Sendai virus RNA synthesis and nucleocapsid formation in the presence of cycloheximide

Role of the virus nucleoprotein in the regulation of lymphocytic choriomeningitis virus transcription and RNA replication

The prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) has a bisegmented negative-strand RNA genome. Each segment carries two viral genes in opposite orientation and separated by an intergenic region (IGR). The RNA-dependent RNA polymerase (RdRp) L of LCMV produces subgenomic mRNA and full-length genomic and antigenomic RNA species in two different processes termed transcription and replication, respectively. It is widely accepted that intracellular nucleoprotein (NP) levels regulate these two processes. Intracellular NP levels increase during the course of the infection, resulting in the unfolding of secondary RNA structures within the IGR. Structure-dependent transcription termination at the IGR is thereby attenuated, promoting replication of genome and antigenome RNA species. To test this hypothesis, we established a helper-virus-free minigenome (MG) system where intracellular synthesis of an S segment analogue from a plasmid is driven by RNA polymerase I. Cotransfection with two additional plasmids expressing the minimal viral trans-acting factors L and NP under control of RNA polymerase II allowed for RNA synthesis mediated by the intracellularly reconstituted LCMV polymerase. Both processes, transcription and replication, were strictly dependent on NP. However, both were equally enhanced by incrementally increasing amounts of NP up to levels in the range of those in LCMV-infected cells. Our data are consistent with a central role for NP in transcription and replication of the LCMV genome, but they do not support the participation of NP levels in balancing the two processes.

Increased expression of the N protein of respiratory syncytial virus stimulates minigenome replication but does not alter the balance between the synthesis of mRNA and antigenome

A popular model for RNA synthesis by nonsegmented negative-strand RNA viruses is that transcription and RNA replication are executed by the same polymerase complex and that there is a dynamic balance between the two processes that is mediated by the nucleocapsid N protein. According to this model, transcription occurs until sufficient soluble N protein accumulates to initiate encapsidation of the nascent RNA product, which somehow switches the polymerase into a readthrough replicative mode. This model was examined for respiratory syncytial virus (RSV) using a reconstituted transcription and RNA replication system that involves a minireplicon and viral proteins that are expressed intracellularly from transfected plasmids. Preliminary experiments showed that reconstituted RNA replication was highly productive, such that on average each molecule of plasmid-supplied minigenome that became encapsidated was amplified 10- to 50-fold. N protein was increased on its own or in concert with the phosphoprotein P and in the presence or absence of the M2 ORF1 transcription elongation factor. The maximum level of N and P protein expression achieved from plasmids equalled or exceeded that obtained in RSV-infected cells. Increased levels of N protein stimulated RNA replication. This is consistent with the idea that RNA replication is dependent on the availability of N protein for encapsidation, which is one postulate of the model. The M2 ORF1 protein had no detectable effect on RNA replication under the various conditions of expression of N and P, which confirmed and extended previous results. However, there was no evidence of a significant switch in positive-sense RNA synthesis from transcription (synthesis of mRNAs) to RNA replication (synthesis of antigenome). The synthesis of positive-sense antigenome and mRNA appeared to occur at a fixed ratio, with mRNA being by far the more abundant product.

Gene expression of nonsegmented negative strand RNA viruses

Nonsegmented negative strand RNA viruses comprise major human and animal pathogens in nature. This class of viruses is ubiquitous and infects vertebrates, invertebrates, and plants. Our laboratory has been working on the gene expression of two prototype nonsegmented negative strand RNA viruses, vesicular stomatitis virus (a rhabdovirus) and human parainfluenza virus 3 (a paramyxovirus). An RNA-dependent RNA polymerase (L and P protein) is packaged within the virion which faithfully copies the genome RNA in vitro and in vivo; this enzyme complex, in association with the nucleocapsid protein (N), is also involved in the replication process. In this review, we have presented up-to-date information of the structure and function of the RNA polymerases of these two viruses, the mechanisms of transcription and replication, and the role of host proteins in the life-cycle of the viruses. These detailed studies have led us to a better understanding of the roles of viral and cellular proteins in the viral gene expression.

