Virion Properties

Virions are flexuous filaments 690 - 700 nm long. Virion S_20w is 166S for Wheat streak mosaic virus (WSMV). Virions contain a positive sense ssRNA, ca. 8.5 kb in size, with a 3’-poly(A) terminus. The viral CP is a single peptide of ca. 298 amino acids for WSMV and 265 amino acids for Brome streak mosaic virus (BStMV). The Mr estimated by electrophoresis is ca. 32 x 10³. No lipid or carbohydrate are reported.

The complete nucleotide sequence of the BStMV genome consists of 9672 nts excluding the 3’-terminal poly(A) tail. Sequence analysis reveals an ORF of 3093 amino acids. The structure and organization of the BStMV and WSMV genomes appear to be similar to those of other members of the family Potyviridae except the bymoviruses. Most known potyvirus motifs are present in the polyprotein of BStMV. However, motifs in the putative helper-component and CP of BStMV are incomplete or missing which may account for different vector relations of the tritimoviruses. The WSMV CP sequence shows limited (22 - 25%) homology with CP sequences of some aphid-transmitted potyviruses. WSMV shows significant amino acid sequence homology with aphid-transmitted potyviruses in the cylindrical inclusion protein and portions of the nuclear inclusion proteins. WSMV RNA has been translated in vitro into several large proteins immunoprecipitable with WSMV CP antiserum, suggesting that WSMV uses a proteolytic processing strategy to express functional proteins such as the CP. Antiserum to Tobacco etch virus (TEV) 58 x 10³ nuclear inclusion protein also reacts with WSMV in vitro translation products. There is an in vitro translation product that is precipitated with antiserum to HC-Pro helper component of Tobacco vein mottling virus. Despite similarities with other members of the family Potyviridae, comparative sequence analyses show limited similarities between the nine mature proteins and those of other members of the family Potyviridae. WSMV has CP molecules of 42, 36 and 32 x 10³ in size; the two smaller proteins are subsets of the 42 x 10³ protein.

Moderately immunogenic. The tritimoviruses are serologically related to the rymoviruses, but not to the other members of the family Potyviridae.

Cytology

Characteristic cytoplasmic cylindrical ("pinwheel") inclusions are present in infected cells.

Host Range

WSMV has a wide host range within the Graminae. BStMV has a narrow host range, also restricted to the Graminae. .

Transmission

Transmitted by eriophyid mites in a persistent manner and transmissible experimentally by mechanical inoculation and by grafting.

• Genome sequence relatedness.

- CP amino acid sequence identity less than ca. 80%,

- nucleotide sequence identity less than 85% over whole genome,

- different polyprotein cleavage sites.

• Natural host range.

- host range may be related to species but usually not helpful in identifying species. May delineate strains.

• Pathogenicity and cytopathology.

- different inclusion body morphology,

- lack of cross protection,

- seed transmissibility, or lack thereof,

- some aspects of host reaction may be useful (e.g., resistance genes, different responses in key host species).

• Mode of transmission.

- different primary vectors, but vector species not of use in identification to virus species.

• Antigenic properties.

- serological differences.

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