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Type Species |
(NoV) |
Virions are nonenveloped, roughly spherical in shape, 29 to 32 nm in diameter and have icosahedral symmetry (T = 3). No distinct surface structure is seen by electron microscopy of negatively-stained preparations (Fig. 1). Empty shells are seldom seen in virus preparations.
Physicochemical and Physical Properties
Virion Mr is about 9 
106; S20w is about 135 to 140S. Virion buoyant density in CsCl is 1.30 to 1.34 g/cm3 (varies with species). Infectivity of aqueous suspensions is stable to extraction with chloroform. Infectivity of Nodamura virus (NoV), Black beetle virus (BBV), or Flock house virus (FHV) is stable at room temperature in 1% sodium dodecyl sulfate but Boolarra virus (BoV) is inactivated. Virions are stable at acid pH. The RNA content of the virion is about 16%.
The genome consists of two molecules of positive sense ssRNA: RNA1 (Mr 1.1 106) and RNA2 (Mr 0.48 106). Both molecules are required for infectivity and both are encapsidated in the same virus particle. Both RNA molecules are capped at their 5
-ends with cap zero structures and lack poly(A) tails at their 3-ends. RNA 3-ends cannot be chemically derivatized even after treatment with denaturing solvents, indicating that the expected 3-terminal-OH groups are unreactive.
The capsid consists of 180 protein subunits (protomers) arranged on a T = 3 surface lattice. Each protomer is composed of a single capsid protein (CP) (protein ) or the two products of its cleavage (proteins and ). For FHV, these proteins are: protein : Mr 44 103, 407 amino acid residues; protein : Mr 40 103, amino acid residues 1-363; protein : Mr 4 103, amino acid residues 364-407. Morphogenesis involves the formation of a non-infectious provirion which acquires infectivity by autocatalytic cleavage of protein to form proteins and . Maturation cleavage is often incomplete and virions typically contain residual uncleaved protein . The N-termini of proteins and are blocked by an unidentified entity.
None reported.
None reported.
Genome Organization and Replication
Alphanodaviruses replicate in the cytoplasm of infected cells (Fig. 2). RNA synthesis is resistant to actinomycin D. Infected cells contain three ssRNAs: RNA1 (Mr 1.1 106; 3.1 kb); RNA2 (Mr 0.48 106; 1.4 kb) and a subgenomic RNA3 (Mr 0.13 106; 0.39 kb), whose nucleotide sequence corresponds to the 3-end of RNA1 (387 nts in the case of FHV). RNA3 is not packaged into virions; its 3-end is chemically unreactive like those of RNAs 1 and 2. RNA1 encodes protein A (Mr 112 103), which is the catalytic subunit of the viral RNA-dependent RNA polymerase. RNA2 encodes protein , the CP precursor (Mr 44 103). Depending on virus species, RNA3 encodes one or two small proteins (proteins B1 and B2, Mr 11 103) of unknown function. B1 is encoded in the same ORF as protein A whereas B2 is encoded in an overlapping ORF. BoV RNA3 does not encode protein B1 but all known alphanodavirus RNA3s encode protein B2. Protein B2 appears to be essential for productive infection but its role in RNA replication is unclear. Cells transfected with isolated RNA1 synthesize RNA1 and overproduce RNA3, but do not make RNA2. RNA2 replication strongly inhibits synthesis of RNA3 and the translation of RNA2 suppresses the translation of RNA1.
NoV, BBV, FHV and BoV are cross-reactive by double-diffusion precipitin tests but all four members represent different serotypes (neutralization titer of each antiserum less than 0.5% in heterotypic crosses).
All species of the alphanodaviruses were isolated in nature from insects, although serological data suggest that NoV also naturally infects pigs and perhaps herons. NoV seems to be unique among the nodaviruses in its ability to infect both vertebrates and invertebrates. It is also very unusual in being able to kill both insect and mammalian hosts. The other alphanodaviruses do not show strict specificity for particular insect hosts.
In the laboratory, most alphanodaviruses can be propagated in larvae of the common wax moth, Galleria mellonella, where they cause paralysis and death. NoV, isolated from mosquitoes, also grows in suckling mice but not in cultured cells of Drosophila melanogaster. FHV, BBV, and BoV grow well in cultured Drosophila melanogaster cells and form plaques on monolayers of these cells. Defective-interfering particles are readily formed unless the viruses are passaged at low multiplicity of infection. Persistent infections, with subsequent resistance to superinfection, occur readily in cultured Drosophila melanogaster cells. NoV multiplies poorly in cell culture but can be propagated by transfecting insect or vertebrate cell cultures with virion RNA at temperatures below about 34°C.
NoV is transmissible to suckling mice by Aedes aegypti mosquitoes. It causes paralysis and death when injected into suckling mice, but no disease in adult animals. In their insect hosts, the alphanodaviruses typically cause stunting, paralysis, and death.
List of Species Demarcation Criteria in the Genus
The following criteria can be applied to the demarcation of species within the alphanodavirus genus:
1. |
Biological properties (host range, vectors, mode of transmission). Since the natural host-ranges of the nodaviruses have generally not been examined in detail but may in some cases be broad, virus isolation from a new host is not, in itself, evidence of a new nodavirus species. | ||||
2. |
Antigenic properties. Antisera raised against different isolates or strains of a single nodavirus species should exhibit high levels of cross-reactivity in Western blot and/or neutralization analyses. Lower levels of cross-reactivity in these assays using antisera against all previously recognized nodavirus species can provide evidence of a novel nodavirus. | ||||
3. |
Virion physical/physicochemical characteristics.
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4. |
Structural protein characteristics. The electrophoretic mobilities in SDS-PAGE of the CP precursor or its cleavage products should be compared with those of other nodavirus species. | ||||
5. |
Genome molecular characteristics.
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6. |
Genome sequence characteristics. The nucleotide sequences of the two genomic RNAs should be compared with those of other nodaviruses. Because the nodavirus genome is segmented, reassortment can occur and the two genome segments may have distinct evolutionary lineages. |
Application of these criteria. In practice, while criteria 1-5 above may be suggestive of a new species, definitive demarcation is based on the nucleotide sequence of the viral CP gene. The two closest recognized species are BBV and FHV, whose RNA2 sequences show 80% identity at the nucleotide level and 87% identity at the amino acid sequence level. Their RNA1 sequences, however, are 99% identical.
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, genome sequence accession numbers [ ], and assigned abbreviations ( ) are:
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Black beetle virus |
RNA1 [X02396] RNA2 [X00956] |
(BBV) |
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Boolarra virus |
RNA2 [X15960] |
(BoV) |
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Flock house virus |
RNA1 [X77156] RNA2 [X15959] |
(FHV) |
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Gypsy moth virus |
(GMV) | |
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(Lymantria ninayi virus (Greenwood)) |
(LNV) | |
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Manawatu virus |
(MwV) | |
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New Zealand virus |
(NZV) | |
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(Drosophila line virus) |
(DLV) | |
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Nodamura virus |
RNA2 [X15961] |
(NoV) |
Tentative Species in the Genus
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Pariacato virus |
(PaV) |
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