Analysis of arbovirus ribonucleic acid forms by polyacrylamide gel electrophoresis

De-Coding the Contributions of the Viral RNAs to Alphaviral Pathogenesis

Alphaviruses are positive-sense RNA arboviruses that are capable of causing severe disease in otherwise healthy individuals. There are many aspects of viral infection that determine pathogenesis and major efforts regarding the identification and characterization of virulence determinants have largely focused on the roles of the nonstructural and structural proteins. Nonetheless, the viral RNAs of the alphaviruses themselves play important roles in regard to virulence and pathogenesis. In particular, many sequences and secondary structures within the viral RNAs play an important part in the development of disease and may be considered important determinants of virulence. In this review article, we summarize the known RNA-based virulence traits and host:RNA interactions that influence alphaviral pathogenesis for each of the viral RNA species produced during infection. Overall, the viral RNAs produced during infection are important contributors to alphaviral pathogenesis and more research is needed to fully understand how each RNA species impacts the host response to infection as well as the development of disease.

An in vitro assay to study chikungunya virus RNA synthesis and the mode of action of inhibitors

Chikungunya virus (CHIKV) is a re-emerging mosquito-borne alphavirus that causes severe persistent arthralgia. To better understand the molecular details of CHIKV RNA synthesis and the mode of action of inhibitors, we have developed an in vitro assay to study CHIKV replication/transcription complexes isolated from infected cells. In this assay (32)P-CTP was incorporated into the CHIKV genome, subgenomic (sg) RNA and into a ~7.5 kb positive-stranded RNA, termed RNA II. We mapped RNA II, which was also found in CHIKV-infected cells, to the 5' end of the genome up to the start of the sgRNA promoter region. Most of the RNA-synthesizing activity, negative-stranded RNA and a relatively large proportion of nsP1 and nsP4 were recovered from a crude membrane fraction obtained by pelleting at 15,000 G: . Positive-stranded RNA was mainly found in the cytosolic S15 fraction, suggesting it was released from the membrane-associated replication/transcription complexes (RTCs). The newly synthesized RNA was relatively stable and remained protected from cellular nucleases, possibly by encapsidation. A set of compounds that inhibit CHIKV replication in cell culture was tested in the in vitro RTC assay. In contrast to 3'dNTPs, chain terminators that acted as potent inhibitors of RTC activity, ribavirin triphosphate and 6-aza-UTP did not affect the RNA-synthesizing activity in vitro. In conclusion, this in vitro assay for CHIKV RNA synthesis is a useful tool for mechanistic studies on the RTC and mode of action studies on compounds with anti-CHIKV activity.

A novel viral RNA species in Sindbis virus-infected cells

Sindbis virus (SIN), the type alphavirus, has been studied extensively to identify the viral cis-acting sequences and proteins involved in RNA transcription and replication. However, very little is known about how these processes are coordinated. For example, synthesis of the genomic RNA and the subgenomic mRNA depends on the minus strand. Do these activities occur independently on different templates, or can replication and transcription take place simultaneously on the same template? We describe the appearance of a SIN-specific, plus-sense RNA that is intermediate in size between the genomic and subgenomic RNA species. This RNA, designated RNA II, is observed in a number of different cell lines, both early and late in infection. The number of RNA II species, their sizes, and their abundances are influenced by the subgenomic promoter. We have mapped the 3' end of RNA II to a site within the subgenomic promoter, four nucleotides before the initiation site of the subgenomic mRNA. Our results indicate that the appearance of RNA II is correlated with subgenomic mRNA transcription, such that strong or active promoters tend to increase the abundance of RNA II, relative to weak or less active promoters. RNA II is most abundantly detected with the full promoter and is at much lower abundance with the minimal promoter. The possible origins of RNA II are discussed.

