Recombination and complementation between temperature-sensitive mutants of a Bunyavirus, snowshoe hare virus

Whole Genome Sequencing Analysis of African Orthobunyavirus Isolates Reveals Naturally Interspecies Segments Recombinations between Bunyamwera and Ngari Viruses

Bunyamwera virus is the prototype of the Bunyamwera serogroup, which belongs to the order Bunyavirales of the Orthobunyavirus genus in the Peribunyaviridae family. Bunyamwera is a negative-sense RNA virus composed of three segments S, M, and L. Genetic recombination is possible between members of this order as it is already documented. Additionally, it can lead to pathogenic or host range improvement, if it occurs with viruses of public health and agricultural importance such as Rift Valley fever virus and Crimea-Congo hemorrhagic fever virus. Here, we characterize five African Orthobunyavirus viruses from different geographical regions. Our results suggest that the five newly characterized strains are identified as Bunyamwera virus strains. Furthermore, two of the five strains sequenced in this study are recombinant strains, as fragments of their segments are carried by Ngari and Bunyamwera strains. Further investigations are needed to understand the functional impact of these recombinations.

A Review of Bunyamwera, Batai, and Ngari Viruses: Understudied Orthobunyaviruses With Potential One Health Implications

Bunyamwera (BUNV), Batai (BATV), and Ngari (NRIV) are mosquito-borne viruses of the Bunyamwera serogroup in the Orthobunyavirus genus of the Bunyaviridae family. These three viruses have been found to cause disease in both livestock animals, avian species, and humans. Thus, these viruses pose a potential threat to human public health, animal health, and food security. This is especially the case in the developing nations, where BUNV and NRIV are found, mainly in Africa. BUNV and BATV are fairly well characterized, while NRIV is not well characterized owing to only sporadic detection in human and animal populations in Africa. Reassortment is common among bunyaviruses, but NRIV is believed to be the only natural reassortant of the Bunyamwera serogroup. It resulted from a combination of BUNV S and L segments and the BATV M segment. This indicates at least some level co-circulation of BUNV and BATV, which have no historically been reported to overlap in geographic distributions. But as these viruses are undercharacterized, there remains a gap in the understanding of how such reassortment could occur, and the consequences of such. Due to their combined wide range of hosts and vectors, geographic distributions, potential severity of associated diseases, and potential for transmissibility between vertebrate hosts, these viruses represent a significant gap in knowledge with important One Health implications. The goal of this review is to report available knowledge of and identify potential future directions for study of these viruses. As these are collectively understudied viruses, there is a relative paucity of data; however, we use available studies to discuss different perspectives in an effort to promote a better understanding of these three viruses and the public and One Health threat(s) they may pose.

Temperature-sensitive mutations for live-attenuated Rift Valley fever vaccines: implications from other RNA viruses

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease endemic to the African continent. RVF is characterized by high rate of abortions in ruminants and hemorrhagic fever, encephalitis, or blindness in humans. RVF is caused by the Rift Valley fever virus (RVFV: genus Phlebovirus, family Bunyaviridae). Vaccination is the only known effective strategy to prevent the disease, but there are no licensed RVF vaccines available for humans. A live-attenuated vaccine candidate derived from the wild-type pathogenic Egyptian ZH548 strain, MP-12, has been conditionally licensed for veterinary use in the U.S. MP-12 displays a temperature-sensitive (ts) phenotype and does not replicate at 41°C. The ts mutation limits viral replication at a specific body temperature and may lead to an attenuation of the virus. Here we will review well-characterized ts mutations for RNA viruses, and further discuss the potential in designing novel live-attenuated vaccines for RVF.

Genomic and phylogenetic characterization of viruses included in the Manzanilla and Oropouche species complexes of the genus Orthobunyavirus, family Bunyaviridae

A thorough characterization of the genetic diversity of viruses present in vector and vertebrate host populations is essential for the early detection of and response to emerging pathogenic viruses, yet genetic characterization of many important viral groups remains incomplete. The Simbu serogroup of the genus Orthobunyavirus, family Bunyaviridae, is an example. The Simbu serogroup currently consists of a highly diverse group of related arboviruses that infect both humans and economically important livestock species. Here, we report complete genome sequences for 11 viruses within this group, with a focus on the large and poorly characterized Manzanilla and Oropouche species complexes. Phylogenetic and pairwise divergence analyses indicated the presence of high levels of genetic diversity within these two species complexes, on a par with that seen among the five other species complexes in the Simbu serogroup. Based on previously reported divergence thresholds between species, the data suggested that these two complexes should actually be divided into at least five species. Together these five species formed a distinct phylogenetic clade apart from the rest of the Simbu serogroup. Pairwise sequence divergences among viruses of this clade and viruses in other Simbu serogroup species complexes were similar to levels of divergence among the other orthobunyavirus serogroups. The genetic data also suggested relatively high levels of natural reassortment, with three potential reassortment events present, including two well-supported events involving viruses known to infect humans.

