Bromoviruses are a group of plant single-stranded RNA viruses that belong to the genus Bromoviridae . Their polyhedral particles of ca. 26 nm in diameter have the icosahedral T-3 surface lattice symmetry (1 ), with 180 identical polypeptides used to build their virion particles. Because of the tripartite nature of their genomic RNA, three different particles exist: one containing one molecule of RNA-1, one containing one molecule of RNA-2, and one containing one molecule each of RNA-3 and RNA-4 (see Fig. 1 ).