Details of DPV and References

DPV NO: 215 July 1979

Family:
Genus:
Species: | Acronym:

Bromovirus group

L. C. Lane Department of Plant Pathology, University of Nebraska, Lincoln, NE 68583, USA

Contents

Type Member

Brome mosaic virus

Main Characteristics

Three types of icosahedral particle with identical diameters (26 nm) and similar sedimentation coefficients (c. 85 S), but with different RNA compositions and slightly differing buoyant densities. The densest type of particle contains RNA-1 (M. Wt c. 1.1 x 106), the lightest particle contains RNA-2 (M. Wt c. 1.0 x 106) and the intermediate type contains RNA-3 and RNA-4 (M. Wt c. 0.75 x 106 and 0.3 x 106). Infection requires all three types of particle or the three largest RNA species. The protein shell has T = 3 icosahedral symmetry and consists of 180 identical subunits (M. Wt c. 2 x 104) clustered into 12 pentamers and 20 hexamers. Thermal inactivation points range from 65° to 95°C. Infectivity survives from a few days to a month in vitro and dilution end-points range from 10-3 to 10-6. The viruses reach and maintain levels of 0.3 to 3 mg per g of plant tissue in their respective natural hosts, in which they induce mottle or mosaic symptoms. The viruses infect several species in the families of their natural hosts, and some species in the genus Chenopodium. Their host ranges are otherwise limited. They are transmissible readily by inoculation with sap and at low frequency by beetles.

Bromovirus properties have been reviewed extensively by Lane (1974).

Members

Members of the bromovirus group together with some of their properties are:

Virus Description no. Strains and synonyms Thermal
inactivation
point*
(°C)		 Isoelectric
point of
particles*
(pH)
Broad bean mottle (BBMV) 101 None 90-95 5.6
Brome mosaic (BMV) 3, 180 Weidelgrasmosaik-Virus
Ryegrass streak virus
Trespenmosaik Virus
Marmor graminis		 70-85 c. 7
Cowpea chlorotic mottle (CCMV) 49 Bean yellow stipple virus†
(distinct strain)		 67-76 c. 4

*Lane, 1974.
†Fulton, Gamez & Scott, 1975.

Geographical Distribution

BMV is common in central USA, eastern Europe and South Africa (Lane, 1974). BBMV has been reported in England (Lane, 1974), Sudan (Murant, Abu-Salih & Goold, 1974) and Portugal (Borges & Louro, 1974); an isolate reported from India (Phatak, 1974) is now found not to be a bromovirus and has been renamed blackgram mottle virus (Scott & Phatak, 1979). CCMV is endemic in south-eastern USA (Lane, 1974) and Central America (Gamez, 1976). The bromoviruses have restricted host ranges; BMV infects a few species in the Gramineae and BBMV and CCMV infect a few species in the Leguminosae. All three viruses give local lesions in some Chenopodium species, which thus serve as useful assay hosts. The viruses can multiply in isolated protoplasts of some species that are apparently immune as whole plants (Furusawa & Okuno, 1978). Detailed host ranges are presented by Lane (1974).

Transmission by Vectors

In the laboratory, BMV (Panarin, 1978; Panarin & Zabavina, 1978; R. W. Toler & L. R. Nault, personal communication), BBMV (Walters & Surin, 1973; Borges & Louro, 1974) and CCMV (Walters & Dodd, 1969) have been transmitted with low efficiency by beetles; transmission of BMV by nematodes is also reported (Schmidt, Fritzsche & Lehmann, 1963; Fritzsche, 1975). It is not known what part vectors play in transmission of bromoviruses in the field, but the viruses can be spread by mechanical injury (McKinney, 1953).