Short-lived minus-strand polymerase for Semliki Forest virus

Semliki Forest virus (SFV)-infected BHK-21, Vero, and HeLa cells incorporated [3H]uridine into 42S and 26S plus-strand RNA and into viral minus-strand RNA (complementary to the 42S virion RNA) early in the infectious cycle. Between 3 and 4 h postinfection, the synthesis of minus-strand RNA ceased in these cultures, although the synthesis of plus-strand RNA continued at a maximal rate. At the time of cessation of minus-strand RNA synthesis, two changes in the pattern of viral protein synthesis were detected: a decrease in the translation of nonstructural proteins and an increase in the translation of the viral structural proteins. Addition of cycloheximide and puromycin to cultures of SFV-infected BHK cells actively synthesizing both viral plus- and minus-strand RNA resulted within 15 to 30 min in the selective shutoff of minus-strand RNA synthesis. Removal of the cycloheximide-containing medium led to the resumption of minus-strand synthesis and to an increased rate of viral RNA synthesis. We conclude that the minus-strand polymerase regulates the rate of SFV plus-strand RNA synthesis by determining the number of minus-strand templates and that the synthesis of the minus-strand templates is regulated at the level of translation by a mechanism which utilizes one or more short-lived polymerase proteins.

Characterization of a small, nonstructural viral polypeptide present late during infection of BHK cells by Semliki Forest virus

BHK cells, late in infection with Semliki Forest virus, were found to contain a small virus-specific polypeptide not found in the mature virion. This polypeptide had an apparent molecular weight of 6,000 and is referred to here as the 6K protein. No [2-3H]mannose was incorporated into 6K, and hence it does not appear to be a glycoprotein. This protein appears to be a primary translation product of the subgenomic 26S mRNA, which encodes the viral structural proteins. The genes encoding the viral structural proteins are arranged on the message in the order of 5'-C-E3-E2-E1-3'. We have found that the gene coding for 6K is located to the 3' side of the gene encoding E2. Subcellular fractionation of pulse-labeled cells infected with Semliki Forest virus demonstrated that 6K, like the viral glycoproteins p62 and E1, was present predominantly in the rough microsomal membrane fraction. 6K appears to be analogous, therefore, to the nonstructural 4.2K protein present in cells infected with Sindbis virus.

Messenger ribonucleoprotein complexes containing in vitro-synthesized 26S and 42S Semliki Forest virus RNA

An extract derived from Semliki Forest virus (SFV) infected cells is described which catalyzes the synthesis of virus-specific RNAs. The newly-synthesized 26S and 42S RNAs are found complexed with protein as messenger ribonucleoproteins (mRNPs). These mRNPs either are non-membrane bound or are associated with large cytoplasmic lipoprotein membranes, and they are found as free mRNPs as well as mRNPs bound to ribosomal subunits, ribosomes, and polysomes. Following treatment with Tween 40 and deoxycholate, membrane-bound mRNPs containing in vitro-synthesized 26S RNA are dissociated and sediment at 33S. These membrane-dissociated mRNPs contain relatively little protein. In contrast, the free or ribosome-bound mRNPs, which are isolated as 30S to 160S particles, remain heterogeneous after detergent treatment and have a much higher protein content. Addition of purified, native 40S ribosomal subunits to the extract leads to the formation of complexes between the added ribosomal subunits and the newly-synthesized viral mRNA. The in vitro-synthesized 26S and 42S RNAs participate in the assembly of translational initiation and elongation complexes.

The effect of ribonuclease on the replicative forms of Sindbis virus RNA

Three species of double-stranded RNA, designated RF I, RF II, and RF III in order of decreasing size (25), are produced by ribonuclease treatment of extracts of chicken embryo cells infected for 6 hours with Sindbis virus. Only one class of replicative form RNA is present in extracts not treated with ribonuclease; this class contains some molecules which can be enzymatically cleaved to produce the other two replicative forms. At a low level of enzyme (0.001 microgram/ml) the major species obtained was RF I, the replicative form of the genome. When the enzyme concentration was increased 10-, 100-, and 1000-fold, there was a progressive increase in the proportions of RF's II and III and a concomitant decrease in the proportion of RF I. The generation of RF's II and III by nuclease resulted in the ratio expected for these two species if they are produced by cleavage of RF I-like molecules. In preparations of isolated double-stranded RNA, only RF I and replicative intermediate RNA were present. Mild nuclease treatment of these preparations converted the replicative intermediates primarily to RF I. Higher enzyme levels generated greater proportions of RF II and RF III, but RF I-like molecules were the major source for these increased proportions. Treatment of the isolated naturally occurring replicative form with 0.01 microgram of ribonuclease per ml cleaved some molecules migrating as RF I during gel electrophoresis into molecules which migrated as RF II and RF III.

