The RNA of influenza virus

Defective Interfering Particles of Negative-Strand RNA Viruses

Viral defective interfering particles (DIPs) were intensely studied several decades ago but research waned leaving open many critical questions. New technologies and other advances led to a resurgence in DIP studies for negative-strand RNA viruses. While DIPs have long been recognized, their exact contribution to the outcome of acute or persistent viral infections has remained elusive. Recent studies have identified defective viral genomes (DVGs) in human infections, including respiratory syncytial virus and influenza, and growing evidence indicates that DVGs influence disease severity and may contribute to viral persistence. Further, several studies have advanced our understanding of key viral and host factors that regulate DIP formation and activity. Here we review these discoveries and highlight key questions moving forward.

Dynamical Behavior of a Stochastic SIRC Model for Influenza A

In this paper, a stochastic SIRC epidemic model for Influenza A is proposed and investigated. First, we prove that the system exists a unique global positive solution. Second, the extinction of the disease is explored and the sufficient conditions for extinction of the disease are derived. And then the existence of a unique ergodic stationary distribution of the positive solutions for the system is discussed by constructing stochastic Lyapunov function. Furthermore, numerical simulations are employed to illustrate the theoretical results. Finally, we give some further discussions about the system.

Influenza A in Bovine Species: A Narrative Literature Review

It is quite intriguing that bovines were largely unaffected by influenza A, even though most of the domesticated and wild animals/birds at the human-animal interface succumbed to infection over the past few decades. Influenza A occurs on a very infrequent basis in bovine species and hence bovines were not considered to be susceptible hosts for influenza until the emergence of influenza D. This review describes a multifaceted chronological review of literature on influenza in cattle which comprises mainly of the natural infections/outbreaks, experimental studies, and pathological and seroepidemiological aspects of influenza A that have occurred in the past. The review also sheds light on the bovine models used in vitro and in vivo for influenza-related studies over recent years. Despite a few natural cases in the mid-twentieth century and seroprevalence of human, swine, and avian influenza viruses in bovines, the evolution and host adaptation of influenza A virus (IAV) in this species suffered a serious hindrance until the novel influenza D virus (IDV) emerged recently in cattle across the world. Supposedly, certain bovine host factors, particularly some serum components and secretory proteins, were reported to have anti-influenza properties, which could be an attributing factor for the resilient nature of bovines to IAV. Further studies are needed to identify the host-specific factors contributing to the differential pathogenetic mechanisms and disease progression of IAV in bovines compared to other susceptible mammalian hosts.

Defective viral genomes are key drivers of the virus-host interaction

Viruses survive often harsh host environments, yet we know little about the strategies they utilize to adapt and subsist given their limited genomic resources. We are beginning to appreciate the surprising versatility of viral genomes and how replication-competent and -defective virus variants can provide means for adaptation, immune escape and virus perpetuation. This Review summarizes current knowledge of the types of defective viral genomes generated during the replication of RNA viruses and the functions that they carry out. We highlight the universality and diversity of defective viral genomes during infections and discuss their predicted role in maintaining a fit virus population, their impact on human and animal health, and their potential to be harnessed as antiviral tools.

This Review describes recent findings on the biogenesis and the role of defective viral genomes during replication of RNA viruses and discusses their impact on viral dynamics and evolution.

Defective (interfering) viral genomes re-explored: impact on antiviral immunity and virus persistence

Defective viral genomes (DVGs) are natural products of virus replication that occur in many positive and negative sense RNA viruses, including Ebola, dengue and respiratory syncytial virus. DVGs, which have severe genomic truncations and require a helper virus to replicate, have three well-described functions: interference with standard virus replication, immunostimulation, and establishment of virus persistence. These functions of DVGs were first described almost 50 years ago, yet only recent studies have shown the molecular intersection between their immunostimulatory and pro-persistence activities. Here, we review more than half a century of scientific literature on the immunostimulatory and pro-persistence functions of DVGs. We highlight recent advances in the field and the critical role DVGs have in both the acute and long-term virus-host interactions.

