Virion Properties

Morphology

Virions are flexuous filaments, 680 - 900 nm long and 11 - 13 nm wide, with helical symmetry and a pitch of about 3.4 nm. Particles of some viruses are longer in the presence of divalent cations than in the presence of EDTA.

Physicochemical and Physical Properties

Virion S20w is 150 - 160S; density in CsCl is 1.31 g/cm³; E^0.1%1 cm, 260 nm = 2.4 - 2.7.

Nucleic Acid

Virions contain a single molecule of linear, positive sense ssRNA, about 9.7 kb in size (Mr 3.0 - 3.5 x 10⁶); virions contain 5% RNA by weight. RNA molecules have poly (A) tracts at their 3'-ends. A genome-linked protein of Mr 24 x 10³ is covalently linked at or near the 5' terminus.

Proteins

Virions contain a single CP, Mr 30 to 47 x 10³ in size. The CP of most isolates of the type species, PVY, contains 267 amino acids.

 Figure 2: Generic genomic map of a member of the genus Potyvirus. The RNA genome is represented by thin lines and an open box which represent untranslated and translated segments of the ssRNA, respectively. Activities associated with these products are shown. VPg, genome-linked viral protein covalently attached to the 5' terminal nucleotide (represented by the filled oval at the 5'-end); P1, a protein with a proteolytic activity responsible for cleavage at Phe-Ser (O); HC-Pro, a protein with aphid transmission helper component activity and proteolytic activity responsible for cleavage at a Gly-Gly (_); Pro, serine-like proteolytic activity responsible for cleavage at Gln-(Ser/Gly) (_). Some of these proteins of particular member viruses of the family Potyviridae aggregate to form inclusion bodies during infection. The protein involved and the particular type of inclusion body is shown above the genetic map; AI, amorphous inclusion; CI, cylindrical-shaped inclusion body found in the cytoplasm; NIa and NIb, small and large nuclear inclusion proteins, respectively, which aggregate in the nucleus to form a nuclear inclusion body.

Virions are moderately immunogenic; there are serological relationships among many members. One monoclonal antibody reacts with most aphid-transmitted potyviruses. The CP amino acid sequence homology among aphid-transmitted viruses is 40-70%. Some species are serologically related to species in the genera Rymovirus and Bymovirus.

Many individual viruses have a narrow host range, but a few infect species in up to 30 host families. The viruses are transmitted by aphids in a non-persistent manner and are transmissible experimentally by mechanical inoculation. Some isolates are inefficiently transmitted by aphids and others are not transmissible by aphids at all. This is apparently due to mutations within the helper component and/or CP cistrons. Some viruses are seed-transmitted.

• Genome sequence relatedness.

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

- nucleotide sequence identity of 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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