Ecology and Control

The factors that determine natural spread of bromoviruses have not been investigated. In the central USA, clones of Bromus inermis infected with BMV are common and persist indefinitely; the virus spreads slowly and is commonest in populated areas. The distribution of BBMV in the field suggests spread within the crop from scattered point sources of infection (Bawden, Chaudhuri & Kassanis, 1951). CCMV-infected plants are distributed apparently at random; disease incidence is not notably increased adjacent to artificially inoculated plots suggesting that the virus is not normally transmitted from plant to plant within a field (Demski, 1978). There is one report of seed transmission of BBMV (Murant, Abu-Salih & Goold, 1974).

Relations with Cells and Tissues

The bromoviruses infect nearly all the tissues of their hosts (Lane, 1974), except the endosperm of the seed (Proll, 1965). Both protein and nucleic acid are synthesized predominantly in the cytoplasm. A variety of inclusions (spherical, crystalloid, vacuolate) can be detected by light microscopy of infected tissue (Christie & Edwardson, 1977). Crystalline arrays of virus particles can often be detected in infected tissue by electron microscopy. Proliferation of the endoplasmic reticulum and nuclear membranes can be detected in the early stages of virus replication (Burgess, Motoyoshi & Fleming, 1974; Kim, 1977). Apparent multiplication of BMV can be detected by electron microscopy in the wheat curl mite, Aceria tulipae, which is not a vector of the virus (Paliwal, 1972).

Properties of Particles

The protein shells of the bromoviruses consist of 180 identical protein subunits arranged in a T = 3 icosahedral lattice. The subunits are clustered into 12 pentamers and 20 hexamers (Finch & Klug, 1967). The RNA penetrates into the protein shell. It does not completely fill the interior of the particle: there is a central cavity, c. 8 nm in diameter (White & Fischbach, 1973). High concentrations of salt (Bancroft, 1970) or low concentrations of sodium dodecyl sulphate (Boatman & Kaper, 1976) disrupt the particles of bromoviruses into protein and nucleic acid constituents. The bromoviruses are the only isometric plant viruses known to be permeable to low M. Wt solutes (White & Fischbach, 1973), including nucleic acid-binding dyes (Adolph, 1975; L. C. Lane, unpublished data).

Above pH 6.5 and in the absence of divalent cations, the particles swell and the nucleic acid becomes susceptible to nucleolytic digestion (Lane, 1974). In the swollen state, the N-terminal portion of the coat protein, which is rich in basic amino acids, becomes susceptible to proteases but the remaining C-terminal polypeptide retains the ability to form shells (Agrawal & Tremaine, 1972). The N-terminus is probably involved in nucleic acid binding (Tremaine, Ronald & Agrawal, 1977) and may be similar to the N-terminus of the coat protein of tomato bushy stunt virus, which is interpenetrated by the virus RNA, but has no regular structure (Harrison et al., 1978). Amino acid sequencing of BMV protein is almost completed (Moosic, 1978), and partial sequences of CCMV protein are known (Tremaine et al., 1977). Physical properties of the bromoviruses have been compiled by Lane (1974).

Genome Properties

The bromoviruses contain four RNA species of c. 1.1 x 106 (RNA-1), 1.0 x 106 (RNA-2), 0.75 x 106 (RNA-3) and 0.3 x 106 (RNA-4). The genome consists of the three largest RNA species, which are separately encapsidated (Lane & Kaesberg, 1971). With BMV, RNA-1 and RNA-2 are each translated in vitro into a single protein, of M. Wt 1.2 x 105 and 1.1 x 105 respectively (Shih & Kaesberg, 1976); RNA-3 contains two genes, the one towards the 3' terminus coding for the coat protein (M.Wt 2 x 104), and that near the 5' terminus coding for a second protein (M. Wt 3.5 x 104); translation in vitro yields only the second protein (Kaesberg, 1976). RNA-4 is a messenger for the coat protein (Shih & Kaesberg, 1973) but it is not part of the genome; it is derived in vivo from RNA-3. In protoplasts infected with BMV, four new proteins are synthesized (Sakai, Dawson & Watts, 1979), corresponding in size with those found in vitro. Translation of CCMV RNA species was similar to that of BMV, both in vitro (Davies & Kaesberg, 1974) and in vivo (Sakai et al., 1977), except that no protein corresponding to the RNA-1 product of BMV was found. No comparable information is available for BBMV and no function is established for its RNA-3 (Hull, 1972).

