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Type Species |
(MCMV) |
Virions sediment at an S20w of 109S. The genomic RNA is about 4.4 kb and contains four ORFs. The polymerase is larger than those encoded other species in the family Tombusviridae. Like the panicoviruses, the machlomovirus polymerase has an amino terminal extension fused to the rest of the polymerase that exhibits sequence conservation with the family Tombusviridae. The 3 proximally located CP ORF is expressed in vivo from the 1.1 kb sgRNA. No larger sgRNA has been detected to explain the expression of the two small internally located ORFs. The overall size and organization of the genome is quite similar to that of the genus Panicovirus. However, genomes of the genus Machlomovirus encode an additional 5 proximally located ORF encoding a Mr 32 103 protein of unknown function. The virus is restricted to monocotyledonous hosts.
Virions are approximately 30 nm in diameter and exhibit icosahedral symmetry (Fig. 9). 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
Mr of virions is 6.1 106; S20w is 109S; buoyant density in CsCl is 1.365 g/cm3. Virions are insensitive to ether, chloroform and non-ionic detergents. Virions are stable in vitro for up to 33 days and the thermal inactivation point of virions is between 80-85°C. Virions are stable at pH 6 and lower. Virions are stabilized by divalent cations.
Virions contain a single molecule of infectious linear positive-sense ssRNA. The RNA is 4437 nts in length. The 5-end of the RNA is capped with a m7GpppA residue. The RNA does not contain a 3-terminal poly(A) tract. A 1100 nts sgRNA is also packaged into virions at a very low frequency.
The virion is probably composed of 180 copies of a single CP species made up of 238 amino acids (Mr 25.1 103).
None reported.
None reported.
Genome Organization and Replication
The genomic RNA contains four ORFs (Fig. 10). ORF1 is capable of encoding a Mr 32 103 protein. ORF2 can encode a Mr 50 103 protein. Readthrough of the ORF2 amber termination codon allows for translation to continue into ORF 2RT, yielding a Mr 111 103 protein. A Mr 111 103 protein is produced by translation of virion RNA in in vitro. ORF3 encodes a Mr 9 103 protein whose carboxyl terminus is like those of proteins encoded by similarly located small ORFs in the genomes of the genera Avenavirus, Carmovirus, Necrovirus, and Panicovirus. Assuming readthrough of the ORF3 opal termination codon, a Mr 33 103 protein would be produced. ORF4 encodes the Mr 25.1 103 CP. A sgRNA of 1.1 kb synthesized in infected cells is the translational template for CP. The functions of ORF1 and ORF3 encoded proteins and the ORF3 readthrough product are not known. The ORF2 encoded protein and its readthrough product are thought to be the viral polymerase.
The virus is moderately to highly immunogenic. Various serological variants have been identified. Antisera yield a single precipitin line in agar gel-diffusion assays.
In nature, the virus systemically infects varieties of maize (Zea mays). In the laboratory, the virus is restricted to members of the family Graminae.
The virus is readily transmitted by mechanical inoculation. The virus is also seed-transmitted. Kansas and Nebraska isolates can be transmitted by six species of chrysomelid beetles in the laboratory. A Hawaiian isolate is transmitted by thrips.
The virus has been reported in Argentina, Mexico, Peru, and the United States. Within the United States, the virus is restricted to the Republican River valley of Kansas and Nebraska, and to Kauai, Hawaii.
Pathogenicity, Association with Disease
Maize chlorotic mottle virus (MCMV) causes a mild mosaic on maize in nature. When plants are also infected with one of several Graminae-specific potyviruses, a severe necrotic disease results, termed corn lethal necrosis.
List of Species Demarcation Criteria in the Genus
Not applicable.
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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Maize chlorotic mottle virus (284) |
[X14736] |
(MCMV) |
Tentative Species in the Genus
None reported.
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