Closteroviridae

This chapter focuses on Closteroviridae family whose member genuses are Closterovirus, Ampelovirus, and Crinivirus. The virions are helically constructed filaments with a pitch of the primary helix in the range of 3.4–3.8 nm, containing about 10 protein subunits per turn of the helix and showing a central hole of 3–4 nm. The very flexuous and open structure of the particles is the most conspicuous trait of members of the family. The virions have a diameter of about 12 nm and their length ranges from 650 nm in case of species with fragmented genome, to over 2000 nm in case of species with monopartite genome. The fragility of virions and a tendency to end-to-end aggregation contribute to the fact that a range of lengths is often given for single viruses. The virions of several species are degraded by CsCl and are unstable in high salt concentration, resist moderately high temperatures and organic solvents, but are sensitive to RNase and chelation. Regardless of the genome type, monopartite or fragmented, virions contain a single molecule of linear, positive sense, single stranded RNA, constituting 5–6% of the particle weight. The structural proteins of most members of the family consist of a major CP and of a diverged copy of it denoted minor CP (CPm), with a size ranging from 22 to 46 kDa (CP) and 23 to 80 kDa (CPm). The members of the family have one of the largest genomes among plant viruses because of sequence duplication and acquisition of nonviral coding sequences such as protease, and HSP70 protein via RNA recombination.