12 nm, constructed from helically arranged protein subunits in a primary helix with a pitch of 3.4 nm and between 9 and 10 subunits per turn (Fig. 1).
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Type Species |
(ASGV) |
Virions are flexuous filaments, 640-700 12 nm, constructed from helically arranged protein subunits in a primary helix with a pitch of 3.4 nm and between 9 and 10 subunits per turn (Fig. 1).
Physicochemical and Physical Properties
S20w of Apple stem grooving virus (ASGV) particles is approximately 112S, isoelectric point is about pH 4.3 at ionic strength 0.1 M, and the electrophoretic mobility is 10.3 and 6.5 10-5cm2/second/V, respectively, at pH 7.0 and 6.0 (ionic strength 0.1 M).
Virions contain linear positive sense ssRNA, 6.5-7.4 kb in size, constituting about 5%, by weight, of virions. The RNA is polyadenylated at its 3
-end. The complete nucleotide sequence of ASGV (three isolates) and of Cherry virus A (CVA) genomic RNA was determined. Isolates of ASGV from different hosts show wide variations in the sequence of a 284-aa region of ORF1-encoded protein, between the polymerase and capsid protein (CP) domains.
Virions are composed of a single protein (Mr 
24-27 103). Nonstructural proteins include a 36-52 103 protein with sequence homology with putative movement proteins, and a large replication-associated protein fused with the CP, with conserved methyltransferase, helicase and RNA polymerase motifs, expressed as a precursor with probable maturation through proteolysis.
None reported.
None reported.
Genome Organization and Replication
The genomic RNA of all sequenced viruses has the same structural organization, and two ORFs (Fig. 2). ORF1 encodes a putative 240-266 103 protein followed by an untranslated region of 142 nts upstream of the 3-poly(A) tail. ORF2 is nested within ORF1 near its 3-end, and encodes a protein with a Mr of 36-52 103. ORF1-encoded product has homologies with putative polymerase proteins of the “alpha-like” supergroup of RNA viruses. Although the CP cistron is located in the C-terminal end of ORF1, and ORF2 is nested within ORF1, the strategy of expression of both CP and putative movement protein (MP) may be based on sgRNA production, as suggested by the analysis of dsRNA patterns from infected tissues. dsRNAs of ASGV consist of five major bands with a size of approximately 6.5, 5.5, 4.5, 2.0 and 1.0 kbp, respectively. The 6.5 kbp species probably represents the double-stranded form of the full-length genome, whereas the 2.0 and the 1.0 kbp species may be the double-stranded forms of sgRNAs coding for the putative MP and the CP, respectively. Replication is likely to occur in the cytoplasm, in which virus particles accumulate in discrete bundles.
Virions are moderately antigenic. There is no apparent serological relationship between species.
ASGV is pathogenic to pome fruits and citrus in which it induces stock/scion incompatibility, i.e., top-working disease of apple and bud union crease syndrome of citrus. It also infects lily. CVA is found in cherry but no disease has been associated with it.
No vectors are known. ASGV was transmitted through seed to progeny seedlings of Chenopodium quinoa, and lily. ASGV, CVA, and Nandina stem pitting virus (NSPV) are transmitted by grafting. NSPV has not been transmitted by sap inoculation, but by slashing stems with a partially purified virus preparation.
Geographic distribution ranges from wide to restricted according to the virus. ASGV has been recorded from most areas where apples are grown, and is widespread in citrus in China, Japan, United States, Australia, and South Africa. LCLV occurs in England and possibly in continental Europe and the United States. NSPV is found only in the United States and CVA in Germany.
No distinct cytological alterations have been observed in infected cells. Virus particles occur in bundles in mesophyll and phloem parenchyma cells, but not in the epidermis and sieve elements.
List of Species Demarcation Criteria in the Genus
The criteria demarcating species in the genus are:
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Natural host range, |
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Serological specificity (all known species are serologically unrelated), |
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Amino acid sequences of CP and putative MP differing by more than 10%. |
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, CMI/AAB description numbers ( ), genome sequence accession numbers [ ], and assigned abbreviations ( ) are:
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Apple stem grooving virus (31) (Citrus tatter leaf virus) |
[D14995, D16681, D16368, D14455, AB004063] |
(ASGV) |
|
Cherry virus A |
[X82547] |
(CVA) |
|
Lilac chlorotic leafspot virus (202) |
(LiCLV) |
Tentative Species in the Genus
|
Nandina stem pitting virus |
(NSPV) |
Phylogenetic Relationships within the Genus
Not available.
Members of the genus Capillovirus have the same particle morphology as those of the family Closteroviriridae and the genera Trichovirus and Vitivirus. Similarities exist between members of the genera Capillovirus, Trichovirus and Vitivirus in amino acid sequence around conserved helicase and polymerase motifs, in their putative MP and CP, and in genome expression strategy. The genome organization, however, is remarkably different.
Capillo: from Latin capillus, a hair.
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