Tritimoviruses are transmitted by mites of a different species from those that transmit rymoviruses. They form a separate cluster in phylogenetic analyses (Stenger et al., 1998).
Virions are flexuous filaments 690–700 nm long.
Physicochemical and physical properties
Virion Sedimentation coefficient S20,w is 166S for wheat streak mosaic virus (WSMV).
Virions contain a positive-sense ssRNA of about 9.4–9.6 kb with a 3′-poly(A) terminus.
The viral CP is a single peptide of about 349 aa for WSMV, 337 aa for oat necrotic mottle virus (ONMV), and 320 aa for brome streak mosaic virus (BrSMV). The Mr estimated by electrophoresis is 42 kDa.
Genome organization and replication
The oat necrotic mottle virus (ONMV) genome consists of 9,346 nt excluding the 3′-terminal poly(A) tail (Figure 1.Tritimovirus). Sequence analysis reveals an ORF of 3,023 aa. The structure and organization of the ONMV genome is similar to those of other members of the family Potyviridae except the bymoviruses (Figure 2.Potyviridae). Most known potyvirus motifs are present in the polyprotein of ONMV. However, motifs in the putative helper-component and CP of BrSMV are incomplete or missing, which may account for different vector relations of tritimoviruses. The WSMV CP sequence shows limited (22–25%) identity with CP sequences of some aphid-transmitted potyviruses. WSMV shows significant aa sequence identity with aphid-transmitted potyviruses in the cylindrical inclusion protein and portions of the nuclear inclusion proteins. WSMV RNA is translated in vitro into several large proteins that can be immunoprecipitated 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 kDa nuclear inclusion protein also reacts with in vitro translation products of WSMV. An in vitro translation product is precipitated with an antiserum to HC-Pro helper component of an isolate of the species Tobacco vein mottling virus (genus Potyvirus). Comparative sequence analyses show similarities with other members of the family Potyviridae, but these are limited to the nine mature proteins. WSMV is especially susceptible to proteinases in planta and has CP molecules of 42, 36 and 32 kDa; the two smaller proteins are parts of the 42 kDa protein.
|Figure 1.Tritimovirus Schematic diagram of the oat necrotic mottle virus (ONMV) genome. The polyprotein ORF is indicated by the large open box divided into putative mature proteins. The pretty interesting Potyviridae protein (PIPO) is represented by a small box. The untranslated regions (UTR) are represented by lines on each end of the large ORF. Activities of mature proteins are postulated by analogy with genus Potyvirus. Conventions are as for the potyvirus genome organization map (Figure 2.Potyviridae). Not to scale.
The viruses only affect hosts in the family Poaceae, but while WSMV has a wide host range, BrSMV and ONMV have narrow ones.
WSMV and BrSMV are transmitted by eriophyid mites in a semi-persistent manner. The WSMV HC-Pro is required for mite transmission. All tritimoviruses are transmissible experimentally by mechanical inoculation.
Moderately immunogenic. WSMV and oat necrotic mottle virus (ONMV) are serologically related to each other, but not to the other members of the family Potyviridae.
Species demarcation criteria
See discussion under family description.