Interplay between influenza A virus and host factors: targets for antiviral intervention

Influenza A viruses (IAVs) pose a major public health threat worldwide. Recent experience with the 2013 H7N9 outbreak in China and the 2009 "swine flu" pandemic have shown that antiviral vaccines and drugs fall short of controlling the spread of disease in a timely and effective manner. Major problems include rapid emergence of drug-resistant influenza virus strains and the slow process of vaccine production. With the threat of a highly pathogenic H5N1 bird-flu pandemic looming large, it is crucial to develop novel ways of combating influenza A viruses. Targeting the host factors critical for influenza A virus replication has shown promise as a strategy to develop novel antiviral molecules with broad-spectrum protection. In this review, we summarize the role of currently identified host factors that play a critical role in the influenza A virus life cycle and discuss the most promising candidates for anti-influenza therapeutics.

Genetics, evolution, and the zoonotic capacity of European Swine influenza viruses

The European swine influenza virus lineage differs genetically from the classical swine influenza viruses and the triple reassortants found in North America and Asia. The avian-like swine H1N1 viruses emerged in 1979 after an avian-to-swine transmission and spread to all major European pig-producing countries. Reassortment of these viruses with seasonal H3N2 viruses led to human-like swine H3N2 viruses which appeared in 1984. Finally, human-like swine H1N2 viruses emerged in 1994. These are triple reassortants comprising genes of avian-like H1N1, seasonal H1N1, and seasonal H3N2 viruses. All three subtypes established persistent infection chains and became prevalent in the European pig population. They successively replaced the circulating classical swine H1N1 viruses of that time and gave rise to a number of reassortant viruses including the pandemic (H1N1) 2009 virus. All three European lineages have the capacity to infect humans but zoonotic infections are benign.

Expression of IMP1 enhances production of murine leukemia virus vector by facilitating viral genomic RNA packaging

Murine leukemia virus (MLV)-based retroviral vector is widely used for gene transfer. Efficient packaging of the genomic RNA is critical for production of high-titer virus. Here, we report that expression of the insulin-like growth factor II mRNA binding protein 1 (IMP1) enhanced the production of infectious MLV vector. Overexpression of IMP1 increased the stability of viral genomic RNA in virus producer cells and packaging of the RNA into progeny virus in a dose-dependent manner. Downregulation of IMP1 in virus producer cells resulted in reduced production of the retroviral vector. These results indicate that IMP1 plays a role in regulating the packaging of MLV genomic RNA and can be used for improving production of retroviral vectors.

Some properties of influenza virus nucleocapsids

Nucleocapsids released from influenza virions by sodium deoxycholate sedimented heterogeneously in sucrose gradients. Highly infectious virus (complete) preparations yielded nucleocapsids with peak distributions at 64 and 56S; von Magnus type virus (incomplete) lacked 64S nucleocapsids. Treatment of influenza virus nucleocapsids with pancreatic ribonuclease rendered the associated viral ribonucleic acid (RNA) molecules acid-soluble, indicating that capsid proteins do not completely surround the viral RNA's. However, the capsid proteins remained associated after enzymatic hydrolysis of the RNA, as judged by persistently high sedimentation rates. Sedimentation rates of viral nucleocapsids reflected the sedimentation rates of the associated RNA's: 64S nucleocapsids contained 18S RNA, whereas 56S nucleocapsids contained 15S RNA, although in both cases RNA's sedimenting at 4 to 13S were also recovered. Furthermore, just as incomplete virions lacked 64S nucleocapsids, they also lacked 18S RNA. These findings support the hypothesis that the influenza virus genome is divided among several distinct pieces of RNA.