Heterologous interference in Aedes albopictus cells infected with alphaviruses

Maximum amounts of 42S and 26S single-stranded viral RNA and viral structural proteins were synthesized in Aedes albopictus cells at 24 h after Sindbis virus infection. Thereafter, viral RNA and protein syntheses were inhibited. By 3 days postinfection, only small quantities of 42S RNA and no detectable 26S RNA or structural proteins were synthesized in infected cells. Superinfection of A. albopictus cells 3 days after Sindbis virus infection with Sindbis, Semliki Forest, Una, or Chikungunya alphavirus did not lead to the synthesis of intracellular 26S viral RNA. In contrast, infection with snowshoe hare virus, a bunyavirus, induced the synthesis of snowshoe hare virus RNA in both A. Ablpictus cells 3 days after Sindbis virus infection and previously uninfected mosquito cells. These results suggested that at 3 days after infection with Sindbis virus, mosquito cells restricted the replication of both homologous and heterologous alphaviruses but remained susceptible to infection with a bunyavirus. In superinfection experiments the the alphaviruses were differentiated on the basis of plaque morphology and the electrophoretic mobility of their intracellular 26S viral RNA species. Thus, it was shown that within 1 h after infection with eigher Sindbis or Chikungunya virus, A. albopictus cells were resistant to superinfection with Sindbis, Chikungunya, Una, and Semliki Forest viruses. Infected cultures were resistant to superinfection with the homologous virus indefinitely, but maximum resistance to superinfection with heterologous alphaviruses lasted for approximately 8 days. After that time, infected cultures supported the replication of heterologous alphaviruses to the same extent as did persistently infected cultures established months previously. However, the titer of heterologous alphavirus produced after superinfection of persistently infected cultures was 10- to 50-fold less than that produced by an equal number of previously uninfected A. albopictus cells. Only a small proportion (8 to 10%) of the cells in a persistently infected culture was capable of supporting the replication of a heterologous alphavirus.

Origin of the actinomycin D insensitive RNA species in Aedes albopictus cells

In the presence of actinomycin D or a combination of actinomycin D and either camptothecin or alpha-amanatin. Aedes albopictus cells synthesize a variety of single stranded RNA species. These actinomycin D resistant species are ethidium bromide sensitive and they are present in the cell cytoplasm in an RNase resistant structure which has the sedimentation and buoyant density characteristics of mitochondria. Twelve actinomycin D insensitive RNA species can be detected by electrophoresis in 7M urea and 11 of these bind to oligo(dT)-cellulose. An identical set of oligo(dT)-cellulose binding RNA species is obtained when A. albopictus cells are labeled in the presence of camptothecin alone. The actinomycin D insensitive RNA species which bind to oligo(dT)-cellulose hybridize to mitochondrial DNA. These data indicate that the actinomycin D insensitive RNA species have a mitochondrial origin and are not associated with the replication of an inapparent contaminating virus.

Mechanism for control of synthesis of Semliki Forest virus 26S and 42s RNA

When cells infected with the Semliki Forest virus (SFV) mutant ts-4 were shifted to the nonpermissive temperature, synthesis of 26S RNA ceased, whereas synthesis of 42S RNA continued normally. These two single-stranded SFV RNAs are synthesized in two types of replicative intermediate (RI), 26S RNA in RI(b) and 42S RNA in RI(a). Cessation of 26S RNA synthesis after shift up in temperature was accompanied by loss of RI(b). When infected cells were shifted back down to 27 degrees C, 26S RNA synthesis resumed, coincident with the reappearance of RI(b). In both types of RI, the 42S minus-strand RNA is template for synthesis of plus-strand RNA. In pulse-chase experiments, we obtained RIs labeled only in their minus-strand RNA, and thus could follow the fate of RIs assembled at 27 degrees C when they were shifted to 39 degrees C. Our results show that, after shift up to 39 degrees C, there was a quantitative conversion of RIs in which 26S RNA had been synthesized to RIs in which 42S RNA was synthesized. This conversion of RI(b) to RI(a) was reversible, since RIs in which 26S RNA was synthesized reappeared when the infected cultures were shifted back down to 27 degrees C. We propose that, associated with RI(b), in which 26S RNA is synthesized, there is a virus-specific protein that functions to promote initiation of 26S RNA transcription at an internal site on the 42S minus-strand RNA and to block transcription on the minus strand in this region by the SFV RNA polymerase that had bound and was copying the minus-strand RNA from its 3' end. A ribonuclease-sensitive region would thus result in the sequence adjacent to the one that was complementary to 26S RNA. This virus-specific protein is not a component of the SFV RNA polymerase that continues to transcribe 42S RNA, and it is temperature sensitive in ts-4 mutant-infected cells. When this virus-specific protein is not present on RIs, the SFV polymerase transcribes the whole 42S minus-strand RNA and yields 42S plus-strand RNA.

