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Type Species |
(MRFV) |
Virions exhibit icosahedral symmetry, are 28-32 nm in diameter, and do not have an envelope. Capsomer arrangement is readily seen in electron micrographs (Fig. 1).
Physicochemical and Physical Properties
Purified virus sediments as two components: top component (no RNA) (T) and bottom component (B). S20w are 52 (47-57) (T) and 120 (118-124) (B). Buoyant densities in CsCl are 1.26-1.28 g/cm3 (T) and 1.42-1.46 g/cm3 (B) and in Cs2SO4 are 1.24 g/cm3 (T) and 1.37 g/cm3 (B).
RNA constitutes 25-30% of B particles by weight. Virions contain one molecule of linear positive sense ssRNA, Mr 2.0-2.4 106. The genomic RNA of Oat blue dwarf virus (OBDV) has been sequenced and is 6.5 kb in size. RNA is capped at the 5-end, polyadenylated at the 3-end, and has a high cytidine content.
Virions contain a major protein of about 22 103 and a related minor protein of about 28 103, although virions of one isolate of Bermuda grass etched-line virus (BELV) may contain only a single major coat protein (CP) of about 27 103. The major and minor CPs of OBDV are now known to be 21 and 24.4 103, respectively, on the basis of sequence data.
None reported.
None reported.
Genome Organization and Replication
The genomic RNA of OBDV encodes a 227 103 polyprotein with methyltransferase, papain-like protease, helicase, and polymerase motifs similar to those of other positive strand RNA viruses (Fig. 2). The CP sequences are nested within the C-terminal end of the 227 103 polyprotein. The 21 103 CP is probably translated from a sgRNA, while the 24 103 protein is probably cleaved from the 227 103 polyprotein. At least four major peptides with estimated Mr of up to 227 103 are produced upon cell-free translation of OBDV virion RNA. A tentative model for genome expression has been proposed in which the 227 103 polyprotein is processed to yield three fragments: an N-terminal fragment encompassing the methyltransferase, protease, and helicase domains, a fragment containing the RNA polymerase, and the 24 103 CP.
Virions are moderately immunogenic. Although no serological relationship was reported between a Costa Rican isolate of Maize rayado fino virus (MRFV) and OBDV, a distant relationship does exist between a US isolate of MRFV and OBDV. BELV is serologically related to both MRFV and OBDV.
The viruses generally have narrow host ranges restricted to the family Gramineae. One member, OBDV, has a wider host range that includes dicotyledonous plants. The viruses are transmitted by leafhoppers; mechanical transmission of MRFV using a vascular puncture technique has been reported. Replication of marafiviruses in their vectors is suggested by serial passage experiments and an increase of virus structural proteins in vectors with time after infection.
List of Species Demarcation Criteria in the Genus
Members of the genus are considered distinct species on the basis of host range, serology, vector specificity, and limited sequence information. Using polyclonal rabbit antisera, MRFV, BELV, and OBDV can be distinguished from each other in Ouchterlony gel diffusion tests. MRFV is transmitted by Dalbulus maidis, BELV is transmitted by Aconurella prolixa, and OBDV is transmitted by Macrosteles fascifrons (North America) or M. laevis (Europe). Both MRFV and BELV infect maize, but BELV is known to infect oat, durum, and Johnson grass as well. The host range of OBDV includes oat, durum, and a number of other monocotyledonous and dicotyledonous plants (most notably flax), but not maize. Comparisons of partial CP sequences indicate sequence identities of at least 90% among MRFV isolates, but only 43-47% between OBDV and these isolates. No sequence data are available for BELV.
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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Bermuda grass etched-line virus |
(BELV) | |
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Maize rayado fino virus (220) |
[U97717-U97730] |
(MRFV) |
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Oat blue dwarf virus (123) |
[U87832] |
(OBDV) |
Tentative Species in the Genus
None reported.
Phylogenetic Relationships within the Genus
Not available.
Marafiviruses are members of the “alpha-like” group of plant viruses. Based upon the sequence of OBDV, the genome organization of marafiviruses is similar to that of capilloviruses and tymoviruses. Strong similarities in genome sequence, organization, and expression strategy exist between OBDV and tymoviruses. Replicase polyprotein sequences of OBDV and tymoviruses are more closely related to each other than to those of other viruses. Similarities also exist in genome size, the high cytidine content of the genomic RNA, and virion morphology. On the other hand, most tymoviral genomes form tRNA-like structures at their 3-termini, have only one CP encoded in a unique ORF near the 3-terminus, and have a movement protein ORF that overlaps the replication associated polyprotein ORF near the 5-terminus. The marafiviruses and tymoviruses also differ significantly in their general biology.
Both OBDV and capilloviruses are polyadenylated and encode large polyproteins containing a CP at the C-terminus. However, OBDV does not encode a protein analogous to the capillovirus 36 103 protein, virion morphology is quite different, and OBDV has much greater sequence similarity to the tymoviruses.
Marafi: sigla from maize rayado fino.
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