RNA virus-like particles in pathogenic plant trypanosomatids

A double-stranded RNA (ds RNA) with an approximate size of 4.7 kb was found in 6 Phytomonas isolates specifically associated with plant pathogenicity in coconut trees ("Hartrot" disease) and oil palm ("Marchitez sorpressiva" disease). This ds RNA was not detected in 10 non-pathogenic Phytomonas isolates from different lactiferous plants or in the insect trypanosomatids Crithidia and Herpetomonas. Analysis by electron microscopy of a sucrose gradient fraction containing this ds RNA revealed virus-like particles.

Multipartite nature of potato virus X

Potato virus X (PVX) was among the first viruses to be purified. Nonetheless, properties of the purified virus remain contentious. The literature has been heavily influenced by the concept of a virus as a monopartite entity. Despite the fact that electron micrographs invariably show large proportions of shorter virus particles, the latter are universally ignored. Seven distinct classes of particle lengths were detected. Seven RNA species of approximate sizes 6.4, 3.6, 3.0, 2.1, 1.8, 1.4, and 0.9 kb were extracted from these purified virus preparations. This study shows clearly that shorter PVX particles are not breakage products and indicates that they may reflect fundamental properties of the genome strategy. Furthermore, other potexviruses have been found to contain many shorter particles, and the level of these particles is stable during purification. PVX is generally believed to consist of particles of single length even though the literature does not confirm this conclusion. The notion of a single particle length appears to reflect historical concepts of what a virus should be rather than what PVX is. This report considers whether shorter rods present in virus preparations of PVX are distinctive products of infection. The problem addressed is significant because if affects conclusions concerning the mechanisms of PVX biosynthesis and replication.

Isolation of a Lelystad virus-like agent from British pigs and scanning electron microscopy of infected macrophages

Six, one-week-old gnotobiotic piglets were inoculated with tissues or sera collected from field cases of porcine reproductive and respiratory syndrome. The piglets showed little or no illness, and two that were necropsied at 8 and 9 days post infection appeared grossly normal. However, a Lelystad virus-like agent was isolated from most of the inoculated pigs using porcine alveolar macrophage cultures. Seroconversion to the Lelystad virus was observed and some animals developed microscopically detectable interstitial pneumonias. Scanning electron microscopy was used to study the in vitro cytopathic effect of the Lelystad virus on porcine alveolar macrophages.

Characterization of swine infertility and respiratory syndrome (SIRS) virus (isolate ATCC VR-2332)

The characterization of an isolate of swine infertility and respiratory syndrome (SIRS) virus (ATCC VR-2332) is reported. A commercial cell line (CL2621) was used for the propagation of the virus for all assays. Laboratory studies indicate that this isolate is a fastidious, nonhemagglutinating, enveloped RNA virus. Cesium chloride-purified virions visualized by electron microscopy were spherical particles with an average diameter of 62 nm (range: 48-83 nm) and a 25-30 nm core surrounded by an envelope. Virus replication was restricted to the cytoplasm, as demonstrated by immunofluorescence. The virus did not react serologically with antisera to several common porcine viruses or with antisera to known viruses in the alphavirus, rubivirus, pestivirus, and ungrouped lactic dehydrogenase virus genera of the Togaviridae. However, convalescent sow sera and rabbit hyperimmune sera neutralized the SIRS virus at titers of 1:256 and 1:512, respectively. The virus was stable at 4 and -70 C, but was labile at 37 and 56 C. The properties of this isolate of SIRS virus resemble those of the family Togaviridae but do not match the described genera.

Serotype A and B strains of bean common mosaic virus are two distinct potyviruses

The serological relationships among strains of bean common mosaic virus (BCMV) (genus Potyvirus, family Potyviridae) were investigated by testing 13 isolates of the 10 known BCMV pathotypes with two monoclonal antibodies and six antisera to BCMV strains. In addition, other properties of serologically distinct BCMV strains were compared. Two groups of BCMV strains were obtained by ELISA and Western blot serology: serotype A contained the BCMV strains NL3, NL5, and NL8 and serotype B contained the BCMV strains NL1, NL2, NL4, NL6, US4, NL7, NY15, and Fla. SDS polyacrylamide gel electrophoresis and Western blotting of freshly purified preparations, and of extracts from leaves infected with eleven BCMV strains showed that the apparent molecular mass of the capsid protein of the serotype A isolates NL3, NL5, and NL8 are lower (about M(r) 33,000) than those of the serotype B isolates (M(r) 34,500 to 35,000). The normal lengths of the particles of the serotype A isolates were shorter (810-818 nm) than those of most isolates (except NL6 and NY15) of serotype B (847-886 nm). All isolates studied induced cytoplasmic pinwheel and scroll inclusions. Cells infected with serotype A isolates contained a specific type of proliferated endoplasmic reticulum which was never found in cells infected with serotype B isolates. The capsid protein gene of a representative member of each serotype was cloned and sequenced. Molecular mass calculations based upon nucleotide sequence-derived amino acid sequences yielded M(r) of 29,662 and 32,489 for the capsid proteins of the serotype A isolate NL8 and the serotype B isolate NL4, respectively. Comparison of the coat-protein sequences showed considerable differences at the N-termini whereas the core regions and the C-termini appeared to be highly conserved. Marked differences were also observed within the 3' non-coding regions of cloned cDNAs of NL 4 and NL 8. The striking differences between the two serotypes of BCMV strongly suggest that they be classified as two distinct potyviruses which naturally infect Phaseolus beans.