In vitro replication of Sendai virus wild-type and defective interfering particle genome RNAs

A system for studying the in vitro replication of the genome RNAs of Sendai virus and its defective interfering particle DI-H has been developed. Cytoplasmic extracts of baby hamster kidney cells infected with wild-type Sendai virus or coinfected with wild-type Sendai virus plus DI-H were prepared after lysolecithin treatment at 12 h postinfection. The extracts supported the transcription of six viral mRNAs as well as the replication of the Sendai virus 50S (wild-type) and 14S DI-H genome RNAs and their encapsidation into nucleocapsids in the absence of de novo protein synthesis. RNA replication in vitro represented more than 50% of total RNA synthesis, a relative level higher than that found in the infected cell. The proteins required for Sendai virus RNA replication were present in a soluble protein pool at the time of extract preparation. Depletion of the protein pool by prior treatment of infected cells with cycloheximide inhibited subsequent in vitro genome replication without affecting transcription. The cytoplasmic extract may be separated by high-speed centrifugation into two components: the Sendai virus wild-type and DI-H nucleocapsid templates containing the RNA and associated NP, L, and P proteins and the soluble protein fraction containing primarily the P, NP, and M viral proteins with trace amounts of the L, HN, Fo, and nonstructural C proteins. The isolated intracellular DI-H nucleocapsid template alone cannot replicate its RNA, but when recombined with the Sendai virus soluble protein fraction it catalyzes the replication and encapsidation of viral RNAs. The initiation of RNA replication in vitro can be demonstrated because detergent-disrupted purified DI-H virions replicate both positive- and negative-strand RNAs in the presence, but not in the absence, of the soluble protein fraction from an extract of infected cells.

Characterization of La Crosse virus small-genome transcripts

With restriction fragments from DNA clones of the La Crosse virus S genome segment, the 3' end of the S mRNA was located by S1 nuclease mapping near a polyuridine tract, approximately 100 nucleotides, from the end of the S genome. Genome replication in La Crosse virus-infected cells was abolished by the drug cycloheximide, similar to other negative-strand RNA viruses. However, the synthesis of S mRNA could not be detected in cells pretreated with cycloheximide, suggesting that ongoing protein synthesis is required for La Crosse virus genome transcription and replication. Primer extension experiments in the presence of chain-terminating nucleoside triphosphates demonstrated that the 5' end of the La Crosse virus S mRNA begins 10 to 14 nucleotides before the 3' end of the S genome segment, suggesting that the La Crosse virus S mRNA is initiated on a host primer. A hypothesis consistent with these unexpected findings is presented.

Identification of four complementary RNA species in Akabane virus-infected cells

The analysis of RNA extracted from purified Akabane virus demonstrated the presence of three size classes of single-stranded RNAs with sedimentation coefficients of 31S (large, L), 26S (medium, M), and 13S (small, S). Molecular weights of these RNA species were estimated to be 2.15 X 10(6), 1.5 X 10(6), and 0.48 X 10(6) for the L, M, and S RNAs, respectively. Hybridization analysis involving viral genomic RNA and RNA from virus-infected cells resulted in the identification of four virus-specific cRNA species in infected cells. These cRNAs were found to be nonpolyadenylated by their inability to bind to oligodeoxythymidylate-cellulose. Kinetic analysis of cRNA synthesis in infected cells at various times postinfection suggested that cRNA synthesis could be detected as early as 2 h postinfection and that maximal synthesis occurred at 4 to 6 h postinfection. The RNAs synthesized in infected cells could be partially resolved by sucrose density gradient centrifugation. The RNA fraction that cosedimented with the S segment of viral genomic RNA yielded two duplex RNA species when hybridized with viral genomic RNA, suggesting the presence of two small cRNA species. Specific hybridization with individual viral genomic RNAs confirmed that two species of cRNA are coded by the S RNA segment. Analysis of cRNA synthesis in the presence of the protein synthesis inhibitors cycloheximide and puromycin indicated that cycloheximide completely inhibited virus-specific RNA synthesis early and late in infection, whereas a very low level of synthesis occurred in the presence of puromycin. The inhibitory effects of these drugs were found to be reversible when the drugs were washed from the cells. It is concluded that continued protein synthesis is required for cRNA synthesis to proceed in Akabane virus-infected cells.