Identification of two severe fever with thrombocytopenia syndrome virus strains originating from reassortment

Recently, a novel bunyavirus, severe fever with thrombocytopenia syndrome virus (SFTSV), was isolated in central China. The virus can cause multi-clinical symptoms: severe fever, thrombocytopenia, leukocytopenia, with a mortality rate of ~10%. Several studies show that SFTSV could undergo rapid evolution via gene mutation and homologous recombination. However, as an important evolutionary force for segmented-genome viruses, reassortment has not been reported in SFTSV. In this study, we identified two SFTSV strains of which the S segment has different origin from M and L, suggesting that reassortment might be potential force driving rapid change of SFTSV. This result might shed new light on the evolutionary behavior of the novel virus.

A natural M RNA reassortant arising from two species of plant- and insect-infecting bunyaviruses and comparison of its sequence and biological properties to parental species

Reassortment allows multicomponent viruses to exchange genome segments, a process well-documented in the vertebrate- and arthropod-infecting members of the family Bunyaviridae but not between distinct species of the plant- and insect-infecting members of the genus Tospovirus. Genome sequence comparisons of a virus causing severe tospovirus-like symptoms in Florida tomato with Groundnut ringspot virus (GRSV) and Tomato chlorotic spot virus (TCSV) demonstrated that reassortment has occurred, with the large (L) and small (S) RNAs coming from GRSV and the medium (M) RNA coming from TCSV (i.e. L(G)M(T)S(G)). Neither parental genotype is known to occur in the U.S. suggesting that L(G)M(T)S(G) was introduced as a reassortant. L(G)M(T)S(G) was transmitted by western flower thrips (Frankliniella occidentalis [Pergande]), and was not able to overcome the Sw5 resistance gene of tomato. Our demonstration of reassortment between GRSV and TCSV suggests caution in defining species within the family Bunyaviridae based on their ability to reassort.

Chapter 5. In vivo analysis of the decay of transcripts generated by cytoplasmic RNA viruses

The field of RNA decay has grown extensively over the last few years and numerous decay pathways have been identified and characterized. This is a truly powerful machinery for both regulation and quality control of gene expression. It is very likely that the transcripts of RNA viruses must successfully confront this arsenal of enzymes and RNA binding factors in order to establish a productive infection. This interface is an understudied branch of virology that needs to be explored if we are to fully comprehend the molecular biology of virus-cell interactions. Research in this area has the potential to increase our understanding of the fundamentals of both mRNA stability and viral biology, perhaps leading to novel antiviral approaches. This chapter discusses methods for examining the half-lives of viral RNAs during natural infection, including purification of the viral transcripts and subsequent analysis of both deadenylation and decay. Additionally, a hybrid selection protocol for identifying viral-specific small RNAs that are generated during infection by the RNAi branch of the cellular RNA decay machinery is described.

Batai and Ngari viruses: M segment reassortment and association with severe febrile disease outbreaks in East Africa

Ngari virus is an orthobunyavirus recently recognized as a reassortant between Bunyamwera virus and an as yet unidentified M segment donor. Analysis of M segment sequences of Batai and Ilesha viruses revealed 95% deduced amino acid identity between Batai virus and Ngari virus. These findings suggest Batai virus as the donor of Ngari virus M segment sequence. Analysis of Batai virus-related African isolates identified UgMP-6830, isolated from mosquitoes in Uganda, as an isolate of Batai virus. KV-141, isolated during a febrile disease outbreak in Sudan, was identified as another isolate of Ngari virus, emphasizing a role of this reassortant virus in severe human illness throughout East Africa.

Analysis of the medium (M) segment sequence of Guaroa virus and its comparison to other orthobunyaviruses

Guaroa virus (GROV), a segmented virus in the genus Orthobunyavirus, has been linked to the Bunyamwera serogroup (BUN) through cross-reactivity in complement fixation assays of S segment-encoded nucleocapsid protein determinants, and also to the California serogroup (CAL) through cross-reactivity in neutralization assays of M segment-encoded glycoprotein determinants. Phylogenetic analysis of the S-segment sequence supported a closer relationship to the BUN serogroup for this segment and it was hypothesized that the serological reaction may indicate genome-segment reassortment. Here, cloning and sequencing of the GROV M segment are reported. Sequence analysis indicates an organization similar to that of other orthobunyaviruses, with genes in the order GN-NSm-Gc, and mature proteins generated by protease cleavage at one, and by signalase at possibly three, sites. A potential role of motifs that are more similar to CAL than to BUN virus sequences with respect to the serological reaction is discussed. No discernable evidence for reassortment was identified.