RNA synthesized in calicivirus-infected cells is atypical of picornaviruses

RNA labeled with [3H]uridine from Vero cells infected with San Miguel sea lion virus in the presence of actinomycin D was analyzed by glycerol density gradient sedimentation and polyacrylamide gel electrophoresis. The predominant single-stranded RNA (36S, 2.6 x 10(6) molecular weight) was genome size. There was also a prominent 22S, 1.1 x 10(6)-molecular weight, single-stranded component and one or more double-stranded or partially double-stranded classes. Replicative forms, sedimenting at 18S, contained single-stranded RNA corresponding to the larger-molecular-weight class. All classes of intracellular RNA and virion RNA were polyadenylated. These findings and results with pig kidney cells infected with vesicular exanthema of swine virus and feline cells infected with feline calicivirus indicate that caliciviruses exhibit a strategy of replication different from typical picornaviruses and supports removal of the caliciviruses from the family Picornaviridae.

Control of protein synthesis in Semliki forest virus-infected cells

Protein synthesis in Semliki forest virus-infected chicken embryo cells was studied by labeling them with [35S]methionine for short periods at different times after infection, with or without synchronization of protein synthesis by the hypertonic block technique. The rate of host-cell protein synthesis declined almost linearly in inverse correlation to the increase in the amount of virus specific RNA. At 5.5 h postinfection, the host-cell protein synthesis was reduced by about 70%. The viral structural proteins were detectable with certainty at 3.5 h postinfection, and their rate of synthesis increased linearly parallel to the amount of their messenger, the 26S RNA. This suggests that the rate of synthesis of the structural proteins is controlled at the level of transcription. The rate of synthesis of the virus-specific nonstructural proteins attained its maximum between 3 and 4 h postinfection and declined thereafter, wheras the amount of their messenger, the 42S RNA, continued to increase linearly in the cells. Thus, the messenger activity of the 42S RNA is reduced in the late phase of infection compared with its activity in the early phase.

Free and membrane-bound polyribosomes in BHK cells infected with Sindbis virus

The data presented in the paper demonstrate that in BHK cells infected with Sindbis virus virtually all the 42S mRNA not in nucleocapsid is associated with free polyribosomes, whereas the 26S mRNA is distributed between free and membrane-bound polyribosomes. We suggest that the 26S RNA polyribosomes are bound to the membranes through the nascent chains of the B1 protein and that a large percentage of 26S RNA polyribosomes free in the cytoplasm may be due to the small amount of rough endoplasmic reticulum in BHK cells. In addition, we found that intracellular nucleocapsid is in the nonmembrane fraction of the cytoplasm of infected cells.

Replication of semliki forest virus: polyadenylate in plus-strand RNA and polyuridylate in minus-strand RNA

The 42S RNA from Semliki Forest virus contains a polyadenylate [poly(A)] sequence that is 80 to 90 residues long and is the 3'-terminus of the virion RNA. A poly(A) sequence of the same length was found in the plus strand of the replicative forms (RFs) and replicative intermediates (RIs) isolated 2 h after infection. In addition, both RFs and RIs contained a polyuridylate [poly(U)] sequence. No poly(U) was found in virion RNA, and thus the poly(U) sequence is in minus-strand RNA. The poly(U) from RFs was on the average 60 residues long, whereas that isolated from the RIs was 80 residues long. Poly(U) sequences isolated from RFs and RIs by digestion with RNase T1 contained 5'-phosphorylated pUp and ppUp residues, indicating that the poly(U) sequence was the 5'-terminus of the minus-strand RNA. The poly(U) sequence in RFs or RIs was free to bind to poly(A)-Sepharose only after denaturation of the RNAs, indicating that the poly(U) was hydrogen bonded to the poly(A) at the 3'-terminus of the plus-strand RNA in these molecules. When treated with 0.02 mug of RNase A per ml, both RFs and RIs yielded the same distribution of the three cores, RFI, RFII, and RFIII. The minus-strand RNA of both RFI and RFIII contained a poly(U) sequence. That from RFII did not. It is known that RFI is the double-stranded form of the 42S plus-strand RNA and that RFIII is the experimetnally derived double-stranded form of 26S mRNA. The poly(A) sequences in each are most likely transcribed directly from the poly(U) at the 5'-end of the 42S minus-strand RNA. The 26S mRNA thus represents the nucleotide sequence in that one-third of the 42S plus-strand RNA that includes its 3'-terminus.