RNA synthesis by Newcastle disease virus temperature-sensitive mutants in two RNA-negative complementation groups

The temperature-sensitive RNA-negative mutants of Newcastle disease virus comprise two complementation groups, group A (seven members) and group E (one member). The RNA-synthesizing activities of four representative members of group A and the single member of group E were compared with the activity of the wild type. These mutants were defective to varying extents in primary transcription at the nonpermissive temperature, ranging from mutant A1, which had no activity, to mutant E1, which lost only 50% of its activity. All of the mutants were also defective in a postprimary transcriptive process since after preincubation at the permissive temperature in the presence of cycloheximide, there was no subsequent RNA synthesis at the nonpermissive temperature upon removal of the cycloheximide. Similarly, in experiments in which cycloheximide was not used, shifts from the permissive temperature to the nonpermissive temperature before 3 h postinfection did not result in RNA synthesis. However, later shifts to the nonpermissive temperature did allow RNA synthesis. With the exception of mutant A1, all of the mutants maintained this RNA-synthetic ability for at least 3 h, suggesting that RNA synthesis from progeny genomes was not the major postprimary transcriptive defect in these mutants. In contrast, the RNA-synthetic ability of mutant A1 rapidly decayed at the nonpermissive temperature, suggesting that the A gene product is involved in RNA synthesis from progeny genomes. The postprimary transcriptive defect(s) of the other mutants may be in the processing or stability of a protein, in the processing of mRNA, or in replication. Plaque-forming revertants (ts+) of all of the mutants coreverted for RNA synthesis. This finding strengthens the relationship between temperature sensitivity for plaquing and both the primary and postprimary RNA-negative phenotypes.

Plus and minus strand leader RNAs in negative strand virus-infected cells

Sendai virus and VSV minus strand genome RNAs, labeled specifically at their 3' ends with RNA ligase, were used as probes to detect leader RNA--that is, short transcripts (approximately 50 nucleotides) complementary to the exact 3' end of the minus strand genome. These probes have allowed the detection of plus strand leader RNAs in both Sendai virus and VSV-infected cells as well as in the virion transcriptase reactions. The use of a similar probe, prepared from the self-complementary ends of DI genome RNA and containing the 3' end of the plus strand antigenome RNA, has allowed the detection of a minus strand leader RNA of identical size in VSV-infected cells. Since the presence of DI genomes could not be detected by analytical sucrose gradient centrifugation in these VSV-infected cells, this minus strand leader RNA is apparently synthesized on the template formed by the exact 3' end of the antigenome RNA.

Effect of cordycepin (3'-deoxyadenosine) on virus-specific RNA species synthesized in Newcastle disease virus-infected cells

Cordycepin (3'-deoxyadenosine) has no effect on the size or relative proportions of Newcastle disease virus-specific 18-22S mRNA species nor on the amount or size of the polyadenylic acid associated with them. Cordycepin does, however, cause an inhibition of incorporation of [3H]uridine into 50S virus-specific RNA relative to 18-22S RNA. This inhibition is probably not a direct effect of the drug on the synthesis of 50S viral RNA. Like cycloheximide, another drug which inhibits 50S RNA accumulation in paramyxovirus-infected cells, cordycepin inhibits protein synthesis as measured by amino acid incorporation. It is likely that the inhibition of 50S RNA accumulation is a secondary effect of protein synthesis inhibition. This is supported by the finding that concentrations of cordycepin and cycloheximide, which inhibit protein synthesis to the same extent, have the same effect on the ratio of 50 to 18-22S virus-specific RNA.

Relationships among the polypeptides of Newcastle disease virus

We have studied the relationships among the polypeptides of Newcastle disease virus by using both kinetic and tryptic peptide analyses. The results of our tryptic peptide analyses suggest that there are at least six unique viral polypeptides--L, HN, FO(F), NP, M, and a 47,000-dalton polypeptide. The small virion glycopolypeptide F is related to FO, a glycopolypeptide found only in infected cells. In addition, several smaller polypeptides, including a 53,000-dalton polypeptide found both in purified virions and in infected cells, are related to the nucleocaspid protein. Kinetic analysis of each viral polypeptide reveals that all of the major viral polypeptides, with the possible exception of L, are stable after an amino acid chase. A precursor-product relationship between FO and F was not demonstrable by pulse-chase experiments. Also, almost the same relative amount of F, the putative product, was present in infected cultures after either 5 or 30 min of radioisotopic labeling. These results suggest that FO is processed rapidly.

