Taxonomic Structure of the Family
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Closteroviridae |
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Virions are very flexuous filaments with helical symmetry, open particle structure, and distinct cross-banding with a pitch of 3.4-3.8 nm (Fig. 1). There are about 10 protein subunits per turn of the helix. Particles of all members of the family are about 12 nm in diameter, but their length varies according to the genus and/or the individual species. Virus particles, members of the genus Closterovirus (monopartite genome) are 1250-2000 nm long, whilst those of the genus Crinivirus (bipartite genome) have two modal lengths, i.e., 650-800 and 700-900 nm. Particles of Beet yellows virus (BYV) and Citrus tristeza virus (CTV) of the genus Closterovirus are coated at an extremity by the coat protein (CP) analogue expressed by a CP duplicate gene (CPd) giving rise to a distinct structure for which the names “rattlesnake” or “heterodimeric” were used. Because the CPd occurs in all members so far sequenced of both genera, this terminal structure may be a general trait of the family.
Physicochemical and Physical Properties
Virions of both genera usually sediment as a single band in sucrose or Cs2SO4 gradients. S20w ranges from 96S to 140S, buoyant density in CsCl is 1.30-1.34 g/cm3, and in Cs2SO4 is 1.24-1.27 g/cm3. Virions of most species are degraded by CsCl and are unstable in high salt concentration. Virions resist moderately to high temperatures (thermal inactivation is around 45-55°C) and organic solvents, but are sensitive to RNase and chelation.
Regardless of the genome type, monopartite or bipartite, virions contain a single molecule of linear, positive sense, ssRNA, constituting 5- 6% of the particle weight. Genome size is related to particle length. In monopartite species (genus Closterovirus) genome size ranges from 15.5 kb (BYV) to 19.3 kb (CTV), the largest genome among positive-strand RNA plant viruses. Genome size of the bipartite Lettuce infectious yellows (LIYV, Crinivirus) is 15.3 kb. The 5 end of the genome is likely to be capped, the 3 end is not polyadenylated and does not possess a tRNA-like structure, but may have several hairpins structures. The genomic RNAs of three species of the genus Closterovirus, i.e., BYV, CTV (four isolates from California, Florida, Spain and Israel, respectively), and Little cherry virus (LChV), and one member of the genus Crinivirus, i.e., LIYV have been completely sequenced. Partial to almost complete sequences are available for Grapevine leafroll-associated virus 2 (GLRaV-2) and Grapevine leafroll-associated virus 3 (GLRaV-3), Beet yellow stunt virus (BYSV), Carnation necrotic fleck virus (CNFV), and Beet pseudo yellows virus (BPYV) from the genus Closterovirus, Tomato infectious chlorosis (TICV) and Sweet potato chlorotic stunt (SPCSV) from the genus Crinivirus.
Virions of all members of the family are composed of a major CP with a Mr ranging from 22 to 46 103, according to the individual species, but the capsid of BYV and CTV incorporates also the divergent CP analogue to one of the particle extremities (CPd). Structural proteins of some of the species; BYV, Carnation necrotic fleck virus (CNFV) and Lilac chlorotic leafspot virus (LCLV), lack tryptophan, which is reflected in their high A260/A280 ratio (1.4-1.8). The reverse is true for the CP of CTV, which contains a significant amount of tryptophan (A260/A280 = 1.21-1.22). Nonstructural proteins common to all members of the family are: (1) a large polypeptide (Mr 295-349 103) containing the conserved domains of papain-like protease (P-PRO), methyltransferase (MT), and helicase (HEL); (2) a Mr 48-57 103 protein with all sequence motifs of viral RNA-dependent RNA polymerases (RdRp) of the “alpha-like” supergroup of positive-strand ssRNA viruses; (3) a Mr 6 103 hydrophobic protein with membrane-binding properties; (4) a Mr 59-70 103 homologue of the cellular HSP70 heat-shock proteins, thought to be implicated in the cell-to-cell movement of the viruses, (5) a Mr 55-64 103 product.
None reported.
None reported.
Genome Organization and Replication
Closteroviruses have the largest genome among positive-strand ssRNA plant viruses. The genome is monopartite for the members of the genus Closterovirus and bipartite for those in the genus Crinivirus. The genome of both genera is characterized by the presence of unique genes coding for a homologue of the HSP70 proteins and for an analogue of the CP. The genome organization, the number and relative position of the ORFs varies with the genus and/or individual viral species. In aphid-transmitted members of the family (BYV, CTV, BYSV), the CPd is upstream of the CP, whereas the reverse is true with whitefly-transmitted (LIYV, SPCSV, Cucumber chlorotic spot virus, CCSV) and mealybug-transmitted (GLRaV-3, LChV) viruses. The ORFs coding for the Mr 6 103 small hydrophic protein, the HSP70 homologue, the 55-64 product, the CP, and its CPd, form a five-gene module conserved among members of the family. The genome expression strategy is based on: (1) proteolytic processing of the polyprotein encoded by ORF1a; (2) +1 ribosomal frameshift for the expression of RdRp domain encoded by ORF1b; (3) expression of the downstream ORFs via the formation of a nested set of 3 co-terminal sgRNAs. dsRNA patterns are very complex and variable among species, reflecting the different number and size of the ORFs present in individual genomes. Replication occurs in the cytoplasm, possibly in association with membranous vesicles and vesiculated mitochondria.
Virion proteins are moderately antigenic. Most of the species are serologically unrelated or distantly related to one another. Monoclonal antibodies have been produced to a number of members, i.e., CTV, GLRaV-1 to -6, Pineapple mealybug wilt-associated virus (PMWaV) and SPCSV. Polyclonal antisera to CTV, LChV, GLRaV-3, and SPCSV have been raised from fusion proteins produced in bacterial expression systems.
The natural and experimental host ranges of individual virus species are restricted. Disease symptoms are of the yellowing type (i.e., stunting, rolling, yellowing or reddening of the leaves, small and late ripening fruits), or pitting and/or grooving of the woody cylinder. Infection is systemic, but usually limited to the phloem, which may necrotize to a varying extent.
Few species are transmissible by mechanical inoculation, though with difficulty. In vegetatively propagated crops, long distance virus dissemination is primarily through infected propagating material. Transmission through seeds is very rare. Natural vectors are aphids, whiteflies (Bemisia, Trialeurodes), pseudococcid (Pseudococcus, Planococcus; Phenacoccus, Saccharicoccus, and Dysmicoccus), and coccids (Pulvinaria, Neopulvinaria, and Parthenolecanium) mealybugs. Transmission is semi-persistent regardless of the type of vector.
Geographical distribution varies from restricted to widespread, depending on the viral species, most of which occur in temperate or subtropical regions.
Virions are usually found in the phloem (sieve tubes, companion cells, phloem parenchyma), occasionally in the mesophyll and epidermis. Ultrastructural modifications consist in membrane proliferation, vesiculation of chloroplasts, degeneration and vesiculation of mitochondria, and formation of inclusion bodies. These are made up of aggregates of virions or membranous vesicles, or a combination of the two. Virions accumulate in conspicuous cross-banded fibrous masses or, more typically, in more or less loose bundles intermingled with single or clustered membranous vesicles. Inclusions of this type are one of the hallmarks of the family. The vesicles contain a fibrillar network and derive either from the endoplasmic reticulum, or from peripheral vesiculation of mitochondria.
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