The 5' termini of all four RNA molecules of BMV are ‘capped’ with 7-methyl guanosine (Dasgupta, Harada & Kaesberg, 1976). The 5' terminus of RNA-4 binds to ribosomes and the coat protein initiator codon is ten nucleotides from the end (Dasgupta et al., 1975). The 3' termini of all bromovirus RNA molecules accept tyrosine in a reaction catalysed by plant aminoacyl tRNA synthetases (Kohl & Hall, 1974). Partial digestion of each of the BMV RNA molecules gives fragments of 161 nucleotides which retain the tyrosine acceptor activity. The fragments from RNA-3 and RNA-4 have identical sequences and those from RNA-1 and RNA-2 differ only slightly from these; none of them bears much resemblance to tyrosine transfer RNA (Dasgupta & Kaesberg, 1977). The aminoacylation reaction does not function in BMV- RNA directed protein synthesis (Kaesberg, 1976).

The genomes of different bromoviruses are generally incompatible. However, Bancroft (1972) obtained a pseudo-recombinant containing RNA-1 and RNA-2 of BMV and RNA-3 of CCMV; it multiplied very slowly in several species of Chenopodium but not at all in the natural hosts of BMV or CCMV.

Replication

All the bromoviruses replicate in the cytoplasm although there is some evidence for involvement of the nucleus in early stages of replication. Cycloheximide, an inhibitor of cytoplasmic protein synthesis, inhibits viral coat protein synthesis (Gibbs & MacDonald, 1974). The viruses produce double-stranded RNA intermediates (replicative forms) corresponding to the three largest RNA molecules but there is disagreement concerning the existence of a replicative form corresponding to RNA-4 (Philipps, Gigot & Hirth, 1974; Bancroft et al., 1975; Bastin & Kaesberg, 1976). RNA polymerases have been isolated from tissue infected with BMV (Hadidi & Fraenkel-Conrat, 1973; Kummert & Semal, 1977), BBMV (Jacquemin & Lopez, 1974) and CCMV (White & Dawson, 1978). The activities increase during the early stages of replication. The structure and specificity of the polymerases remain in doubt. In vivo, RNA-3 accumulates to a greater extent than the other RNA species; the proportion of RNA-4 increases during the infection cycle (Bancroft et al., 1975; Dawson, 1978). Three of the four proteins encoded by CCMV can be detected in vivo (Sakai et al., 1977). The largest of the three is last to appear. The bromoviruses can be assembled in vitro from RNA and protein (Bancroft, 1970; Adolph & Butler, 1977). The in vivo assembly process is presumably similar.

Relationships

BMV and CCMV are distantly related serologically (Scott & Slack, 1971) and their coat proteins have similar amino acid sequences (Tremaine et al., 1977); they are similar in most properties except for surface charge and host range. BBMV differs appreciably from BMV and CCMV, to which it is serologically unrelated; its protein has a considerably different amino acid sequence and lacks trytophan (Tremaine et al., 1977). BBMV is more susceptible to proteases than BMV or CCMV (Agrawal & Tremaine, 1972). BBMV is the only bromovirus for which seed transmission has been reported. It is more thermostable than BMV and CCMV and contains less RNA-3.

Affinities with Other Groups

The structures and replication strategies of the bromoviruses and cucumoviruses are very similar. However, the cucumoviruses have larger coat proteins, broader host ranges, broader geographical distributions, different vectors (aphids) and are less salt-stable (e.g. in CsCl solutions). Alfalfa mosaic virus and the ilarviruses have replication strategies similar to those of the bromoviruses, but, in addition to the three genome RNA species, require either coat protein or RNA-4 to initiate infection. These viruses are morphologically distinct from the bromoviruses and are similar to the cucumoviruses in their salt stability.

Figures

References list for DPV: Bromovirus group (215)

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