Genetic variability: the key problem in the prevention and therapy of RNA-based virus infections

Despite extraordinary progress that has recently been made in biomedical sciences, viral infectious diseases still remain one of the most serious world health problems. Among the different types of viruses, those using RNA as their genetic material (RNA viruses and retroviruses) are especially dangerous. At present there is no medicine allowing an effective treatment of RNA-based virus infections. Many RNA viruses and retroviruses need only a few weeks to escape immune response or to produce drug-resistant mutants. This seems to be the obvious consequence of the unusual genetic variability of RNA-based viruses. An individual virus does not form a homogenous population but rather a set of similar but not identical variants. In consequence, RNA-based viruses can easily adapt to environmental changes, also those resulting from immune system response or therapy. The modifications identified within viral genes can be divided into two groups: point mutations and complex genome rearrangements. The former arises mainly during error-prone replication, whereas RNA recombination and generic reassortment are responsible for the latter. This article shortly describes major strategies used to control virus infections. Then, it presents the various mechanisms generating the genetic diversity of RNA-based viruses, which are most probably the main cause of clinical problems.

Molecular and biochemical studies of the evolution, infection and transmission of insect bunyaviruses

Members of the Bunyaviridae family of RNA viruses (bunyaviruses, hantaviruses, nairoviruses, phleboviruses and uukuviruses) have been studied at the molecular and genetic level to understand the basis of their evolution and infection in vertebrate and invertebrate (arthropod) hosts. With the exception of the hantaviruses, these viruses infect and are transmitted by a variety of blood-sucking arthropods (mosquitoes, phlebotomines, gnats, ticks, etc.). The viruses are responsible for infection of various vertebrate species, occasionally causing human disease, morbidity and mortality (e.g. Rift Valley fever, Crimean-Congo haemorrhagic fever, Korean haemorrhagic fever). Genetic and molecular analyses of bunyaviruses have established the coding assignments of the three viral RNA species and documented which viral gene products determine host range and virulence. Ecological studies, with molecular techniques, have provided evidence for bunyavirus evolution in nature through genetic drift (involving the accumulation of point mutations) and shift (RNA-segment reassortment).

Bunyavirus-vector interactions

Recent advances in the genetics and molecular biology of bunyaviruses have been applied to understanding bunyavirus-vector interactions. Such approaches have revealed which virus gene and gene products are important in establishing infections in vectors and in transmission of viruses. However, much more information is required to understand the molecular mechanisms of persistent infections of vectors which are lifelong but apparently exert no untoward effect. In fact, it seems remarkable that LAC viral antigen can be detected in almost every cell in an ovarian follicle, yet no untoward effect on fecundity and no teratology is seen. Similarly the lifelong infection of the vector would seem to provide ample opportunity for bunyavirus evolution by genetic drift and, under the appropriate circumstances, by segment reassortment. The potential for bunyavirus evolution by segment reassortment in vectors certainly exists. For example the Group C viruses in a small forest in Brazil seem to constitute a gene pool, with the 6 viruses related alternately by HI/NT and CF reactions, which assay respectively M RNA and S RNA gene products (Casals and Whitman, 1960; Shope and Causey, 1962). Direct evidence for naturally occurring reassortant bunyaviruses has also been obtained. Oligonucleotide fingerprint analyses of field isolates of LAC virus and members of the Patois serogroup of bunyaviruses have demonstrated that reassortment does occur in nature (El Said et al., 1979; Klimas et al., 1981; Ushijima et al., 1981). Determination of the genotypic frequencies of viruses selected by the biological interactions of viruses and vectors after dual infection and segment reassortment is an important issue. Should a virus result that efficiently interacts with alternate vector species, the virus could be expressed in different circumstances with serious epidemiologic consequences. Dual infection of vectors with different viruses is not unlikely, because many bunyaviruses are sympatric in nature. For example, the Ae. trivittatus-cottontail rabbit and the Ae. triseriatus-squirrel arbovirus cycles are sympatric in the ecotone between their respective grassland and forest ecosystems (LeDuc, 1979). Should a LaCrosse virus variant or reassortant evolve that was efficiently vectored by Ae. trivittatus mosquitoes, significantly more human infections with La Crosse virus would likely occur. Unlike Ae. triseriatus, Ae. trivittatus mosquitoes are not restricted to forested areas and consequently are more likely to encounter and to feed upon humans.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative pathogenicity and antigenic cross-reactivity of Rift Valley fever and other African phleboviruses in sheep

