The Sendai paramyxovirus accessory C proteins inhibit viral genome amplification in a promoter-specific fashion

Inhibition of Sendai virus genome replication due to promoter-increased selectivity: a possible role for the accessory C proteins

The role of the negative-stranded virus accessory C proteins is difficult to assess because they appear sometimes as nonessential and thereby of no function. On the other hand, when a function is found, as in the case of Sendai virus, it represents an enigma, in that the C proteins inhibit replication under conditions where the infection follows an exponential course. Furthermore, this inhibitory function is exerted differentially: in contrast to the replication of internal deletion defective interfering (DI) RNAs, that of copy-back DI RNAs appears to escape inhibition, under certain experimental conditions (in vivo assay). In a reexamination of the C effect by the reverse genetics approach, it was found that copy-back RNA replication is inhibited by C in vivo as well, under conditions where the ratio of C to copy-back template is increased. This effect can be reversed by an increase in P but not L protein. The "rule of six" was differentially observed in the presence or absence of C. Finally, a difference in the ability of the replicating complex to tolerate promoter modifications in RNA synthesis initiation was shown to occur in the presence or the absence of C as well. We propose that C acts by increasing the selectivity of the replicating complex for the promoter cis-acting elements governing its activity. The inhibitory effect of C becomes the price to pay for this increased selectivity.

A point mutation in the Sendai virus accessory C proteins attenuates virulence for mice, but not virus growth in cell culture

A mutant Sendai virus (SevMVC), which grows much better than its progenitor virus (SeVM) in cell culture, but, in strong contrast to SeVM, is totally avirulent for mice, has been described. SeVMVC contains two amino acid substitutions relative to SeVM, namely, F170S in the C protein and E2050A in the L protein. We have examined which substitutions were responsible for the above phenotypes by exchanging the C gene of our reference strain Z with those of SeVH (another reference strain), SeVM, and SeVMVC, in turn. We have found that the F170S mutation in the CMVC protein is responsible both for enhanced replication in cell culture and for avirulence in mice. Avirulence appeared to be due to restricted viral replication primarily after day 1, implicating some aspect of innate immunity in this process. The SeV C proteins thus appear to be required for multiple cycles of replication in mice.

The Sendai virus C protein binds the L polymerase protein to inhibit viral RNA synthesis

The Sendai virus nested set of C proteins which are expressed in an alternative open reading frame from the P mRNA has been shown to downregulate viral RNA synthesis. Utilizing a glutathione S-transferase (gst) C fusion protein (gstC), we have shown that C protein forms a complex with the L, but not the P, subunit of the viral RNA polymerase. When P, L, and gstC are coexpressed, an oligomer of P, through its interaction with L, is also bound to beads. Since binding of C to L in the P-L complex does not disrupt P binding, the C and P binding sites appear to be different. GstC binding to L occurs only when the proteins are coexpressed in the same cell. The gstC, but not gst, protein inhibits viral transcription in vitro, showing that the fusion protein retains biological function. Pulse-chase experiments of the various complexes show that L protein synthesized alone has a half-life of 1. 2 hr, which is increased 12.5-fold by binding P, but is not significantly increased by binding gstC. Analyses of complex formation with truncations of L protein show that the C-terminal 1333 amino acids of L are not required for binding C. The dose-response curves show that replication of the genomic DI-H RNA is more sensitive to inhibition by C protein than is the synthesis of DI leader RNA, suggesting that the downregulation of RNA synthesis may be more complex than just the inhibition of the initiation of RNA synthesis.

Minimum protein requirements for transcription and RNA replication of a minigenome of human parainfluenza virus type 3 and evaluation of the rule of six

A reconstituted transcription and RNA replication system for human parainfluenza virus type 3 (HPIV3) was developed using components expressed intracellularly from transfected plasmids driven by T7 RNA polymerase supplied by a vaccinia virus recombinant. The system is based on a negative-sense analog of HPIV3 genomic RNA in which the viral genes were deleted and replaced with that encoding bacterial chloramphenicol acetyl transferase (CAT). The N, P, and L proteins expressed from cotransfected plasmids were necessary and sufficient to direct efficient transcription and RNA replication. Transcription yielded subgenomic polyadenylated mRNA, which was isolated by oligo(dT) chromatography. RNA replication yielded a mini-antigenome and progeny minigenome, which were shown to be encapsidated based on resistance to digestion with micrococcal nuclease. A panel of cDNAs was constructed to encode minigenomes which differed in length by single-nucleotide increments. Transcription and RNA replication in the reconstituted system were most efficient for the minigenome whose length was an even multiple of six. Both RNA replication and transcription appeared to be governed by the rule. However, minigenomes whose lengths were one nucleotide greater than or less than an even multiple of six also were very active, especially in RNA replication, indicating that the rule was not absolute.