Gold labelling of RNA in virus-induced mitochondrial vesicles in the sheep blowfly Lucilia cuprina

Membranous vesicles are thought to be the replication site for viral RNA of many plant and animal viruses. A relatively rare site of virus-associated vesiculation is that of the mitochondrial outer membrane. In this study, virus-induced mitochondrial vesicles of the blowfly, Lucilia cuprina, were labelled with ribonuclease/gold and an antibody against double stranded RNA (anti-polyinosinic:polycytidylic acid). Both methods showed the presence of RNA in the vesicles thus indicating they may be a site for viral RNA replication in Lucilia.

Structure of spherical viruses

This review summarizes some of the results obtained from the crystal structure determination of viruses. The structural similarities of viral coat proteins is discussed, as well as some structural results related to the regulation of viral disassembly.

Phylogeny of capsid proteins of rod-shaped and filamentous RNA plant viruses: two families with distinct patterns of sequence and probably structure conservation

Computer-assisted comparative analysis of all available amino acid sequences of the capsid proteins of positive strand RNA plant viruses with helical capsids is described. Two distinct families of homologous proteins were delineated through statistically significant sequence similarities, one including the capsid proteins of rod-shaped viruses (tobamo-, tobra-, hordei-, and furoviruses) and the other those of filamentous viruses (poty-, bymo-, potex-, carla-, and closteroviruses). It was concluded that the capsid proteins of all rod-shaped viruses, on the one hand, and filamentous viruses, on the other hand, evolved from common ancestors. Analysis of residue conservation patterns in the capsid proteins of rod-shaped viruses revealed maintenance of the hydrophobic core and of the (putative) salt bridge between conserved Arg and Asp residues. Sequence comparisons within the filamentous virus family expanded the observations on the relationship between the capsid proteins of potex-, carla-, poty-, and bymoviruses. Grouping of the beet yellows closterovirus capsid protein sequence, recently determined in this laboratory (Agranovsky et al., J. Gen. Virol., 1991, 72, 15-23), with those of potex- and carlaviruses was demonstrated. The coat protein of another closterovirus, apple chlorotic leaf spot virus, appeared to constitute a distinct phylogenetic lineage. Despite the lack of significant overall similarity, comparison of the alignments of the capsid proteins of the two families suggested formation of analogous salt bridges.

A RNA virus in cells from Culicoides variipennis

A virus was detected in cells (designated CuVa) cultured from one laboratory colony of the biting midge, Culicoides variipennis. By electron microscopy (30 nm), nonenveloped, icosahedral virions arranged separately and in crystalline matrix arrays were seen in the cytoplasm but not in the nucleus of CuVa cells. Separation by 10% polyacrylamide gel electrophoresis revealed multiple bands of viral-induced double-stranded RNA. Inoculation of this virus onto different cell lines and intracranially into suckling mice revealed no detectable pathology. Immunoperoxidase staining using polyclonal antibody determined that the virus is infectious to toad cells, bovine endothelial cells, bovine kidney cells, mosquito cells, and cells (designated KC) initiated from another laboratory colony of C. variipennis. KC cells infected with this virus were coinfected with bluetongue virus with no decrease in bluetongue virus titer.

