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Ipomoviruses are distinguished from other genera by their mode of transmission being whiteflies, and by phylogenetic analyses.
Virions are flexuous filaments 800–950 nm long.
Virion S20,w is 155S for sweet potato mild mottle virus (SPMMV).
Virions contain a single molecule of linear, positive sense ssRNA, about 9.7 kb in size; virions contain 5% RNA by weight.
The viral CP is a single polypeptide of 302–378 aa (35–41 kDa).
Ipomoviruses exhibit unusual structural variability. The structure and organization of the SPMMV genome is similar to that of members of Potyvirus (Figure 2.Potyviridae), but some motifs of HC-Pro and CP characteristic of Potyvirus are incomplete or missing, which may account for its vector relations. The unusually large P1 protein (83 kDa) of SPMMV contains no obvious AlkB domain and hence differs from that found in members of the genus Brambyvirus. Cucumber vein yellowing and squash vein yellowing viruses (CVYV and SqVYV) differ from SPMMV by encoding two P1-like serine proteases (P1a and P1b) but no HC-Pro. P1b functions as a suppressor of RNA silencing (Li et al., 2008). Cassava brown streak virus (CBSV) differs from SPMMV by having no HC-Pro and, also, from CVYV and SqVYV by having only P1b which suppresses silencing (Figure 1.Ipomovirus). Additionally, CBSV contains a Maf/HAM1-like sequence recombined into the NIb/CP junction, which can accommodate heterologous genes in engineered infectious potyvirus clones. Homology of HAM1h with cellular Maf/HAM1 NTP pyrophosphatases suggests that HAMh1 might intercept non-canonical NTPs to reduce mutation rates of viral RNA.
Figure 1.Ipomovirus. Genomic maps of the ipomoviruses sweet potato mild mottle virus (SPMMV), squash vein yellowing virus (SqVYV) and cassava brown streak virus (CBSV). The ssRNA genome is represented by a line and an open box representing the ORF translated into a polyprotein. Conventions are as for the potyvirus genome organization map (Figure 2.Potyviridae). Activities of most mature proteins are postulated by analogy with genus Potyvirus. CVYV and SqVYV contain two P1-like serine proteases (P1a and P1b), of which P1b functions as a suppressor of RNA silencing. CBSV also contain P1b which suppresses silencing and, additionally, carries a Maf/HAM1-like sequence recombined into the NIb/CP junction. HAMh1 might intercept non-canonical NTPs to reduce mutation rates of viral RNA.
Moderately immunogenic. No serological relationships with other members of the family Potyviridae have been found.
The natural host range of SPMMV is wide, with more than nine families susceptible, whereas the host range of CBSV, CVYV and SqVYV is less known apart from the hosts which they have been found to infect in the field (Winter et al., 2010).
CBSV, CVYV and SqVYV are transmitted by the whitefly Bemisia tabaci in a non-persistent manner. B. tabaci may also be the vector of SPMMV, but this is not fully confirmed. All ipomoviruses are transmissible experimentally by mechanical inoculation and by grafting.
Ipomo: from Ipomea and mosaic, the symptoms induced on the host of the type species.
Coccinia mottle virus
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