Genome nucleotide lengths that are divisible by six are not essential but enhance replication of defective interfering RNAs of the paramyxovirus simian virus 5

For some members of the Paramyxoviridae family of negative strand RNA viruses, efficient genome replication only occurs when the total genome length is a multiple of six (6N length, where N is any integer). To determine if this "rule of six" requirement applied to the replication of the prototype paramyxovirus simian virus 5 (SV5), defective interfering (DI) RNA genomes were generated by sequential undiluted passage of virus in tissue culture. Molecular cloning and nucleotide sequence analysis of 10 RNA genomes revealed a series of copyback DI RNAs with chain lengths between 449 and 1365 bases, but only 4 of the 10 naturally occurring RNA genomes were of 6N length. Many of the cloned DI genomes could be grouped into two distinct nested sets, with the members of each set having the same polymerase crossover junctions and extent of terminal complementarity but differing from each other by internal deletions. One of these nested sets of genomes consisted of novel DI RNAs that contained a pentameric stretch of nontemplated adenosine residues inserted precisely at the polymerase crossover junction. A reverse genetics system was established in which SV5 DI genomes were replicated in vivo entirely by cDNA-derived components. Using this system, two naturally occurring SV5 DI RNAs were examined in a mutational analysis to determine the role of genome length on SV5 RNA replication. The progressive insertion of one to six nucleotides into a 6N length DI genome (852 bases) resulted in a reduction in replication for RNAs that contained one to four additional bases (approximately 35-50% of WT levels), followed by an increase back to WT replication levels for genomes that were altered by five and six base insertions (approximately 70 and 100% of WT levels, respectively). An insertion of five nucleotides into a second non-6N length DI RNA (499 total bases) created a genome length that was a multiple of six (504 bases) and led to a approximately 10-fold stimulation of replication over that of the unaltered genome. Together, these results indicate that there was a clear influence of 6N genome length on SV5 DI RNA replication, but the stringency of this replication requirement appeared to be less than that found previously for other paramyxoviruses. This work completes the testing of the rule of six replication requirement for representatives of each of the four genera of the Paramyxoviridae family and indicates that the preference for replication of 6N length RNA genomes varies between the individual paramyxoviruses.

Normal cellular replication of Sendai virus without the trans-frame, nonstructural V protein

The Sendai virus V protein is a nonstructural trans-frame protein in which a highly conserved cys-rich Zn2+-binding domain is fused to the N-terminal half of the P protein via mRNA editing. Using a recently developed system in which infectious virus is recovered from cDNA, we have engineered a virus in which a translation stop codon was placed at the beginning of the V ORF. Translation of the V(stop) mRNA yields a W-like protein, i.e., a protein composed of the N-terminal half of the P protein alone which is naturally expressed at low levels from the P gene. This V-minus but W-augmented virus was found to replicate normally in cell culture and embryonated chicken eggs. The Sendai virus V protein is thus an accessory protein, and the cys-rich Zn2+-binding domain is likely to function in a specialized role during virus propagation.

The paramyxovirus, Sendai virus, V protein encodes a luxury function required for viral pathogenesis

The Sendai virus (SeV) V protein is characterized by the unique cysteine-rich domain in its carboxy-terminal half which is fused to the amino-terminal half of the P protein, but its function has remained enigmatic. The V protein-directing mRNA is generated by a remarkable process known as mRNA editing involving the pseudotemplated addition of a single G residue at a specific septinucleotide locus in the P gene, whereas the unedited exact copy encodes the P protein. Here, we introduced two nucleotide changes in the septinucleotide motif (UUUUCCC to UUCUUCC) in a full-length SeV cDNA and were able to recover a virus from the cDNA, which was devoid of mRNA editing and hence unable to synthesize the V protein. Compared with the parental wild-type virus with regard to gene expression, replication and cytopathogenicity in various cell lines in vitro, the V(-) virus was found to be either potentiated or comparable but never attenuated. The V(-) virus, however, showed markedly attenuated in vivo replication capacity in and pathogenicity for mice. Thus, though categorized as a nonessential gene product, SeV V protein encodes a luxury function required for in vivo pathogenicity.