Methylation of messenger RNA of Newcastle disease virus in vitro by a virion-associated enzyme

Purified Newcastle disease virus contains an enzyme that incorporates the methyl group from S-adenosyl-L-methionine into RNA synthesized in vitro by the virion-associated RNA polymerase (RNA nucleotidyltransferase). Incorporation of radioactivity from S-adenosyl-L-[methyl-3H]methionine was totally dependent upon RNA synthesis. The methylation reaction was completely inhibited by S-adenosyl-L-homocysteine, suggesting the transfer of only the methyl group of S-adenosyl-methionine to RNA products. Velocity sedimentation and hybridization of the in vitro product RNA indicated that both [3H]methyl and [32P]GMP labels resided in single-stranded 18S RNA molecules which were virus specific. Approximately 1 to 2 methyl groups were incorporated per RNA molecule. DEAE-cellulose chromatography of product RNA after alkaline hydrolysis suggested that the 5' terminus was the site of methylation.

Replication of measles virus: continued synthesis of nucleocapsid RNA and increased synthesis of mRNA in the presence of cycloheximide

The effect of cycloheximide on virus specific RNA synthesis in Vero cells infected with either wild-strain (Edmonston) or subacute sclerosing panencephalitis strain measles virus was investigated. At 3 days postinfection, cells treated with cycloheximide (2.6 x 10(-4) M) and then exposed to [(3)H]uridine showed a marked increase in labeled virus-specific RNA. A major portion of this incremental labeled RNA was putative viral mRNA which sedimented at 16, 22, and 30S. Five distinct classes of polyribosomes, which were not evident in untreated cells, were found in cycloheximide-treated cells and each contained similar species of virus-specific RNA. Viral nucleocapsid RNA, 50 and 18S, was synthesized and encapsidated in the presence of cycloheximide. The latter observation is in apparent contrast to reports that cycloheximide inhibits replication of RNA of classical paramyxoviruses, and may indicate that mechanisms for replicating RNA of measles virus are different from those for replicating RNA of paramyxoviruses.

Effects of L-glutamine deprivation on growth of HVJ (Sendai virus) in BHK cells

l-Glutamine requirement for viral maturation was found in BHK-HVJ cells, a cell line of baby hamster kidney cells persistently infected with HVJ (Sendai virus). Synthesis of envelope protein in BHK-HVJ cells was markedly suppressed by deprivation of l-glutamine, whereas development of nucleocapsid (S) antigen was less affected. More detailed examination of this phenomenon was carried out by using a cytolytic system. Growth of HVJ in BHK cells cultured in media deprived of various amino acids was investigated, and omission of l-glutamine from culture medium resulted in a marked inhibitory effect on the release of infectious virus and synthesis of envelope protein, although synthesis of virus-specific RNA and nucleocapsid antigen in the cells was readily detected. When l-glutamine was restored to the culture medium, infectious virus and envelope protein could be detected. l-Glutamic acid, l-aspartic acid, or l-alanine could be substituted for l-glutamine. Effects of l-glutamine deprivation on HVJ growth in several other cells were also investigated. The growth of HVJ in the cells other than BHK and FL cells was not suppressed by lack of l-glutamine. Growth of Sindbis virus in BHK cells was also markedly retarded in the absence of l-glutamine.

RNA synthesis of vesicular stomatitis virus. V. Interactions between transcription and replication

The temperature-sensitive mutant ts114 of vesicular stomatitis virus is temperature-sensitive in both primary and secondary transcription, but not in the replication of 40S RNA. The synthesis of 40S RNA is specifically inhibited when protein synthesis is shut off. The addition of cycloheximide to cells infected by ts114, rapidly inhibits RNA replication at the permissive and nonpermissive temperatures. However, the addition of cycloheximide at the nonpermissive temperature also results in almost complete recovery of transcription to the level found at the permissive temperature. Other inhibitors of protein synthesis applied at the nonpermissive temperature result in the same recoverability of the apparent temperature-sensitive lesion. Ribonucleic acid products synthesized by ts114 at the nonpermissive temperature in the presence of cycloheximide are identical to virus-specific messenger RNA by the criteria of size and complementarity to virion RNA. When mutant-infected cells are shifted to the nonpermissive temperature, incubated for a period of time, and then treated with cycloheximide, the ratio of transcriptive to replicative activity varies depending on the time at which radioactive precursor is added to cells. This suggests an interdependence between replication and transcription during virus-specific RNA synthesis.