Genome packaging in influenza A virus

The negative-sense RNA genome of influenza A virus is composed of eight segments, which encode 12 proteins between them. At the final stage of viral assembly, these genomic virion (v)RNAs are incorporated into the virion as it buds from the apical plasma membrane of the cell. Genome segmentation confers evolutionary advantages on the virus, but also poses a problem during virion assembly as at least one copy of each of the eight segments is required to produce a fully infectious virus particle. Historically, arguments have been presented in favour of a specific packaging mechanism that ensures incorporation of a full genome complement, as well as for an alternative model in which segments are chosen at random but packaged in sufficient numbers to ensure that a reasonable proportion of virions are viable. The question has seen a resurgence of interest in recent years leading to a consensus that the vast majority of virions contain no more than eight segments and that a specific mechanism does indeed function to select one copy of each vRNA. This review summarizes work leading to this conclusion. In addition, we describe recent progress in identifying the specific packaging signals and discuss likely mechanisms by which these RNA elements might operate.

Explaining the high mutation rates of cancer cells to drug and multidrug resistance by chromosome reassortments that are catalyzed by aneuploidy

The mutation rates of cancer cells to drug and multidrug resistance are paradoxically high, i.e., 10(-3) to 10(-6), compared with those altering phenotypes of recessive genes in normal diploid cells of about 10(-12). Here the hypothesis was investigated that these mutations are due to chromosome reassortments that are catalyzed by aneuploidy. Aneuploidy, an abnormal number of chromosomes, is the most common genetic abnormality of cancer cells and is known to change phenotypes (e.g., Down's syndrome). Moreover, we have shown recently that aneuploidy autocatalyzes reassortments of up to 2% per chromosome per mitosis because it unbalances spindle proteins, even centrosome numbers, via gene dosage. The hypothesis predicts that a selected phenotype is associated with multiple unselected ones, because chromosome reassortments unbalance simultaneously thousands of regulatory and structural genes. It also predicts variants of a selected phenotype based on variant reassortments. To test our hypothesis we have investigated in parallel the mutation rates of highly aneuploid and of normal diploid Chinese hamster cells to resistance against puromycin, cytosine arabinoside, colcemid, and methotrexate. The mutation rates of aneuploid cells ranged from 10(-4) to 10(-6), but no drug-resistant mutants were obtained from diploid cells in our conditions. Further selection increased drug resistance at similar mutation rates. Mutants selected from cloned cells for resistance against one drug displayed different unselected phenotypes, e.g., polygonal or fusiform cellular morphology, flat or three-dimensional colonies, and resistances against other unrelated drugs. Thus our hypothesis offers a unifying explanation for the high mutation rates of aneuploid cancer cells and for the association of selected with unselected phenotypes, e.g., multidrug resistance. It also predicts drug-specific chromosome combinations that could become a basis for selecting alternative chemotherapy against drug-resistant cancer.

Approximately 150 nucleotides from the 5' end of an influenza A segment 1 defective virion RNA are needed for genome stability during passage of defective virus in infected cells

Defective influenza A virus RNAs analyzed in two studies so far possess at least 80-90 nucleotides from the 5' end of the virion RNA segment and more typically around 200 nucleotides, whereas the 3' sequence could be as short as 25 nucleotides (P. A. Jennings et al., Cell 34, 619-627; 1983; S. D. Duhaut and N. J. Dimmock, Virology 247, 241-253, 1998). To determine the biological significance of the highly conserved 5' sequence, we constructed plasmids that expressed a naturally occurring defective segment 1 RNA from A/equine/Newmarket/7339/79 (EQV, H3N8) or modified RNAs with lesser amounts of the 5' end. These had terminal 5' sequences of 220 nucleotides (POLI-220), 150 nucleotides (POLI-150), 80 nucleotides (POLI-80), and 30 nucleotides (POLI-30). Their remaining sequence came from the 3' end of virion RNA, and all were exactly 445 nucleotides in length. After transfection with one of the RNA-expressing POLI plasmids and plasmids encoding PB1, PB2, PA, and NP proteins, Vero cells were infected with a helper influenza virus of one of three different subtypes (the parental H3N8, an H2N2, or an H1N1 virus). Progeny infectious and presumptive progeny defective virus in the resulting tissue culture fluids were then passaged serially to new cultures up to 10 times. We found that POLI-220 and POLI-150 RNAs proved stable on passage and POLI-80 RNA was detected intermittently, while POLI-30 was not detected beyond passage three. Data were essentially reproducible with the three helper viruses and in two cell lines. It thus appears that the terminal 5' 150 nucleotides are necessary for influenza virion RNA molecules to be replicated and packaged consistently in cell culture. The possible functional significance of the 5' sequence is discussed.

