L. C. Lane
Department of Plant Pathology, University of Nebraska, Lincoln, NE 68583, USA
Brome mosaic virus
Three types of icosahedral particle with identical diameters (26 nm) and similar sedimentation
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
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
. 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 of the bromovirus group together with some of their properties are:
||Strains and synonyms
|Broad bean mottle (BBMV)
|Brome mosaic (BMV)
|| 3, 180
Ryegrass streak virus
|Cowpea chlorotic mottle (CCMV)
||Bean yellow stipple virus
Fulton, Gamez & Scott, 1975.
Geographical Distribution etc
BMV is common in central USA, eastern Europe and South Africa
BBMV has been reported in England
(Murant, Abu-Salih & Goold, 1974
) and Portugal
(Borges & Louro, 1974
an isolate reported from India
) is now found not to be a bromovirus
and has been renamed
blackgram mottle virus
(Scott & Phatak, 1979
). CCMV is endemic in
) and Central America
). 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)
Association with Vectors
In the laboratory, BMV
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
). It is not
known what part vectors play in transmission of bromoviruses in the field, but the viruses can be
spread by mechanical injury
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
). 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
), except the endosperm of the seed
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
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
The protein shells of the bromoviruses consist of 180 identical protein subunits arranged in a
= 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
) 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
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).
The bromoviruses contain four RNA species of c.
1.1 x 106
1.0 x 106
(RNA-2), 0.75 x 106
(RNA-3) and 0.3 x 106
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
(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
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
(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
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
(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
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.
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
(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
(Jacquemin & Lopez, 1974
) and CCMV
(White & Dawson, 1978
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
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
Adolph & Butler, 1977
assembly process is presumably similar.
Relationships within the Taxon
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
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.
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