ICTV Virus Taxonomy Profile : Nodaviridae

The family Nodaviridae includes two genera, Alphanodavirus and Betanodavirus. The family name derives from the Japanese village of Nodamura where Nodamura virus was first isolated from Culex tritaeniorhynchus mosquitoes. Virions are nonenveloped and spherical in shape with icosahedral symmetry (T=3) and diameters ranging from 25 to 33 nm. The genome consists of two molecules of single-stranded positive-sense RNA: RNA1 and RNA2. The virion capsid consists of 180 protein subunits arranged on a T=3 surface lattice. Alphanodaviruses infect insects, whereas betanodaviruses are pathogens of fish. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the taxonomy of the Nodaviridae, which is available at www.ictv.global/report/nodaviridae.


VIRION
Virions are non-enveloped, roughly spherical in shape, 25-33 nm in diameter and have icosahedral symmetry (T=3) (Table 1, Fig. 1) [1].Electron microscopy of negatively stained betanodaviruses shows surface projections; these are not observed in alphanodaviruses.Virion buoyant density in CsCl ranges from 1.30 to 1.36 g cm À3 .Virions are stable to pH values ranging from 2 to 9 and are resistant to heating at 56 C for 30 min.

GENOME
The genome of nodaviruses consists of two molecules of positive-sense single-stranded RNA; RNA1 (3.1 kb) encoding protein A, an RNA-dependent RNA polymerase of about 112 kDa (983-1014 amino acids), and RNA2 (1.4 kb) encoding protein a, the capsid protein precursor.Both RNA molecules are encapsidated in the same virus particle, and both are required for infectivity.Both molecules are capped at their 5¢-ends and lack poly(A) tails at their 3¢ends [2].

REPLICATION
Virus replication is cytoplasmic.Infected cells contain single-stranded RNAs corresponding to RNA1 and RNA2, as well as the subgenomic RNA3 (387 nucleotides), which derives from the 3¢-terminus of RNA1 and is not packaged into virions (Fig. 2).In addition to RNA-dependent RNA polymerase activity, protein A binds to and drives invagination of the outer mitochondirial membrane to provide the compartment where RNA replication occurs.RNA3 encodes either one or two proteins; protein B2 (11 kDa) is encoded by all nodaviruses in a reading frame overlapping that of protein A, and is a suppressor of RNA interference.

PATHOGENICITY
Alphanodavirus infection results in the stunting, paralysis and death of the insect host.The infection of fish by betanodaviruses such as striped jack nervous necrosis virus or redspotted grouper nervous necrosis virus causes neural necrosis, encephalopathy or retinopathy and is associated with behavioural abnormalities and high mortality, posing significant problems for marine aquaculture [3,4].

TAXONOMY
The members of the two genera in this family infect insects (Alphanodavirus) [5] or fish (Betanodavirus) [6].Different species encode capsid proteins that differ at >20 % of nucleotide or >13 % of amino acid positions (Alphanodavirus) or >15 % of nucleotide and >12 % of amino acid positions (Betanodavirus).There is limited sequence identity between the capsid proteins of members of different genera, or with those of a number of unclassified nodaviruses isolated from prawns, nematodes and insects.
Typical member: Striped jack nervous necrosis virus (RNA1: AB056571; RNA2: AB056572), species Striped jack nervous necrosis virus, genus Betanodavirus Virion Non-enveloped spherical particles, 25-33 nm in diameter, with or without surface projections Genome Bi-partite single-stranded positive-sense RNA of 3.1 kb (RNA1) and 1.4 kb (RNA2) with 5¢-terminal caps but without poly(A) tails Replication Cytoplasmic within virus-induced invaginations on the outer mitochondrial membrane Translation From capped genomic and subgenomic RNAs Host range Natural hosts are insects (Alphanodavirus) or fish (Betanodavirus) Taxonomy Two genera, each including four or more species Some nodaviruses also express protein B1 (11 kDa), which corresponds to the C-terminal region of protein A and is of unknown function.Maturation of non-infectious provirions involves the autocatalytic cleavage of protein a into proteins b (39 kDa) and g (4 kDa).

Table 1 .
Characteristics of the family Nodaviridae