Homologous and heterologous haemagglutination-inhibition (HAI), complement-fixation (CF), immunodiffusion (ID) and mouse neutralization tests were performed with the Lunyo (LUN) and a Zimbabwean strain of Rift Valley fever (RVF) virus, the prototype and a South African strain of Arumowot (AMT) virus and prototype strains of Gordil (GOR), Saint-Floris (SAF) and Gabek Forest (GF) viruses, using immune mouse ascitic fluids prepared against these viruses. Reactions of identity occurred in all tests between LUN and the Zimbabwean strains of RVF and between the two strains of AMT virus. Otherwise, cross-reactions occurred between all the phleboviruses in HAI tests, while reactions in CF, ID and neutralization tests were monospecific for virus serotypes, except that weak cross-reaction occurred between GOR and SAF viruses in CF and ID tests. Four sheep infected subcutaneously with the Zimbabwean strain of RVF virus developed transient fever, viraemia, leucopaenia, relative thrombocytopaenia, haemoconcentration and raised serum enzyme levels, which indicated that the sheep had developed necrotic hepatitis. Disseminated focal necrotic hepatitis was confirmed in a sheep killed for examination on day 4 post-infection. The other three sheep recovered uneventfully after only mild depression and anorexia. Groups of three sheep infected with SAF, GOR, AMT and GF viruses had no demonstrable viraemia or other sign of infection or illness, except that the sheep infected with AMT developed mild fever lasting less than 24 h. Antibody responses were monitored at intervals over a period of 24 weeks in all sheep by homologous and heterologous HAI, CF and cell culture neutralization (CPENT) tests. Homologous antibody responses were marked in the RVF-infected sheep and their sera cross-reacted strongly in HAI tests with antigens of the other viruses. The sera of the RVF-infected sheep cross-reacted less markedly in CF and CPENT tests. Homologous antibody responses were poor in all the sheep infected with phleboviruses other than RVF, and the cross-reactivity of their sera for RVF antigen or virus was negligible. All sheep were challenged with RVF virus 48 weeks after their initial infection. The sheep which had originally been infected with RVF virus were immune and developed neither fever nor viraemia. All other sheep developed fever, viraemia and antibodies to RVF virus. It was concluded that the African phleboviruses, other than RVF, are unlikely to cause disease in livestock or to induce antibodies which could cause confusion in the diagnosis of RVF.

Virulence of La Crosse virus is under polygenic control

To identify which RNA segments of the California serogroup bunyaviruses determine virulence, we prepared reassortant viruses by coinfecting BHK-21 cells with two wild-type parents, La Crosse/original and Tahyna/181-57 viruses, which differed about 30,000-fold in virulence. The progeny clones were screened by polyacrylamide gel electrophoresis to ascertain the phenotype of the M and S RNA segments, and RNA-RNA hybridization was used to determine the genotype of selected clones. Two or three clones of each of the six possible reassortant genotypes were characterized quantitatively for neuroinvasiveness by determining the PFU/50% lethal dose (LD50) ratio after subcutaneous injection into suckling mice. The reassortants fell into two groups. (i) Six of seven reassortants with a La Crosse M RNA segment were as virulent as the parent La Crosse virus (about 1 PFU/LD50); the one exception was strikingly different (about 1,000 PFU/LD50) and probably represents a spontaneous mutant. (ii) The seven reassortants with a Tahyna M RNA segment were about 10-fold more virulent than the parent Tahyna virus (median 1,600 PFU/LD50 for reassortants and 16,000 PFU/LD50 for Tahyna virus). A comparative pathogenesis study in suckling mice of one reassortant virus and the parent Tahyna virus confirmed the greater neuroinvasiveness of the reassortant virus. From these data it was concluded that the M RNA segment was the major determinant of virulence, but that the other two gene segments could modulate the virulence of a nonneuroinvasive California serogroup virus.

Ambisense RNA genomes of arenaviruses and phleboviruses

This chapter reviews the evidence that shows that arenaviruses and members of one genus of the Bunyaviridae (phleboviruses) have some proteins coded in subgenomic, viral-sense mRNA species and other proteins coded in subgenomic, viral-complementary mRNA sequences. This unique feature is discussed in relation to the implications it has on the intracellular infection process and how such a coding arrangement may have evolved. The chapter presents a list of the known members of the arenaviridae, their origins, and the vertebrate hosts from which isolates have been reported. It discusses the structural components, the infection cycle, and genetic attributes of arenaviruses. In order to determine how arenaviruses code for gene products, the S RNA species of Pichinde virus and that of a viscerotropic strain of LCM virus (LCM-WE) have been cloned into DNA and sequenced. The arenavirus S RNA is described as having an ambisense strategy, to denote the fact that both viral and viral-complementary sequences are used to make gene products. The chapter discusses the infection cycle, the structural and genetic properties of bunyaviridae member.