Viral RNA polymerases

Recent progress in molecular biological techniques revealed that genomes of animal viruses are complex in structure, for example, with respect to the chemical nature (DNA or RNA), strandedness (double or single), genetic sense (positive or negative), circularity (circle or linear), and so on. In agreement with this complexity in the genome structure, the modes of transcription and replication are various among virus families. The purpose of this article is to review and bring up to date the literature on viral RNA polymerases involved in transcription of animal DNA viruses and in both transcription and replication of RNA viruses. This review shows that the viral RNA polymerases are complex in both structure and function, being composed of multiple subunits and carrying multiple functions. The functions exposed seem to be controlled through structural interconversion.

Synthesis of RNA segments 1-3 during generation of incomplete influenza A (fowl plague) virus

Incomplete influenza A virus (fowl plague Dobson strain) was prepared by undiluted passage in primary chick embryo fibroblast cells. Analysis of released virus RNA revealed a deficiency in RNA segments 1-3, characteristic of incomplete virus formation. The virus yield from a high multiplicity infection with standard virus always showed this deficiency, even when analysed as early as 6 hours post-infection, whereas infection at low multiplicity gave rise to virus indistinguishable in RNA composition from the parent virus. The relative amounts of intracellular, non-polyadenylated, complementary RNA (template RNA) were found to reflect accurately the eventual RNA composition of released virus, and were altered in phase with PFU:HAU ratio, throughout a von Magnus cycle.

Extra RNAs of von Magnus particles of influenza virus cause reduction of particular polymerase genes

Extra RNAs, or RNA species other than eight gene segments, in von Magnus particles of the influenza virus WSN strain were studied by polyacrylamide gel electrophoresis and oligonucleotide mapping. From the original virus stock, various cloned stocks were obtained, each giving rise to a characteristic set of extra RNAs. One cloned virus stock contained a large number of von Magnus particles. The RNA pattern was characterized by two prominent extra RNAs (X1 and X2) and a decrease in the content of two polymerase genes, P1 and P2. Segregation of the two extra RNAs was carried out by coinfection of cells with a von Magnus particle and infectious virions. The results showed that the presence of one of the extra RNAs (X2) was associated with a reduction in the amount of the P1 gene and that the presence of the other extra RNA (X1) was associated with a reduction in the amount of the P2 gene. Oligonucleotide mapping showed that both extra RNAs, X1 and X2, were derived from the P1 gene. The results suggested that an extra RNA did not necessarily cause the reduction of the progenitor polymerase gene, but might cause the reduction of another polymerase gene.

Characterization of virus-like particles produced by an influenza A virus

The influenza strain 413 1,1 segregated as a stable recombinant during passage of the isolate 19/N which was obtained after double infection of chick embryo fibroblasts by virus N and the fowl plague virus (FPV) mutant ts 19. Its gene constellation was determined by molecular hybridization. Upon infection of chick embryo cells by this recombinant strain, two particle populations of high (H) and low (L) buoyant densities were produced. By biological and biochemical parameters, the H-population (delta = 1.22 g/cm3) cannot be distinguished from standard infectious influenza virus. In contrast, the noninfectious L-particles (delta = 1.14 g/cm3) lack all virus-specific glycoproteins (HA, NA) as well as the matrix protein M and are visualized by electron microscopy as spikeless particles. Significant changes in the quantitative composition of the phospholipid bilayer are evident as compared to the H-particles. In addition to the previously characterized eight genes both populations contain a variety of smaller RNA fragments which hybridize with complementary RNA and presumably represent degradation products of full-length genes.