Semliki Forest virus capsid protein associates with the 60S ribosomal subunit in infected cells

Semlike forest virus capsid protein cosedimented with the large ribosomal subunit at 60S in sucrose gradients after treatment of cytoplasm from infected cells with Triton X-100 and EDTA. In CsCl gradients the capsid protein banded with the subunit at a density of 1.56 to 1.57 g/cm3. Most of the capsid protein could be detached from the 60S structure by treatment with 0.8 M KCl. The ribonucleoprotein of the 26S RNA had a sedimentation value of 53S and a density of 1.50 g/cm3 and could thus be separated from the 60S structure. The data suggest that the capsid protein binds to the large ribosomal subunit, but not to the viral 26S RNA.

Initiation of translation directed by 42S and 26S RNAs from Semliki Forest virus in vitro

The proteins synthesized in vitro in response to 42S and 26S RNAs from Semliki Forest virus were labeled with formyl-[35S]methionine from initiator tRNA. One protein which comigrated with viral capsid protein was labeled under the direction of 26S RNA, and only one labeled peptide was detected after digestion with trypsin. Further digestion with pronase gave rise to the dipeptide fMet-AsN. Several labeled polypeptides were found in the 42S RNA directed product and these had molecular weights of up to 150,000. However, tryptic digestion of the product yielded only one formylmethionyl-labeled peptide, which had a different mobility from that directed by the 26S RNA. Further digestion with pronase gave a single dipeptide, fMet-Ala. This indicates that nonstructural proteins as large as 150,000 daltons are probably synthesized from one initiation site on the 42S RNA. Translation starting from the internal initiation site on the 42S RNA, which is equivalent to that on the 26S RNA, could not be detected under the conditions used. Internal initiation sites which are similarly inactive have also been detected in other viral RNAs (e.g., brome mosaic virus, tobacco mosaic virus, and polyoma 19S RNA) and this suggests that, although eukaryotic mRNAs can contain more than one initiation site for protein synthesis, only the site nearer the 5' terminus is active in vitro.

Simultaneous translation of structural and nonstructural proteins from Semliki-forest-virus RNA in two eukaryotic systems in vitro

The Semliki Forest virus genome, 42-S RNA, and the virus-specific intracellular 26-S RNA were translated in two cell-free protein-synthesising systems, the wheat germ extract, and a partially purified system from mammalian tissues. The 26-S RNA directed the synthesis of structual proteins only, as revealed by tryptic peptide mapping. About 75--80% of the radioactivity in the products comigrated with capsid and about 4--8% with envelope protein peptides. All the capsid peptides and the full-sized capsid protein were found in the products in vitro, no complete envelope protein was formed and fewer than half of the envelope peptides were detected. This result is consistent with reports that there is only one initiation site for the translation of virus structural proteins, and that the capsid protein is N-terminal in the polyprotein followed by envelope proteins. The systems programmed with 42-S RNA yielded virtually the same structural peptides. However, the bulk of the radioactivity was in peptides which did not comigrate with the structural ones. These peptides were mostly associated with relatively small-sized products. This shows that Semliki Forest virus 42-S RNA has at least two initiation sites, one for the structural proteins and the other(s) for the nonstructural proteins.