Cell-free translation of paramyxovirus messenger RNA

Polypeptides corresponding in electrophoretic mobility to virion polypeptides 1, 3, and 5 were made in a reticulocyte cell-free system to which 18S RNA from Sendai virus-infected cells was added. Immune precipitation was used to select relevant polypeptides from endogenous products. The cell-free product corresponding to virion polypeptide 3 (the nucleocapsid structure unit) was the most abundant; its tryptic peptides comigrated electrophoretically with tryptic peptides of polypeptide 3 isolated from virions. Other sedimenting classes of RNA from infected cells were tested; only the 28S fraction showed slight activity. Virion 50S RNA was inactive. These findings support the hypothesis that complementary RNA transcripts of paramyxovirion RNA are the templates for viral proteins.

Separation of the messenger RNAs of Newcastle disease virus by gel electrophoresis

We have separated the 18-22S putative messenger RNA of Newcastle disease virus into seven species ranging in molecular weight from 0.55 to 1.53 x 10(6) using sodium dodecyl sulfate-acrylamide-gel electrophoresis at relatively high concentrations of acrylamide and for a relatively long time. Studies of the number and molecular weights of the proteins and the 18-22S RNAs of the virus suggests that these RNAs are in the right molecular weight range to code for the known proteins of Newcastle disease virus. In preliminary studies using this separation technique, we have demonstrated that: (a) there is no difference between the 18-22S RNA made during a normal infection and when genome replication is blocked; and (b) there is a strain-specific difference between the RNAs of Newcastle disease virus-AV and Newcastle disease virus-HP.

Measles virus ribonucleic acid and protein synthesis: effects of 6-azauridine and cycloheximide on viral replication

Cycloheximide and 6-azauridine were employed to study the time course of measles virus protein and nucleic acid syntheses in AV3 cells. Synthesis of ribonucleic acid (RNA) essential for infectivity was first detected at 6 hr and increased concurrently with the formation of essential protein. Maximum levels of virus-specific RNA and protein were present by 18 hr, a time when only 5% of progeny virus was detected. Essential RNA and protein syntheses preceded the formation of infectious virus by at least 10 to 12 hr. The time course of RNA and protein syntheses essential for the formation of complement-fixing (CF) antigen and salt-dependent agglutinin (SDA) was also determined. RNA synthesis essential for the formation of SDA was first detected at 2 hr and was present maximally by 6 hr, whereas SDA-protein increased concurrently with the protein essential for infectivity. This suggested that the last protein essential for infectivity may be SDA. RNA synthesis essential for the formation of CF antigen was first detected at 4 hr, while CF-protein increased at 5 hr and preceded SDA-protein and protein essential for infectivity by approximately 3 hr. Reversal of inhibition of protein synthesis by cycloheximide indicated that early protein synthesis (1 to 3 hr) was required for the formation of infectious virus. The data suggest that the relatively long eclipse period observed with measles virus is related to a long maturation period rather than to late formation of early proteins, viral RNA, or structural proteins.

Ribonucleic acid synthesis of vesicular stomatitis virus. IV. Transcription by standard virus in the presence of defective interfering particles

Exposure of vesicular stomatitis virus-infected Chinese hamster ovary cells to cycloheximide results in the complete transcription of virion ribonucleic acid (RNA) into only 28S and 13 to 15S viral-specific RNA species. These RNA are identical to viral messenger RNA by the following criteria: size, single-strandedness, complementarity to virion RNA, and formation of messenger ribonucleoproteins. This transcription represents the intracellular enzymatic activity of the virion-associated polymerase and is shown to be dependent on input multiplicity. Intracellular transcription differs from in vitro polymerase activity in having a temperature optimum of 34 to 37 C and in synthesizing 28S as well as 13 to 15S messenger RNA species. Addition of interfering quantities of defective T particles to these cycloheximide-treated cells, either an hour before or at the same time as standard B particles of vesicular stomatitis virus, does not alter the rate of transcription nor does it change the sucrose gradient pattern of the viral RNA species. From these results it is concluded that the RNA of defective T particles does not serve a transcriptive function and probably interferes through the replicative mechanism for virion RNA synthesis.