Genus Necrovirus

Type Species	Tobacco necrosis virus A	(TNV-A)

Distinguishing Features

Virions sediment as a single component with an S_20w of 118S. The genomic RNA is about 3.8  kb in size and contains four ORFs. A fifth potential smaller ORF is predicted within the 3-leader of the type species TNV-A. The genome organization and expression strategy are similar to members of the genus Carmovirus. Necroviruses have a small CP that forms smooth virions and is phylogenetically related to the Sobemovirus CP. This feature distinguishes the necroviruses from carmoviruses, which have a larger CP with a protruding domain.

Virion Properties

Morphology

Virions are approximately 28  nm in diameter and exhibit a T = 3 icosahedral symmetry. The shell has a smooth appearance (Fig. 11).

Physicochemical and Physical Properties

The virus sediments as one component with S_20w of 118S, and has a buoyant density of 1.40  g/cm³ in CsCl. The particle has an Mr of 7.6 10⁶. The thermal inactivation point of TNV is between 85 and 95°C. Virion isoelectric point is pH 4.5. Virions are insensitive to ether, chloroform and non-ionic detergents.

Nucleic Acid

Virions contain one molecule of infectious linear positive sense ssRNA. The type species RNA is 3684  nts. The 5-end of the RNA does not have a covalently linked virion protein and is uncapped, possessing a ppA . . . terminus. The RNA does not contain a 3 terminal poly(A) tract. The virion packages exclusively the genomic RNA. Three virus specific dsRNA species are found in infected cells. The size of the largest virus specific dsRNA corresponds to that of the genomic RNA. The second largest 1.6  kbp and the smallest 1.3  kbp dsRNAs correspond to sgRNA-1 and -2 respectively.

Proteins

The virion is composed of 180 copies of a single CP species. This protein has 268-275 amino acids and has a Mr of 29-30 10³.

Lipids

None reported.

Carbohydrates

None reported.

Genome Organization and Replication

The genomic RNA contains four ORFs (Fig. 12). However, TNV-A also contains a small 3 proximal ORF5. ORF1 is capable of encoding an Mr 22-23 10³ peptide. Readthrough of the ORF1 amber termination codon allows translation to continue into ORF1-RT for the expression of an Mr 82 10³ polypeptide. The Mr 82 10³ protein is predicted to be the RNA-dependent RNA polymerase. ORF2 can encode an Mr 7-8 10³ polypeptide implicated in cell-to-cell movement. ORF3 encodes an Mr 6-7 10³ polypeptide that also may be involved in movement. ORF4 encodes the Mr 29-30 10³ CP. ORF5, present only in the TNV-A, encodes a Mr 6.7 10³ protein. Two sgRNAs of 1.6 and 1.3  kb are synthesized in infected cells. The smaller sgRNA is the translational template for CP and the larger is the translational template for the ORF2 and possibly ORF3 products. The function of the ORF5 product is not known. Crystalline aggregates of virions are prominent in infected cells. Sometimes patches of electron-dense amorphous material can be seen. The satellite virus readily forms crystalline arrays.

Antigenic Properties

Member viruses are moderately immunogenic. Species can be distinguished serologically. Antisera yield a single precipitin line in agar gel-diffusion assays.

Biological Properties

Host Range

Necroviruses have wide host ranges that include monocotyledonous and dicotyledonous plants. In nature, infections are typically restricted to roots. Experimental inoculations usually cause necrotic lesions on the inoculated leaves, but rarely result in systemic infection.

Transmission

Virions are readily transmitted by mechanical inoculation. Member viruses are transmitted naturally by the chytrid fungus Olpidium brassicae.

List of Species Demarcation Criteria in the Genus

	Extent of serological relationship as determined by immunodiffusion and/or ELISA,
	Extent of sequence identity between relevant gene products,   Less than 62% amino acid sequence identity of the CP,   Less than 76% amino acid sequence identity of the polymerase,
	Transmission by a fungal vector,
	Natural host range,
	Artificial host range reactions.

List of Species in the Genus

Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, CMI/AAB description numbers ( ), genome sequence accession numbers [ ], and assigned abbreviations ( ) are:

Species in the Genus

Chenopodium necrosis virus		(ChNV)
Leek white stripe virus	[X94560]	(LWSV)
Olive latent virus 1	[X85989]	(OLV-1)
Tobacco necrosis virus A (14)	[M33002]	(TNV-A)
Tobacco necrosis virus D (14)	[D00942, U62546]	(TNV-D)

Tentative Species in the Genus

Carnation yellow stripe virus	(CYSV)
Lisianthus necrosis virus	(LNV)