Isolation and characterization of double stranded RNA containing infectious viral genome RNA from cells infected with Semliki Forest virus

A double stranded virus specific RNA sedimenting at about 19S on sucrose density gradients has been isolated from BHK-21 cells infected with Semliki Forest virus (SFV). The molecule consists of double stranded RNA (ds RNA) since it is labeled with 3H-uridine, is soluble in 2 M LiCl, resistant against treatment with DNase and RNase at 2 X SSC, hydrolyzed by alkali treatment, has a sharp thermal melting point at 89 degrees in 1/10SSC, and an extended appearance under non denaturing conditions in the electronmicroscope. The following findings show that it consists of intact, infectious 42S RNA similar or identical to the genome RNA of SFV complexed to a complementary 42S minus strand RNA: 1. Denaturation converts the ds RNA into molecules cosedimenting with 42S RNA isolated from SFV particles. 2. About 50% of the radioactivity of 3H-uridine labeled 42S RNA molecules generated from 19S ds RNA by denaturation hybridizes to 42S viral RNA. 3. The specific infectivity of denatured 19S ds RNA is about half of that of similarly treated viral 42S RNA. Further properties of this molecule are discussed.

Protein synthesis in BHK-21 cells infected with semliki forest virus

[3H]leucine-labeled proteins synthesized in BHK-21 cells infected with Semliki Forest virus were fractionated by polyacrylamide gel electrophoresis (PAGE). Cellular and virus-specific proteins were identified by difference analysis of the PAGE profiles. The specific activity of intracellular [3H-A1leucine was determined. Two alterations of protein synthesis, which develop with different time courses, were discerned. (i) In infected cultures an inhibition of overall protein synthesis to about 25% of the protein synthesis in mock-infected cultures develops between about 1 and 4 h postinfection (p.i.). (ii) The relative amount of virus-specific polypeptides versus cellular polypeptides increases after infection. About 80% of the proteins synthesized at 4 h p.i. are cellular proteins. Since significant amounts of nontranslocating robosomes in polyribosomes were not detected up to 7 h p.i., the inhibition of protein synthesis is not caused by inactivation of about 75% of all polyribosomes but by a decreased protein synthetic activity of the majority of polyribosomes. Indirect evidence indicates that an inhibition of elongation and/or release of protein synthesis develops in infected cells, which is sufficient to account for the observed inhibition of protein synthesis. Inhibition of over-all protein synthesis developed when virus-specific RNA began to accumulate at the maximal rate. This relationship was observed during virus multiplication at 37, 30, and 25 C. A possible mechanism by which synthesis of virus-specific RNA in the cytoplasm could inhibit cellular protein synthesis is discussed. Indirect evidence and analysis of polyribosomal RNA show that the increased synthesis of virus-specific protein is brought about by a substitution of cellular by viral mRNA in the polyribosomes.

Tryptic peptide analysis on nonstructural and structural precursor proteins from Semliki Forest virus mutant-infected cells

Analysis of [35S]methionine-labeled tryptic peptides of the large proteins induced by temperature-sensitive mutants of Semliki Forest virus was carried out. The 130,000-molecular-weight protein induced by ts-2 and ts-3 mutants contained the peptides of capsid protein and of both major envelope proteins E1 and E2. The ts-3-induced protein with molecular weight of 97,000 contained peptides of the capsid and envelope protein E2 but not those of E1. Two proteins with molecular weights of 78,000 and 86,000 from ts-1-infected cells did not contain the peptides of the virion structural proteins. They are evidently expressions of the nonstructural part of the 42S RNA genome of Semliki Forest virus.

Interfering passages of Sindbis virus: concomitant appearance of interference, morphological variants, and trucated viral RNA

Serial passage of Sindbis at high multiplicities of infection resulted in cyclical variations in virus titer. Decreases in virus titer were correlated with the appearance of smaller-sized virions, interference and truncated viral RNA. The smaller particles were 37 nm in diameter, exclusive of the hemagglutinin spikes as compared with a diameter of 50 nm for standard virions. Passages which contained 37-nm partilces also interfered with infectious center formation by standard, plaque-purified virus. Polyacrylamide gel analysis of RNA isolated from virions present in interfering passages demonstrated the sequential appearance of three RNA species smaller than standard RNA with approximate molecular weights of 3.3 X 106, 2.7 X 106, and 2.2 X 106. The 3.3 X 106 RNA was evident in passage 5, by passage 8 both the 3.3 X 106 and 2.7 X 106 RNAs were present, and by passage 13 all three were present with the 2.2 X 106 RNA predominating.