Identification in porcine faeces of a novel virus with a bisegmented double stranded RNA genome

Polyacrylamide gel electrophoresis of nucleic acids extracted from porcine faecal samples revealed in several samples the presence of two discrete bands. The bands were resistant to digestion with of DNase I and RNase T1, but not with RNase A in low salt conditions, indicating that they consisted of double stranded (ds) RNA. The two bands from different samples varied in sizes, in a range between 2.4-2.6 kbp and 1.7-1.9 kbp for the slow and fast moving band respectively. The bands cosedimented in CsCl gradients at an average density of 1.415 g/ml with icosahedral virus particles of a diameter of 34 nm and a triangulation number equal to 3. Aggregates of virus, which appeared to be immunocomplexes, were seen in one sample. From 244 faecal samples collected in one farm, 27 (11.1%) were found to contain the characteristic dsRNA pattern, with a higher prevalence in samples from animals 15 to 35 days old. The agent was equally distributed among samples from diarrhoeic or non-diarrhoeic animals. These results confirm the circulation among pigs of a novel virus, possibly of vertebrates, with a bisegmented double stranded RNA genome, similar to viruses previously described in humans, wild rats, guinea pigs, pigs, and chickens, for which the name "picobirnavirus" has been proposed.

Viral erythrocytic infection in sea bass: virus purification and confirmative diagnosis

The viral erythrocytic infection (VEI) of sea bass (Dicentrarchus labrax) is evidenced by the presence of inclusion bodies in the erythrocytes and erythroblasts. Virus-like particles (VEIV) about 135-150 nm in diameter were observed in erythroblasts of affected fish. No culturable virus could be recovered from the organs and blood of VEI-affected fish, after inoculation of different fish cell lines. Histochemical staining revealed the presence of RNA but not of DNA in the inclusion bodies. Specific, rabbit hyperimmune sera were prepared after extraction and purification by gel chromatography of viral material from pelleted blood cells of infected fish, and evaluated by immunocytochemical assays. Electron microscopic observations revealed the presence of enveloped particles of about 125-150 nm in the antigenic preparation. The antisera specifically reacted to inclusion bodies, cellular membranes and nuclei of VEI-affected erythrocytes in immunofluorescent and peroxidase-antiperoxidase assays. Immunoelectron microscopy in ultrathin sections of head kidney samples from VEI-affected fish showed recognition of inclusion bodies and virus particles. Confirmative diagnostic procedures could be established through the use of specific anti-VEIV sera, which enabled differentiation between similar syndromes described in other fish species.

[The ultrastructure of the interaction of Rickettsia sibirica and R. slovaca with the cells of ixodid ticks]

The ultrastructural aspects of the interaction of R. sibirica and R. slovaca with cells of mites of the species Dermacentor reticulatus, D. marginatus and Ixodes ricinus after their parenteral infection, as well as in the organs of D. marginatus infected naturally in the environment, have been studied. Both rickettsial species have similar morphology in different organs of the vector. These rickettsiae not only multiply, their populations are also partly destroyed in phagolysosomes. The natural mixed infection of R. sibirica and orbivirus in cells of D. reticulatus is described. As shown in this study, both associates pass through the complete ontogenetic cycle of development on the level of the host body and also on the level of an individual cell.

The three-dimensional structure of frozen-hydrated Nudaurelia capensis beta virus, a T = 4 insect virus

The three-dimensional structure of Nudaurelia capensis beta virus (N beta V) was reconstructed to 3.2-nm resolution from images of frozen-hydrated virions. The distinctly icosahedral capsid (approximately 40-nm diameter) contains 240 copies of a single 61-kDa protein subunit arranged with T = 4 lattice symmetry. The outer surface of unstained virions compares remarkably well with that previously observed in negatively stained specimens. Inspection of the density map, volume estimates, and model building experiments indicate that each subunit consists of two distinct domains. The large domain (approximately 40 kDa) has a cylindrical shape, approximately 4-nm diameter by approximately 4-nm high, and associates with two large domains of neighboring subunits to form a Y-shaped trimeric aggregate in the outer capsid surface. Four trimers make up each of the 20 planar faces of the capsid. Small domains (approximately 21 kDa) presumably associate at lower radii (approximately 13-16.5 nm) to form a contiguous, non-spherical shell. A T = 4 model, constructed from 80 trimers of the common beta-barrel core motif (approximately 20 kDa) found in many of the smaller T = 3 and pseudo T = 3 viruses, fits the dimensions and features seen in the N beta V reconstruction, suggesting that the contiguous shell of N beta V may be formed by intersubunit contacts between small domains having that motif. The small (approximately 1800 kDa), ssRNA genome is loosely packed inside the capsid with a low average density.

Eimeria species which infect the chicken contain virus-like RNA molecules

There is increasing support for the presence of viruses and virus-like particles inside protozoan cells. This study describes viral-like RNA molecules that have been detected in two species of Eimeria that infect the chicken. The RNA molecule identified in E. maxima has been characterized: subcellular fractionation studies have shown that the RNA is present in the cytoplasm, probably as an abundant ribonucleoprotein that is insensitive to RNAse A treatment. Electron microscopy has demonstrated that this RNA molecule is double stranded. In addition, all E. maxima strains examined so far contain this RNA molecule.

