Family: Bromoviridae
Genus: Bromovirus
Distinguishing features
Beetle vectors are recorded for most bromoviruses but the efficiency of such transmission is low. A long-standing report of transmission by nematodes has yet to be confirmed.
Virion
Morphology
Virions are polyhedral, and all the same size, with a diameter of 27 nm.
Physicochemical and physical properties
Virions prepared below pH 6.0 have S20,w of 88S, are stable to high salt and low detergent concentrations, and are nuclease- and protease-resistant. At pH 7.0 and above, virions swell to a diameter of 31 nm, S20,w decreases to 78S, salt and detergent stability decreases dramatically, and protein and RNA are susceptible to hydrolytic enzymes. This swelling is accompanied by conformational changes of the capsid that are detectable by physical and serological means.
Nucleic acid
RNA 3′-termini are tRNA-like, are very similar in all viruses sequenced so far, and can be aminoacylated with tyrosine.
Proteins
Virions contain a single coat protein (CP) of 20.3 kDa.
Genome organization and replication
The genome is organized as depicted in Figure 2A.Bromoviridae. Coat protein is not required for activation of the genome. Intra-segmental crossover as well as homologous recombination may occur in brome mosaic virus (BMV) RNAs. Both, viral RNA sequences and host protein contribute to BMV RNA recombination (Kolondam et al., 2015). Data from high throughput sequencing revealed that BMV virions can carry host RNAs that potentially can mediate horizontal gene transfer in plants (Shrestha et al., 2018).
Biology
The natural host range is narrow, limited to a few plant hosts for each species of this genus. All members are thought to be beetle-transmitted. BMV is also transmitted by aphids in a non-persistent manner but with poor efficiency.
Antigenicity
All members are serologically related, with large antigenic differences between species.
Species demarcation criteria
Criteria used for demarcation of species within the genus are host range, serological relationships, comparable replicase proteins (i.e. 1a and 2a proteins) and nucleotide sequence identity between species, which ranges from 50 to 80% depending on the gene used for comparison.