proximally located ORF encoding a protein with Mr 32 
103 of unknown function. The virus is restricted to monocotyledonous hosts.
|
Type Species |
(PMV) |
Virions sediment at S20w 109S. The genomic RNA is 4.3 kb and contains four ORFs. The polymerase is larger than those encoded by members of the family Tombusviridae. Like that of the machlomoviruses, the panicovirus polymerase has an amino terminal extension fused to the rest of the polymerase that is phylogenetically conserved among the family Tombusviridae. The virus produces only a single 1.5 kb sgRNA that is a template for the expression of ORFs 2 and 2FS. A second smaller sgRNA that could be used for the expression of the CP ORF3 and ORF4 has not been identified. The overall size and organization of the genome is similar to that of the genus Machlomovirus. However, viruses in the genus Panicovirus lack the additional 5 proximally located ORF encoding a protein with Mr 32 103 of unknown function. The virus is restricted to monocotyledonous hosts.
Virions are approximately 30 nm in diameter and exhibit icosahedral symmetry (Fig. 13). Detailed structure of virions is not known. Based on CP sequence similarity, it is predicted that the virion is structurally similar to the T = 3 virions of Southern bean mosaic virus (genus Sobemovirus).
Physicochemical and Physical Properties
The virus has one sedimenting component with an S20w of 109S, a buoyant density of 1.365 g/cm3 in CsCl, and a virion Mr of 6.1 106. Virus stored in desiccated tissue retained infectivity after twelve years. The thermal inactivation point of the type strain is 85°C, and 60°C for the St. Augustine grass decline strain. Virions are stable at pH 6 and lower. Virions are insensitive to ether, chloroform and non-ionic detergents. Virions are stabilized by divalent cations.
Virions contain a single molecule of infectious linear positive sense ssRNA. The RNA is 4326 nts in length. The 5-end of the RNA appears not to be capped. The RNA does not contain a 3-terminal poly(A) tract. A 1475 nts sgRNA is also produced in vivo that appears not to be packaged into the virions.
The virion is probably composed of 180 copies of a single CP species (Mr 26 103).
None reported.
None reported.
Genome Organization and Replication
The genomic RNA contains four ORFs (Fig. 14). ORF1 encodes a Mr 48 103 protein. Readthrough of the ORF1 amber termination codon allows the production of a Mr 112 103 protein. Both the Mr 48 103 and a Mr 112 103 polypeptides are produced by translation of virion RNA in vitro. ORF2 encodes a Mr 8 103 protein that is produced by in vitro translation of a transcript representing the subgenomic RNA. Assuming a -1 ribosomal frameshifting event at a canonical ribosomal frameshifting signal at the 3-terminus of ORF2, a fusion protein of Mr 14.6 103 can be produced, which also has been detected upon in vitro translation of the sgRNA transcript. ORF3 encodes the Mr 26 103 CP. ORF4 is within ORF3 in a different reading frame. This ORF encodes a Mr 15 103 polypeptide. The 1.5 kb sgRNA synthesized in infected cells is thought to be the translational template for ORF2, the ORF2 frameshift product (ORF 2FS), ORF3, and the ORF4 gene products. The ORF1 and its amber terminator readthrough product are thought to be the viral polymerase. The ORF2 and its frameshift product are believed to encode proteins involved in virus movement. ORF3 encodes the virus CP. The function of the ORF4 product is not known.
The virus is highly immunogenic. Various serological variants have been identified. Antisera yield a single precipitin line in agar gel-diffusion assays. There are several serological strains of the type strain as well as the serologically distinct St. Augustine grass decline strain.
In nature, the virus is restricted to grass species in the Paniceae tribe of the Poaceae. It is known to cause diseases of note in switch grass (Panicum virgatum), St. Augustine grass (Stenotaphrum secundatum), and centipede grass (Eremochloa ophiuroides). In the laboratory, a number of additional species in the Graminae can be symptomless hosts. Zea mays is used as a propagation and purification host for the type strain. St. Augustine grass (Stenotaphrum secundatum) must be used as a propagation host for the St. Augustine grass decline strain.
The virus is readily transmitted by mechanical inoculation. The virus is typically transmitted by the transport and replanting of infected sod. There is one report that the St. Augustine grass decline strain was seed-transmitted through Setaria italica.
The virus has been reported in the USA and Mexico. The type strain is widely distributed in a number of turf grasses throughout the central United States whereas the St. Augustine grass decline strain is widely distributed throughout the southern United States.
Pathogenicity, Association with Disease
The virus typically forms a systemic mosaic. More severe symptoms, including chlorotic mottling, stunting and seed yield reductions, occur in forage grasses when Panicum mosaic virus (PMV) is in a mixed infection with Panicum mosaic satellite virus. The St. Augustine grass decline strain can cause a severe disease on St. Augustine grass with symptoms being more severe in the hot summer months.
List of Species Demarcation Criteria in the Genus
The list of species demarcation criteria in the genus is:
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Extent of serological relationship as determined by immunodiffusion and/or ELISA, |
|
Extent of sequence identity between relevant gene products, |
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Soil transmission with the aid of a biological vector, |
|
Natural host range, |
|
Artificial host range reactions. |
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:
|
Panicum mosaic virus (177) |
[U55002] |
(PMV) |
Tentative Species in the Genus
|
Molinia streak virus |
(MoSV) |
|
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