Identification and characterization of the virus causing rabbit hemorrhagic disease

Liver tissue from animals that died of rabbit hemorrhagic disease (RHD) was used to identify the causative agent. After extraction of liver homogenates and sucrose density gradient ultracentrifugation, distinct bands were obtained. The respective gradient fractions reacted positively in an enzyme-linked immunosorbent assay as well as in hemagglutination assays and were infective for rabbits. These fractions contained virions which had a diameter of 40 nm and resembled morphologically those of the family Caliciviridae. By immunoblotting, a major structural protein with a molecular weight of 60,000 was identified. Highly pure RNA of about 8 kilobases was isolated from virions. Labeled cDNA synthesized from virion RNA detected two RNAs of 8 and 2 kilobases in Northern (RNA) blots of liver RNA from animals infected with RHD virus. Finally, isolated virion RNA injected into the liver of rabbits produced a disease with clinical symptoms and pathological findings typical of RHD. We conclude that a calicivirus represents the causative agent of RHD.

Isolation and characterization of a RNA-virus like particle from Candida curvata

A virus-like particle (VLP) of 35 nm diameter has been isolated from the lipolytic yeast Candida curvata. The VLP contains a linear, double stranded RNA molecule of 1.55 microns in length.

Characterization of a RNA virus from the parasite Leishmania

We were interested in screening a series of isolates of the protozoan Leishmania for the presence of viruses. The experimental procedure we used was based on an enzymatic assay originally developed for viral RNA-dependent RNA polymerases. Simultaneously, total promastigote nucleic acid preparations were analyzed for the presence of viral genome and/or transcripts. Two isolates, both classified as L. braziliensis guyanensis, were found to be positive for RNA polymerase activity and to carry a large (6 kilobases) RNA species. The polymerase reaction products hybridized to the 6-kilobase RNA, believed to be the viral genome. In conjunction with electron microscopical observations these results indicate the presence of an RNA virus in these Leishmania isolates.

[Electron microscopy in the diagnosis of enteritis in cats]

Over a five-year period (1981-1985) 346 faecal and intestinal samples of cats affected with diarrhoea were studied by electron microscopy. This method revealed the presence of virus in 144 out of 346 (41.6 per cent) samples studied. 117 (81.3 per cent) of these samples contained parvoviruses which were identified by using a specific immune serum (immuno electron microscopy). In addition, rotaviruses (two samples), coronaviruses (thirteen samples), coronavirus-like (ten samples) and picornavirus-like particles (two samples) were detected in the other specimens. The present study shows that electron microscopy is a useful and rapid procedure in the diagnosis of enteritis in cats as well as in other domestic animals.

A virus with a bisegmented double-stranded RNA genome in rat (Oryzomys nigripes) intestines

Examination of the intestinal contents of free-living Oryzomys nigripes rats by PAGE revealed two sharply defined bands that could be stained by ethidium bromide or by silver nitrate with comparable intensities. The molecules forming these bands were susceptible to digestion by pancreatic RNase A but not by RNase T1 or by DNase I. Their lengths were estimated to be about 2.6 and 1.5 kbp, respectively, by comparison with rotavirus SA11 genome segments. They cosedimented in CsCl gradients at a density of 1.39 to 1.40 g/ml, together with uniform particles approximately 35 nm in diameter with indistinct surface structure. It is suggested that these particles represent an as yet undescribed virus with a bisegmented double-stranded RNA genome, for which the name 'picobirnavirus' is proposed.

What does the molecular structure of viruses tell us about viral functions?

The structures of the protein shell of a number of simple icosahedral RNA plant and animal viruses are now known at atomic resolution. In addition, components of viral capsids such as the haemagglutinin and neuraminidase spikes of influenza virus and the hexon unit of adenovirus are known in similar detail. These structures have provided a wealth of information on viral assembly, viral disassembly, the antigenic surface on viruses available to neutralizing antibodies, the host cell receptor attachment site, fusion of viral particles with the host cell, processing of polyproteins during maturation and the manner in which antiviral agents can interfere with the function of a viral capsid.

Isolation of a virus from Mycoplasma pulmonis

A virus designated mycoplasma virus P1 has been isolated from Mycoplasma pulmonis. The virus infects M. pulmonis strain UAB 6510, and a plaque-forming unit assay has been developed. P1 has a tailed, polyhedral morphology with a head diameter of about 28 nm. Nucleic acid isolated from crude preparations of P1 virus contains double-stranded RNA, suggesting that P1 may be the first example of an RNA-containing mycoplasma virus.