Interference between bunyaviruses in Aedes triseriatus mosquitoes

Inhibition of the replication of alternate California serogroup bunyaviruses in Aedes triseriatus mosquitoes has been observed for mosquitoes previously infected with La Crosse (LAC) virus. By contrast, prior infection of mosquitoes with LAC virus did not interfere significantly with the subsequent infection and replication of Guaroa bunyavirus (Bunyamwera serogroup), or heterologous viruses such as West Nile flavivirus, or vesicular stomatitis rhabdovirus.

Nucleotide sequence analyses and predicted coding of bunyavirus genome RNA species

We performed 3' RNA sequence analyses of [(32)P]pCp-end-labeled La Crosse (LAC) virus, alternate LAC virus isolate L74, and snowshoe hare bunyavirus large (L), medium (M), and small (S) negative-stranded viral RNA species to determine the coding capabilities of these species. These analyses were confirmed by dideoxy primer extension studies in which we used a synthetic oligodeoxynucleotide primer complementary to the conserved 3'-terminal decanucleotide of the three viral RNA species (Clerx-van Haaster and Bishop, Virology 105:564-574, 1980). The deduced sequences predicted translation of two S-RNA gene products that were read in overlapping reading frames. So far, only single contiguous open reading frames have been identified for the viral M- and L-RNA species. For the negative-stranded M-RNA species of all three viruses, the single reading frame developed from the first 3'-proximal UAC triplet. Likewise, for the L-RNA of the alternate LAC isolate, a single open reading frame developed from the first 3'-proximal UAC triplet. The corresponding L-RNA sequences of prototype LAC and snowshoe hare viruses initiated open reading frames; however, for both viral L-RNA species there was a preceding 3'-proximal UAC triplet in another reading frame that was followed shortly afterward by a termination codon. A comparison of the sequence data obtained for snowshoe hare virus, LAC virus, and the alternate LAC virus isolate showed that the identified nucleotide substitutions were sufficient to account for some of the fingerprint differences in the L-, M-, and S-RNA species of the three viruses. Unlike the distribution of the L- and M-RNA substitutions, significantly fewer nucleotide substitutions occurred after the initial UAC triplet of the S-RNA species than before this triplet, implying that the overlapping genes of the S RNA provided a constraint against evolution by point mutation. The comparative sequence analyses predicted amino acid differences among the corresponding L-, M-, and S-RNA gene products of snowshoe hare virus and the two LAC virus isolates.

Molecular basis of bunyavirus transmission by mosquitoes: role of the middle-sized RNA segment

In an examination of the molecular basis of oral transmission of bunyaviruses by mosquitoes., La Crosse (LAC), snowshoe hare (SSH), and LAC-SSH reassortant viruses were compared in their ability to be transmitted to laboratory mice by the natural mosquito vector of LAC virus, Aedes triseriatus. Both LAC virus and the reassortment viruses containing the middle-sized (M) segment from the LAC parent were efficiently transmitted. In contrast, SSH virus and reassortment viruses containing the M RNA from the SSH parent were inefficiently transmitted. Thus, the M RNA segment, which codes for the virion glycoproteins, may be a major determinant of oral transmission of bunyaviruses by mosquitoes.

Genetic interactions among viruses of the Bunyamwera complex

Seventy-seven temperature-sensitive (ts) mutants belonging to three antigenically distinct and geographically isolated members of the Bunyamwera complex--Batai virus, Bunyamwera virus, and Maguari virus--have been isolated after 5-fluorouracil treatment. High-frequency recombination was observed, and the mutants of each virus were classified into two groups, which were shown to be equivalent by heterologous recombination experiments. In most combinations heterologous recombination was less efficient than homologous recombination, but all crosses of group I and II mutants yielded viable recombinants. Recombination was an early event. Analysis by polyacrylamide gel electrophoresis of the proteins of the wild-type viruses and recombinant clones obtained from the six possible heterologous combinations of group I and II mutants indicated that recombination occurred by reassortment of genome subunits. Group I appeared to correspond to the genome subunit coding for the N protein, and group II corresponded to the G1/G2 determinant. The G1 (or G2 or both) protein was associated with neutralization specificity and plaque diameter, and the N protein was associated with plaque opacity. Complementation was observed between two nonrecombining mutants of Maguari virus belonging to group I, which may indicate that the N genome subunit codes for an additional protein. There appeared to be no genetic barrier to exchange of genetic material between Batai, Bunyamwera, and Maguari viruses in vitro, and it is concluded that the Bunyamwera complex is potentially a single gene pool if geographical and ecological constraints are discounted.