Homologous interference mediated by defective interfering influenza virus derived from a temperature-sensitive mutant of influenza virus

A temperature-sensitive group II mutant of influenza virus, ts-52, with a presumed defect in viral RNA synthesis, readily produced von Magnus-type defective interfering virus (DI virus) when passed serially (four times) at high multiplicity in MDBK cells. The defective virus (ts-52 DI virus) had a high hemagglutinin and a low infectivity titer, and strongly interfered with the replication of standard infectious viruses (both ts-52 and wild-type ts+) in co-infected cells. Progeny virus particles produced by co-infection of DI virus and infectious virus were also defective and also had low infectivity, high hemagglutinating activity, and a strong interfering property. Infectious viruses ts+ and ts-52 were indistinguishable from ts-52 DI viruses by sucrose velocity or density gradient analysis. Additionally, these viruses all possessed similar morphology. However, when the RNA of DI viruses was analyzed by use of polyacrylamide gels containing 6 M urea, there was a reduction in the amount of large RNA species (V1 to V4), and a number of new smaller RNA species (D1 to D6) with molecular weights ranging from 2.9 X 10(5) to 1.05 X 10(5) appeared. Since these smaller RNA species (D1 to D6) were absent in some clones of infectious viruses, but were consistently associated with DI viruses and increased during undiluted passages and during co-infection of ts-52 with DI virus, they appeared to be a characteristic of DI viruses. Additionally, the UV target size of interfering activity and infectivity of DI virus indicated that interfering activity was 40 times more resistant to UV irradiation than was infectivity, further implicating small RNA molecules in interference. Our data suggest that the loss of infectivity observed among DI viruses may be due to nonspecific loss of a viral RNA segment(s), and the interfering property of DI viruses may be due to interfering RNA segments (DIRNA, D1 to D6). ts-52 DI virus interfered with the replication of standard virus (ts+) at both permissive (34 degrees C) and nonpermissive temperatures. The infectivity of the progeny virus was reduced to 0.2% for ts+ and 0.05% for ts-52 virus without a reduction in hemagglutinin titer. Interference was dependent on the concentration of DI virus. A particle ratio of 1 between DI virus (0.001 PFU/cell) and infectious virus (1.0 PFU/cell) produced a maximal amount of interference. Infectious virus yield was reduced 99.9% without any reduction of the yield of DI viruses Interference was also dependent on the time of addition of DI virus. Interference was most effective within the first 3 h of infection by infectious virus, indicating interference with an early function during viral replication.

Common sequence at the 5' ends of the segmented RNA genomes of influenza A and B viruses

Guanylyl- and methyltransferases, isolated from purified vaccinia virus, were used to specifically label the 5' ends of the genome RNAs of influenza A and B viruses. All eight segments were labeled with [alpha-(32)P]guanosine 5'-triphosphate or S-adenosyl[methyl-(3)H]methionine to form "cap" structures of the type m(7)G(5')pppN(m)-, of which unmethylated (p)ppN- represents the original 5' end. Further analyses indicated that m(7)G(5')pppA(m), m(7)G(5')pppA(m)pGp, and m(7)G(5')pppA(m)pGpUp were released from total and individual labeled RNA segments by digestion with nuclease P1, RNase T1, and RNase A, respectively. Consequently, the 5'-terminal sequences of most or all individual genome RNAs of influenza A and B viruses were deduced to be (p)ppApGpUp. The presence of identical sequences at the ends of RNA segments of both types of influenza viruses indicates that they have been specifically conserved during evolution.

