Cucumber mosaic virus: viral genes as virulence determinants

Taxonomic relationships: Cucumber mosaic virus (CMV) is the type species of the genus Cucumovirus in the family Bromoviridae, which also encompasses the Peanut stunt virus (PSV) and the Tomato aspermy virus (TAV). Nucleotide sequence similarity among these three cucumoviruses is 60%65%. CMV strains are divided into three subgroups, IA, IB and II, based on the sequence of the 5' untranslated region of the genomic RNA 3. Overall nucleotide sequence similarity among CMV strains is approximately 70%-98%. Geographical distribution, host range and symptomatology: CMV is distributed worldwide, primarily in temperate to tropical climate zones. CMV infects more than 1200 species of 100 plant families, including monocot and dicot plants. Symptoms caused by CMV infection vary with the host species and/ or CMV strain, and include mosaic, stunt, chlorosis, dwarfing, leaf malformation and systemic necrosis. CMV disease is spread primarily by aphid transmission in a nonpersistent manner. Physical properties: In tobacco sap, the thermal inactivation point of the viral infectivity is approximately 70 degrees C (10 min), the dilution end-point is approximately 10(-4) and viral infectivity is lost after a few days of exposure to 20 degrees C. Viral infectivity can be retained in freeze-dried tissues and in the form of virions purified using 5 mM sodium borate, 0.5 mM ethylenediaminetetraacetic acid and 50% glycerol (pH 9.0) at -20 degrees C. CMV particles are isometric, approximately 28-30 nm in diameter and are composed of 180 capsid subunits arranged in pentamer-hexamer clusters with T = 3 symmetry. The sedimentation coefficient (s20, w) is c. 98 S and the particle weight is (5.8-6.7) 106 Da. The virions contain 18% RNA. The RNA-protein interactions that stabilize the CMV virions are readily disrupted by sodium dodecylsulphate or neutral chloride salts. Genomic properties: The genomic RNAs are single-stranded messenger sense RNAs with 5' cap and 3' tRNA-like structures containing at least five open reading frames. The viral RNA consists of three genomic RNAs, RNA 1 (c. 3.3 kb), RNA 2 (c. 3.0 kb) and RNA 3 (c. 2.2 kb), and two subgenomic RNAs, RNA 4 (c. 1.0 kb) and RNA 4A (c. 0.7 kb). The 3' untranslated regions are conserved across all viral RNAs. CMV is often accompanied by satellite, noncoding, small, linear RNA that is nonhomologous to the helper CMV. Useful websites: http://www.dpvweb. net/dpv/showdpv. php? dpvno= 400 (general information); http://www.rcsb.org/pdb/explore/explore.do?pdbId= 1f15 (structure).