Genome complexities of the three mRNA species of snowshoe hare bunyavirus and in vitro translation of S mRNA to viral N polypeptide

The genome complexities of the principal intracellular viral complementary RNA species of the snowshoe hare bunyavirus have been analyzed by duplex analyses involving hybridization of complementary RNA to individual 32P-labeled viral RNA species (large, L; medium, M; and small, S), recovery of nuclease-resistant duplexes, and determination of the oligonucleotide fingerprints of the protected 32P-labeled viral sequences. The result for the M RNA (which codes for the glycoproteins G1 and G2; J. R. Gentsch and D. H. L. Bishop, J. Virol. 30:767-770, 1979) indicates that there is a single polycistronic M mRNA. Similar results were obtained for the L and S RNA species. In vitro translation studies with the S complementary RNA species of snowshoe hare virus as well as melted purified S duplexes substantiate earlier genetic and molecular studies (J. R. Gentsch and D. H. L. Bishop, J. Virol. 28:417-419, 1978; J. Gentsch, D. H. L. Bishop, and J. F. Obijeski, J. Gen. Virol. 34-257-268, 1977), which indicate that S mRNA codes for the virion nucleocapsid protein N.

Recombination between snowhoe hare and La Crosse bunyaviruses

We have previously reported heterologous genetic recombination resulting from crosses involving temperature-sensitive (ts) mutants of La Crosse (LAC) group II and snowshoe hare (SSH) group I ts mutants (J. Gentsch, L. R. Wynne, J. P. Clewley, R. E. Shope, and D. H. L. Bishop, J. Virol. 24:893-902, 1977). From those crosses two reassortant viruses having the large/medium/small viral RNA segment genotypes of SSH/LAC/SSH and SSH/LAC/LAC were obtained. In this study it has been found that the reciprocal cross (SSH group II x LAC group I ts mutants) has not yielded the expected LAC/SSH/SSH or LAC/SSH/LAC reassortant viruses. The backcross of a SSH/LAC/SSH group II ts mutant with a LAC group I ts mutant has produced a new reassortant virus, LAC/LAC/SSH, whereas the backcross of SSH/LAC/LAC group I ts mutants with SSH group II ts mutants gave another reassortant, SSH/SSH/LAC. Backcross analyses of LAC/LAC/SSH group I ts mutants with Group II ts mutants of SSH have not yielded the expected LAC/SSH/SSH reassortant virus, nor have backcrosses of SSH/SSH/LAC group II ts mutants with group I ts mutants of LAC virus yielded the expected LAC/SSH/LAC reassortant. Possible reasons why certain reassortant viruses are not produced are discussed. A procedure to screen SSH-LAC reassortant viruses which differ in their virion N polypeptides is described.

In vivo transcription and protein synthesis capabilities of bunyaviruses: wild-type snowshoe hare virus and its temperature-sensitive group I, group II, and group I/II mutants

The in vivo primary and secondary transcription capabilities of wild-type snowshoe hare (SSH) virus and certain of its temperature-sensitive (ts) mutants have been analyzed. The results obtained agree with in vitro studies (Bouloy et al., C.R. Acad. Sci. Paris 280:213-215, 1975; M. Bouloy and C. Hannoun, Virology 69:258-264, 1976; M. Ranki and R. Pettersson, J. Virol. 16:1420-1425, 1975) which have shown that bunyaviruses are negative-stranded RNA viruses with a virion RNA-directed RNA polymerase. The in vivo transcription studies have demonstrated that in the presence of protein synthesis inhibitors (puromycin or cycloheximide) SSH virus can synthesize viral complementary RNA (primary transcription) throughout the infection cycle. The increased levels of viral complementary RNA obtained in the absence of protein synthesis inhibitors (secondary transcription) were not markedly reduced if cells were pretreated with actinomycin D (5 mug/ml), alpha-amanitin (25 mug/ml), or rifampin (100 mug/ml), although progeny virus yields were reduced by up to 80% in the actinomycin D- and rifampin-treated cells. The in vivo transcription capabilities of SSH group I ts mutants at temperatures which were nonpermissive (40 degrees C) for virus replication gave values comparable to those obtained at permissive temperatures (33 degrees C). The SSH group I mutants appear, therefore, to be RNA-positive mutant types. When compared with their transcription capabilities at 33 degrees C, the in vivo transcription abilities of four SSH group II ts mutants (and one double group I/II ts mutant) were found to be more impaired at 40 degrees C than those of the SSH group I ts mutants or wild-type SSH virus at 40 degrees C, although the viral complementary RNA synthetic capabilities of these group II (and group I/II) mutants at 40 degrees C were significantly higher than their primary transcription capabilities (as measured at 33 degrees C in the presence of puromycin or cycloheximide). It was concluded, therefore, that these SSH group II (and double group I/II) ts mutants have an intermediate RNA phenotype. Hybridization studies using (32)P-labeled individual L, M, and S viral RNA species of SSH virus have demonstrated the presence of viral complementary RNA to all three species in extracts of cells infected with SSH ts II-30 and incubated at 33 degrees C (primary and secondary transcription) or 40 degrees C, a nonpermissive temperature for its replication. The results of pulse-labeled in vivo protein analyses indicated that greater quantities of intracellular N protein (coded for by S RNA [J. R. Gentsch and D. H. L. Bishop, J. Virol. 28:417-419, 1978]) than G1 and G2 polypeptides (coded for by M RNA [J. R. Gentsch and D. H. L. Bishop, J. Virol. 30:767-776, 1979]) were present in extracts of cells infected with wild-type SSH virus. In extracts of SSH group I, II, or I/II ts mutant-infected cells incubated at 33 degrees C, N and G1, and for the group II mutant-infected cells, G2, viral polypeptides were detected, whereas in extracts obtained from group I or II mutant virus-infected cells incubated at 40 degrees C, low levels of N and G1 polypeptides were evident.