Complexity and polyadenylic acid content of visna virus 60-70S RNA

The genomic complexity of visna virus was measured by quantitative analysis of 18 RNase T1-resistant oligonucleotides from 60-70S RNA. T1-resistant oligonucleotides were separated by two-dimensional polyacrylamide gel electrophoresis. Visna virus had a genomic complexity of 3.6 X 10(6) daltons, very close to the size of a single 30-40S RNA subunit. It was therefore concluded that the visna virus genome is largely polyploid. Visna virus 60-70S RNA polyadenylic acid segment was purified by T1 RNase digestion followed by oligodeoxythymidylic acid-cellulose column chromatography. It contained over 99% AMP and had a size of about 200 nucleotides. The binding capacities on oligodeoxythymidylic acid-cellulose of native 60-70S RNA and purified 30-40S RNA subunits were examined. It was concluded that two out of three intact subunits contain a polyadenylic acid segment.

Influenza virus genome consists of eight distinct RNA species

The genomic RNA of the avian influenza A virus, fowl plague, was fractionated into eight species by electrophoresis in polyacrylamide-agarose gels containing 6 M urea. The separated 32P-labeled RNA species were characterized by digestion with RNase T1 and fractionation of the resulting oligonucleotides by two-dimensional gel electrophoresis; this demonstrated that each species has a distinct nucleotide sequence. A tentative correlation of each genome RNA species with the virus protein that it encodes was made.

Recombination between a temperature-sensitive mutant and a deletion mutant of Rous sarcoma virus

Cells doubly infected with two mutants of the Schmidt-Ruppin strain of Rous sarcoma virus (RSV), ts68, which is temperature sensitive for cell transformation (srcts), and a deletion mutant, N8, which is deficient in the envelope glycoprotein (env-), produced a recombinant which carried the defects of both parents. The frequency of formation of such a recombinant was exceptionally high and made up 45 to 55% of the progeny carrying the srcts marker. By contrast, the reciprocal recombinant, which is wild type in transformation (srcts) and contains the subgroup A envelope glycoprotein (envA), was almost undetectable. This remarkable difference in the frequency of the formation of the two possible recombinants suggests that a unique mechanism may be involved in the genetic interaction of the two virus genomes, one of which has a large deletion. When an RNA-dependent DNA polymerase-negative variant of the N8 (N8alpha) was crinants also became deficient in the polymerase. Cells infected by the srctsenv- recombinant were morphologically normal at the nonpermissive temperature (41 degrees C) and susceptible to all subgroups of RSV. The rate by which the wild-type RSV transformed the recombinant-preinfected cells was indistinguishable from that of transformation of uninfected chicken cells by the same wild-type virus. This indicates that no detectable interference exists at postpenetration stages between the preinfected and superinfecting virus genomes and confirms that the expression of the transformed state is dominant over the suppressed state.

Transcriptase activity and genome composition of defective influenza virus

Eight genome RNA segments are present in both normal and von Magnustype influenza virus preparations and all species are transcribed by the virion-associated polymerase. Although the RNA polymerase activity and the amount of the three largest RNA segments are reduced in defective influenza virus preparations, these reductions do not appear to be great enough to account for the much greater loss of infectivity.

Influenza virus RNA's in the cytoplasm of chicken embryo cells treated with 3'-deoxyadenosine

3'-Deoxyadenosine (75 to 100 mug/ml) permitted analysis of the cytoplasmic influencza virus-specific RNAs synthesized early in the replicative cycle-a phase that has hitherto been obscurred by host cell RNA synthesis. In addition, late in the cycle (6 to 8) complementary virus-specific RNA's were the predominantly labeled species, suggesting that higher concentrations of 3'-deoxyadenosine selectively inhibit influenza viral genome replication.

Differences in RNA patterns of influenza A viruses

Analysis of the segmented RNAs of influenza A viruses by electrophoresis on polyacrylamide urea slab gels has provided a method for sharper resolution of the number and migration rates of different segments than previously has been possible. Using this system, the RNA genome of influenza A/WSN (HON1) virus can be separated into seven to nine separate bands, depending on whether virus is obtained after high or low multiplicity of infection, and the genome of influenza A/PR/8 (HON1) virus can be resolved into eight bands, six of which migrate differently from comparable RNA bands of WSN virus. Comparision of the RNA patterns produced by influenza A/PR/8 (HON1) and A/England/42/72 (H8n2) virus also reveals major differences in migration speeds of different bands, and analysis of the RNAs of the RNAs of an HON2 recombinant virus derived from these two strains permits the identification of RNA segments which have been derived from one particular parent. By extension of these techniques, it may be possible to define which RNA segment codes for each viral protein and to analyze recombinant strains to identify which genes have been derived from each of its parents.

