sequence between BRSV and HRSV. However, the length of the nucleotides in the intergenic sequences was similar for a given gene junction. As in the case of HRSV, the M2 and L genes of BRSV overlap by 68 nucleotides, suggesting a similar transcription attenuation mechanism. The sequences of the overlap, corresponding to the 3' end of the L gene, were almost identical between BRSV and HRSV.

Molecular cloning and sequence analysis of the mumps virus gene encoding the L protein and the trailer sequence

We have cloned and determined the nucleotide sequences of the seventh gene of the Miyahara strain of mumps virus (MuV) encoding the L protein. The L gene is 6925 nucleotides in length and contains a single long open reading frame which is capable of coding for a protein of 2261 amino acids with a calculated molecular weight of 256,571 Da. The deduced amino acid sequence of the L protein of MuV showed significant homology with those of six other paramyxoviruses, human parainfluenza type 2 virus, Newcastle disease virus, Sendai virus, measles virus, human parainfluenza type 3 virus, and human respiratory syncytial virus. The predicted MuV L protein contained distinct elements thought to be essential for RNA polymerase activity. A noncoding sequence of 24 nucleotides downstream of the presumed polyadenylation site of the L gene showed significant complementarity with the leader sequence composed of 55 nucleotide at the 3' end of the genomic RNA.

Marburg virus, a filovirus: messenger RNAs, gene order, and regulatory elements of the replication cycle

The genome of Marburg virus (MBG), a filovirus, is 19.1 kb in length and thus the largest one found with negative-strand RNA viruses. The gene order - 3' untranslated region-NP-VP35-VP40-GP-VP30-VP24-L-5' untranslated region-resembles that of other non-segmented negative-strand (NNS) RNA viruses. Six species of polyadenylated subgenomic RNAs, isolated from MBG-infected cells, are complementary to the negative-strand RNA genome. They can be translated in vitro into the known structural proteins NP, GP (non-glycosylated form), VP40, VP35, VP30 and VP24. At the gene boundaries conserved transcriptional start (3'-NNCUNCNUNUAAUU-5') and stop signals (3'-UAAUUCUUUUU-5') are located containing the highly conserved pentamer 3'-UAAUU-5'. Comparison with other NNS RNA viruses shows conservation primarily in the termination signals, whereas the start signals are more variable. The intergenic regions vary in length and nucleotide composition. All genes have relatively long 3' and 5' end non-coding regions. The putative 3' and 5' leader RNA sequences of the MBG genome resemble those of other NNS RNA viruses in length, conservation at the 3' and 5' ends, and in being complementary at their extremities. The data support the concept of a common taxonomic order Mononegavirales comprising the Filoviridae, Paramyxoviridae, and Rhabdoviridae families.

The nucleotide sequence of the L gene of Marburg virus, a filovirus: homologies with paramyxoviruses and rhabdoviruses

The nucleotide sequence of the L gene of Marburg virus, strain Musoke, has been determined. The L gene has a single long open reading frame encoding a polypeptide of 2330 amino acids (MW 267,175) that represents the viral RNA-dependent RNA polymerase. The putative transcription start signal (3'CUACCUAUAAUU 5') and the termination signal (3' UAAUUCUUUUU 5') of the gene could be identified. Computer-assisted comparison of the L protein with L proteins of other nonsegmented negative-stranded RNA viruses (Paramyxoviridae: Sendai virus, Newcastle disease virus, human parainfluenza 3 virus, measles virus, human respiratory syncytial virus; Rhabdoviridae: vesicular stomatitis virus, rabies virus) revealed significant homologies primarily in the N-terminal half of the proteins. We have identified three common conserved boxes (A, B, and C) among filo-, paramyxo-, and rhabdovirus L proteins, which are probably involved in the polymerase function. The L proteins can be divided into an N-terminal half, which seems to accommodate the common enzymatic sites, and a C-terminal half carrying virus specific peculiarities. The data presented here suggest a common evolutionary history for all nonsegmented negative-stranded RNA viruses and show that filoviruses are more closely related to paramyxo- than to rhabdoviruses.

RNA viruses: genome structure and evolution

The explosive pace of sequencing of RNA viruses is leading to rapid advances in our understanding of the evolution of these viruses and of the ways in which their genomes are organized and expressed. New insights are coming not only from genomic nucleotide sequence comparisons, but also from direct sequencing of transcribed mRNAs and of RNAs that serve as intermediates in replication.

Nucleotide sequences of the 3' leader and 5' trailer regions of human respiratory syncytial virus genomic RNA

The nucleotide sequences of the 3' extracistronic (leader) and 5' extracistronic (trailer) regions were determined for genomic RNA (vRNA) of human respiratory syncytial virus (RSV) strain A2. To sequence the 3' leader region, vRNA was extracted from purified virions, size-selected, polyadenylated, copied into cDNA, amplified by the polymerase chain reaction, cloned, and sequenced. The 3' leader sequence is 44 nt, which is somewhat shorter than its counterparts (50 to 70 nt) in other nonsegmented negative-strand viruses sequenced to date. The 5' trailer region was mapped and sequenced in part directly by dideoxynucleotide sequencing of vRNA. The sequence was confirmed and completed by analysis of cDNA clones derived from vRNA. The 5' trailer sequence is 155 nt in length, which is substantially longer than its counterparts (40 to 70 nt) in other nonsegmented negative-strand viruses. Ten of the 11 terminal nt of the 3' leader and 5' trailer regions were complementary. Among the other paramyxoviruses, the terminal 5 to 16 nt of the leader and trailer regions are highly conserved, but the corresponding RSV sequences were identical to the others only for the terminal 2 nt of each end. Surprisingly, the termini of the RSV leader and trailer regions were in somewhat better agreement with those of the rhabdoviruses vesicular stomatitis virus and rabies virus, sharing identity for the first 3 or 4 nt.

Rescue of synthetic analogs of respiratory syncytial virus genomic RNA and effect of truncations and mutations on the expression of a foreign reporter gene

The viral genomic RNA (vRNA) of human respiratory syncytial virus is a nonsegmented negative strand that is not infectious alone. To develop methods for complementing synthetic vRNA with viral proteins, a cDNA was constructed to encode a vRNA in which all of the viral protein-coding sequences were removed and replaced with a negative-sense copy of the bacterial chloramphenicol acetyltransferase gene. Upon transfection into respiratory syncytial virus-infected cells, the synthetic vRNA was "rescued" such that it was amplified, expressed, and packaged into infectious virions. A heterologous paramyxovirus, parainfluenza virus 3, was inactive in rescue. Further internal deletions mapped the cis-acting viral sequences required for rescue to two segments totaling 105 nucleotides (nt) derived from the two vRNA ends. Rescue was unaffected by replacement of the 44-nt 3'-terminal leader region with a 50-nt sequence that is complementary to the 5' terminus and represents the 3' end of the positive-sense replicative intermediate RNA. This 5'-end complement was related to the parental leader region only near the 3' terminus (91% or 73% identical for the first 11 or 22 nt, respectively). The addition of 11 heterologous nt to the 3' end of the parental leader region ablated rescue, suggesting that the 3'-proximal conserved domain is required and cannot function from an internal site. However, deletion of the 3'-terminal 3 nt, or a double transition at positions 4 and 5, had no effect on rescue. Thus, the 3'-terminal 5 nt, although conserved between 3' ends of the negative- and positive-sense RNAs, do not appear to be essential.