M viral RNA segment of bunyaviruses codes for two glycoproteins, G1 and G2

Tryptic peptide digests of the two viral glycoproteins (G1 and G2) of snowshow hare (SSH) virus, La Crosse, La Crosse (LAC) virus, and an SSH/LAC recombinant virus which has a large (L)/medium (M)/small (S) RNA segment genome composition of SSH/LAC/SSH were analyzed by ion-exchange column chromatography. The analyses prove that the M RNA species of bunyaviruses codes for the two viral glycoproteins.

Biochemical studies on the Phlebotomus fever group viruses (Bunyaviridae family)

Analyses of the virion polypeptides and genomes of several Phlebotomus fever group viruses, Karimabad, Punta Toro, Chagres, and the sandfly fever Sicilian serotype viruses, have established that they are biochemically similar to the accepted members of the Bunyaviridae family. Like snowshoe hare virus (a member of the California serogroup of the Bunyavirus genus of the Bunyaviridae family), Karimabad, Punta Toro, Chagres, and the sandfly fever Sicilian serotype viruses all have three viral RNA species, designated large (L), medium (M), and small (S). Oligonucleotide fingerprint analyses of Karimabad and Punta Toro virus RNA species indicated that their L, M, and S RNA species are unique. By polyacrylamide gel electrophoresis it was determined for Karimabad virus that the apparent molecular weights of its L, M, and S RNA species are 2.6 X 10(6), 2.2 X 10(6), and 0.8 X 10(6), respectively. For Punta Toro virus, the apparent molecular weights of its L, M, and S RNA species are 2.8 X 10(6), 1.8 X 10(6), and 0.75 X 10(6), respectively. The major internal nucleocapsid (N) protein of Karimabad virus was found to have a molecular weight of 21 X 10(3). A similar polypeptide size class was identified in preparations of sandfly fever Sicilian serotype, Chagres, and Punta Toro viruses. The Karimabad virus glycoproteins formed the external surface projections on virus particles and could be removed from virus preparations by protease treatment. The glycoproteins in an unreduced sample could be resolved into two size classes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. They had apparent molecular weights of 62 X 10(3) and 50 X 10(3) in continuous polyacrylamide gels. When Karimabad virus preparations were reduced with 1% beta-mercaptoethanol, prior to resolution by continuous polyacrylamide gel electrophoresis, all the viral glycoprotein was recovered in a single size class, having an apparent molecular weight of 62 X 10(3). Two or three major virion polypeptides have been identified in preparations of Punta Toro, Chagres, and sandfly fever Sicilian serotype viruses.

The Rift Valley fever epizootic in Egypt 1977-78. 1. Description of the epizzotic and virological studies

From October to December 1977, an extensive epizootic occurred in Egypt resulting in abortions and increased mortality in domestic animals, and severe clinical disease with fatalities in man. Rift Valley Fever (RVF) virus was isolated and identified as the causative agent. In humans, acute febrile, encephalitic, ocular and fatal haemorrhagic diseases were documented as resulting from RVF virus infection. A retrospective serological survey indicated RVF was recently introduced into the area. The 1977 epizootic extensively involved five Governorates. In the summer and autumn of 1978, epizootic RVF reappeared in Egypt and spread to previously uninfected areas. Virological, serological and epidemiological studies, and factors related to the spread of RVF are discussed.

Small viral RNA segment of bunyaviruses codes for viral nucleocapsid protein

Tryptic peptide analyses have been undertaken on the nucleocapsid (N) protein of snowshoe hare (SSH) and La Crosse (LAC) bunyaviruses. Similar analyses have been performed on the N proteins of two recombinant viruses which have the large/medium/small RNA genome configurations: SSH/LAC/LAC and SSH/LAC/SSH. The results provide conclusive evidence that the S RNA of bunyaviruses codes for the the viral N protein.

