Virion Properties

Morphology

Virions are flexuous filaments of two modal lengths, 250 - 300 and 500 - 600 nm; both are 13 nm in width (Fig. 3).

 

Figure 3: Virions of Barley yellow mosaic virus (BaYMV) stained with 1% PTA, pH 7.0. The bar represents 200 nm (from D. Lesemann,).

Physicochemical and Physical Properties

Virion buoyant density in CsCl is 1.28 - 1.30 g/cm³.

Nucleic Acid

Virions contain two molecules of linear positive sense, ssRNA. RNA1 is 7.5 - 8.0 kb (Mr 2.6 x 10⁶) and RNA2 is 3.5 - 4.0 kb (Mr 1.5 x 10⁶) in size; RNA makes up 5% by weight of particles. Both RNA molecules have 3'-terminal poly(A) tracts. There is little base sequence homology between the two RNAs except in the 5' noncoding regions. The CP gene is located in the 3'-proximal region of RNA1.

Proteins

Virions have a single CP with Mr 28.5 - 33 x 10³. The CP of the type species, BaYMV contains 297 amino acids.

The two RNA molecules appear to be translated initially into precursor polypeptides from which functional proteins are derived by proteolytic processing (Fig. 4).

Figure 4: Genomic map of the Bymovirus bipartite genome, using Barley yellow mosaic virus (BaYMV) as an example. The same conventions as for Potyvirus genome organization map (Fig. 2) are employed. The boundaries of possible gene products are represented by vertical lines. Activities of most gene products are postulated by analogy with genus Potyvirus. P1 corresponds to the C-terminal protease of HC-Pro.

The viral proteins are moderately immunogenic; serological relationships exist among members except Barley mild mosaic virus (BaMMV). The CP amino acid sequence homology among members is 35-74%.

Cytology

There are characteristic pinwheel-like inclusions and membranous network structures are formed in the cytoplasm of infected plant cells. No nuclear inclusions are found.

Host Range

The host range of member viruses is narrow, restricted to the host family Graminae.

Transmission

The viruses are transmitted by the plasmodiophoraceous fungus Polymyxa graminis; transmissible experimentally by mechanical inoculation.

• 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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