Quantitation of Semlike Forest virus RNAs in infected cells using 32-P equilibrium labelling

In vitro cultured BHK and HeLa cells were labelled for several cell division cycles with 32-P-phosphate until they were equilibrated with radiophosphorus. After infection with Semliki forest virus (or mock-infection) these cells were analyzed for viral and ribosomal RNA by sucrose gradient centrifugation. From their radioactivities the mass of each RNA species was calculated. It was found that the BHK and HeLa cells contained on average 11.0 plus or minus 3.1 pg and 6.3 plus or minus 1.9 pg of ribosomal RNA (28 S + 18 S) respectively per cell. At the end of the viral growth cycle, i.e. at 8 h post infection the average mass of viral genome produced per cell was 1.0 -1.9 pg and 0.3 - 0.5 pg in BHK and HeLa cells respectively, of which only 1/10 to 1/20 was released as mature virus particles. The amount of the second major virus specific messenger, the 26 S RNA, was estimated from its ratio to the viral genome after labelling with 3-H-uridine in the presence of actinomycin D. These two viral RNAs were found to be present in roughly equimolar amounts.

Replication of Sindbis virus. V. Polyribosomes and mRNA in infected cells

Cells infected with wild-type Sindbis virus contain at least two forms of mRNA, 26S and 49S RNA. Sindbis 26S RNA (molecular weight 1.6 x 10(6)) constitutes 90% by weight of the mRNA in infected cells, and is thought to specify the structural proteins of the virus. Sindbis 49S RNA, the viral genome (molecular weight 4.3 x 10(6)), constitutes approximately 10% of the mRNA in infected cells and is thought to supply the remaining viral functions. In cells infected with ts2, a temperature-sensitive mutant of Sindbis virus, the messenger forms also include a third species of RNA with a sedimentation coefficient of 33S and an apparent molecular weight of 2.3 x 10(6). Hybridization-competition experiments showed that 90% of the base sequences in 33S RNA from these cells are also present in 26S RNA. Sindbis 33S RNA was also isolated from cells infected with wild-type virus. After reaction with formaldehyde, this species of 33S RNA appeared to be completely converted to 26S RNA. These results indicate that 33S RNA isolated from cells infected with either wild-type Sindbis or ts2 is not a unique and separate form of Sindbis RNA.

Involvement of the host cell nuclear envelope membranes in the replication of Japanese encephalitis virus

The distribution of viral ribonucleic acid (RNA) on various cell membrane fractions derived from a porcine kidney cell line infected with Japanese encephalitis virus was investigated. At 40 h postinfection, after virus growth had reached its peak, three viral RNAs, 45S, 27S, and 20S, were associated with the cytoplasmic membranes and intact nuclei. The amount of each RNA associated with the nucleus was two- to fivefold greater than that present with the cytoplasmic membranes. Treatment of washed infected nuclei with 1.0% Triton X-100, which removed the outer nuclear envelope membrane, also removed the viral RNA. When the nucleus was fractionated into nuclear envelope membranes and a large particle fraction which sedimented at 600 x g, nearly all of the viral RNA remained associated with the envelope membranes. The nuclear envelope membranes contained higher viral RNA polymerase activity than the cytoplasmic membranes derived from the same cells. These data suggest that major sites for Japanese encephalitis virus RNA synthesis may be localized on or in very close association with the nuclear envelope membranes.

Deficiency of 60 to 70S RNA in murine leukemia virus particles assembled in cells treated with actinomycin D

Production of particles with the ultrastructural appearance of C-type virions persisted for at least 6 h in actinomycin D-treated cells infected with murine leukemia virus. This phenomenon occurred despite severe inhibition of viral RNA synthesis. Virus particles present in a 6-h harvest sedimented in sucrose gradients with the buoyant density characteristic of RNA tumor viruses (1.16 g/cm(3)) and exhibited high levels of reverse transcriptase activity in response to the exogenous template polyriboadenylic acid.oligo deoxythymidylic acid in the range of untreated controls. However, RNase-sensitive endogenous activity was only (1/5) the level found in controls. This observation correlated with a marked reduction in infectivity. Kinetic studies on the appearance of labeled RNA in banded virions revealed that within the first hour after addition of actinomycin D, particles contained 60 to 70S RNA and two low-molecular-weight RNA species corresponding to 8 and 4S RNA. After approximately 1 h of incubation with actinomycin D, 60 to 70S RNA could not be detected and 4S RNA was the predominant species. These findings suggest that murine leukemia virus particles assembled in the presence of actinomycin D are deficient in 60 to 70S viral RNA.