Formation of recombinants between snowshoe hare and La Crosse bunyaviruses

Wild-type recombinants were obtained at high frequency from coinfections of BHK cells involving temperature-sensitive, conditional-lethal mutants of snowshoe hare (SSH) and La Crosse (LAC) bunyaviruses. Analyses of two of the recombinants indicated that they have the genome compositions SSH/LAC/SSH and SSH/LAC/LAC for their respective L, M, and S virion RNA species. This evidence, together with that for the genetic stability of the recombinants, indicates that they were derived by segment reassortment of the competent genome pieces of the parental viruses. The SSH/LAC/SSH recombinant appears, from polypeptide analysis, to have the SSH type of nucleocapsid protein (N), whereas the SSH/LAC/LAC recombinant has the LAC nucleocapsid protein, suggesting that the viral S RNA codes for the N protein.

Recombination between temperature-sensitive mutants of the arenavirus Pichinde

High-frequency recombination was obtained with temperature-sensitive, conditionally lethal mutants of the arenavirus Pichinde.

Bunyamwera virus-induced polypeptide synthesis

Bunyamwera virus-induced polypeptide synthesis in BSC-1 cell has been studied using polyacrylamide gel electrophoresis and autoradiography. Four virus-induced polypeptides were identified. Their molecular weights were 200 X 10(6) (L), 128 X 10(6) (G1), 31 X 10(6) (G2), and 23 X 10(6) (N). Pulse-chase experiments, short labeling experiments, and experiments using amino acid analogs failed to show evidence of polypeptides processing by proteolytic cleavage. Analysis of the kinetics of synthesis of these polypeptides showed that a clear division into early and late categories could be made, the onset of synthesis of polypeptide N and L rapidly reached a peak and then declined. Polypeptides G1 and G2 were made for several hours; their rate of synthesis then declined. All four polypeptides then continued to be made in relatively small amounts for many hours.

The genome of Uukuniemi virus consists of three unique RNA segments

The three RNA species isolated from virions of Uukuniemi virus, a proposed member of the newly defined Bunyaviridae family, have been characterized by analysis of 32P-labeled ribonuclease T1 oligonucleotides separated on two-dimensional polyacrylamide gels. Each species contains unique oligonucleotides not present in the two others, indicating that the genome of this virus is segmented. Each segment appears to contain a unique primary sequence with little or no overlapping among the segments. The complexities of the RNA segments as calculated from the radioactivity in unique oligonucleotides of defined lengths are about 8000 (L RNA), 3500 (M) and 1900 (S) nucleotides. Since these values are similar to the molecular weights determined by other methods, each size class of RNA corresponds to a single molecular species. The presence of a 5' terminal pppAp ... structure in each RNA segment confirms indications from electron microscopy that the apparently circular RNA segments are not covalently closed. The absence of either a 5' terminal "cap" or 3' terminal poly(A) supports the concept that Uukuniemi virus is a negative strand virus.

Three unique viral RNA species of snowshoe hare and La Crosse bunyaviruses

Two-dimensional gel electrophoreses of RNase T1-derived oligonucleotides of the three individual RNA segments of the bunyavirus snowshow hare virus indicate that its three RNA segments possess distinct nucleotide sequences. The fingerprints of the RNA species of snowshoe hare virus differ from those of the antigenically closely related La Crosse virus. Three viral RNA species have been identified in preparations of Melao and Trivittatus as well as snowshoe hare, Lumbo, and La Crosse bunyaviruses.

Segmented genome and nucleocapsid of La Crosse virus

La Crosse (LAC) virions purified by velocity and equilibrium gradient centrifugation contained three single-stranded RNA species. The three segments had sedimentation coefficients of 31S, 25S, and 12S by sodium dodecyl sulfate-sucrose gradient centrifugation. By comparison with other viral and cellular RNA species, the LAC viral RNAs had molecular weights of 2.9 x 10(6), 1.8 x 10(6), and 0.4 x 10(6). Phenol-sodium dodecyl sulfate-extracted LAC virion RNA was not infectious for BHK-21 cell cultures under conditions in which Sindbis viral RNA was infectious. Treatment of LAC virus with the nonionic detergent Triton X-100 and salt released three nucleocapsid structures, each containing one species of virion RNA. The nucleocapsids had sedimenation coefficients of 115S, 90S, and 65S. Negative-contrast electron microscopy of the nucleocapsids indicated that they were convoluted, supercoiled, and apparently circular. They had a mean diameter of 10 to 12 nm and modal lengths of 200, 510, and 700 nm (some were even longer). By chemical and enzymatic analysis of purified viral RNA, one type of 5' nucleotide (pppAp) present in the proportion of one per RNA segment was identified. After periodate oxidation, each virion RNA species was labeled by reduction with [3H]sodium borohydride. Taken together, these results suggest that although the nucleocapsids appear as closed loops, the viral RNA has free 5' and 3' ends and is, therefore, not circular.