Comparative study of the electrophoretic mobility of the RNA of influenza parent and recombinant strains

The electrophoretic mobility of the RNA of Influenza viruses A/WSN, A/Singapore and their antigenic recombinants X-7 and X-9 was investigated. The genome of each virus studied consisted of seven pieces of RNA. The electrophoretic profile of the influenza virus A/WSN RNA differed from that of A/Singapore but resumbled that of the recombinant X-9 genome. The essential differences were connected with the properties of the fifth fragment of the RNA. The molecular weight of this RNA species of influenza virus A/WSN and X-9 was 5.4 X 10(5) AND 5.3 X 10(5) daltons (d) respectively. The molecular weight of the corresponding component of the influenza viruses A/Sinapore and X-7 RNA was 6.2 X 10(5) and 6.3 X 10(5)d respectively.

Characterization of inactivation of myxoviruses and paramyxoviruses by hydroxylamine, N-methylhydroxylamine and O-methylhydroxylamine

A study of the mechanism of myxovirus and paramyxovirus inactivattion by hydroxylamine, O-methylhydroxylamine and N-methylhydroxylamine was conducted. Influenza A (WSN) was used as the sensitive myxovirus and Newcastle disease virus (NDV-L) was used as the relatively resistant paramyxovirus in certain experiments. Inactivation was found to be rapid (15 minutes) and mose effective at high concentrations (2M). All three compounds significantly decreased the hemagglutination titer of WSN treated at pH smaller than or equal to 5.0. There was no detectable change in NDV hemagglutination titer. Adsorption of hydroxylamine (pH 7.0) inactivated WSN appeared normal; however, the rate of adsorption was decreased when virus was inactivated by (pH 5.0) O-methylhydroxylamine. Equilibrium density gradient centrifugation in potassium tartrate showed no density changes in inactivated virus. WSN inactivated virus. WSN inactivated with 14C-O-methylhydroxylamine and subjected to RNA extraction showed greater than or equal to 35 percent of the 14C in the phenol phases and 21 per cent in the RNA. The 14C-O-methylhydroxylamine associated with the RNA of insensitive NDV was about 3 per cent of that associated with sensitive WSN-RNA. Hydroxylamine has no apparent effect on paramyxovirus (NDV) hemagglutination titer and less 14C-O-methylhydroxylamine is associated with the RNA of this virus. The results suggest these compounds may affect both the RNA and the envelope portion of myxoviruses (WSN) to produce inactivation.

Properties of Visna Virus Particles Harvested at Short Time Intervals: RNA Content, Infectivity, and Ultrastructure

The major RNA component of Visna virus harvested at short intervals of time (5 min) is not the 60 to 70 S RNA but a molecule of higher electrophoretic mobility. This RNA has been isolated and characterized. Its sedimentation coefficient is identical to that of 30 to 40 S RNA subunits obtained by heat denaturation of the 60 to 70 S RNA. In 1.8% acrylamide gels without agarose the electrophoretic mobility of 30 to 40 S RNA subunits present in rapidly harvested virus is slightly lower than that of the subunits obtained by denaturation of the 60 to 70 S RNA; after heat denaturation the mobilities are identical. These free RNA subunits present in early virus particles assemble into a 60 to 70 S RNA complex as shown by following the RNA content of early virus incubated at 37 C for various lengths of time. The rate of this maturation process is slow. There is no difference between the infectivity of immature and mature virus particles. Both particles have a dense core when examined in sections of virus pellets.

Relationship between the RNA polymerase activities from influenza virions and influenza virus-infected cells

Two RNA polymerase activities were characterized in the cytoplasm of influenza Ao/NWS infected cells. Their relationship to the virion-associated RNA polymerase was studied.

Characterization of C-type particles produced by a tissue culture-adapted murine myeloma

C-type particles produced by a tissue culture-adapted BALB/c myeloma were characterized. It was determined that although the particles were morphologically and antigenically similar to murine leukemia and sarcoma virus, the size of their RNA was different, they lacked RNA-dependent DNA polymerase, they were unstable in NET buffer, sucrose and citrate but were stable in glycerol and Earle balanced salt solution, and they behaved differently from oncornaviruses when treated with ether and detergent.

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.

Minor base composition of "70S-associated" 4S RNA from avian myeloblastosis virus

Using a chemical isotope derivative method, we have determined the minor base composition of the "70S-associated" 4S RNA isolated from avian myeloblastosis virus. The minor base composition of this "70S-associated" 4S RNA is strikingly similar to that of the corresponding "free" 4S RNA of the virion. This minor base content, plus the capacity to esterify amino acids, establishes that both of these virion 4S RNA fractions contain transfer RNA. The percentage of minor bases in virion "70S-associated" 4S RNA is, however, much lower than in "free" 4S virion RNA and in myeloblast 4S RNA. The implication is that the "70S-associated" 4S RNA fraction as isolated herein also contains RNA that is not transfer RNA. This latter RNA may represent either degradation products of high molecular weight RNA or indigenous 4S RNA of undetermined function.

The nucleic acid of infectious bronchitis virus

The nucleic acid of infectious bronchitis virus (IBV), like that of other enveloped viruses, consists of discontinuous single stranded RNA. However, unlike many other viruses, there is extreme heterogeneity in the sizes of the RNA fragments, as revealed by centrifugation in sucrose gradients or electrophoresis in polyacrylamide gels. Two principal classes of RNA fragments are present:

A larger class comprising 74.9–85.4 per cent of total RNA and consisting of fragments having molecular weights ranging from 0.5×10⁶ to considerably greater than 3.0×10⁶ daltons and,

A smaller class comprising 9.1–19.7 per cent of total RNA with the size approximately that of ribosomal 4S RNA.

All IBV RNA's were fully susceptible to ribonuclease and had a buoyant density in caesium sulphate identical to that of tobacco mosaic virus RNA. No difference in the RNA profile for IBV was observed from the use of different methods of virus purification. The single-stranded RNA's of poliovirus and tobacco mosaic virus remained undegraded after preparation in the presence of IBV.

Isolation and preliminary characterization of temperature-sensitive mutants of influenza virus

Isolation of temperature-sensitive (ts) mutants was attempted from the WSN strain of influenza A virus which was grown and assayed in MDBK cells. After growth of wild-type virus in the presence of 5-fluorouracil, 15 ts mutants were selected for which the ratio of plaquing efficiency at 39.5 C to that at 33 C was 10(-3) or less. In pairwise crosses of ts mutants, recombination and complementation were either very efficient or undetectable. It is suggested, therefore, that the viral genome consists of physically discrete units and recombination occurs as an exchange of these units. All 15 mutants have been assigned with certainty into five recombination groups. Three mutants are suspected to be double mutants. Any two complementing mutants always recombined with each other, and noncomplementing mutants did not recombine. In physiological tests, mutants showed diverse patterns of functional defects at the nonpermissive temperature. However, it was not always possible to correlate these physiological defects with the results of genetic characterization.

Structure of the ribonucleoprotein of influenza virus

The ribonucleoprotein (RNP) internal components of influenza virus were separated into distinct size classes by sedimentation in glycerol gradients and examined by electron microscopy by using positive staining with uranyl acetate. The large RNP have a peak in length distribution at 90 to 110 nm, the medium, at 60 to 90 nm, and the small, at 30 to 50 nm. These lengths can be correlated with the estimated molecular weights of the ribonucleic acids contained in the various RNP size classes. The RNP structure appears to consist of a strand which is folded back on itself and coiled in a regular double-helical arrangement.