Description of Bunyaviridae
adapted from
ICTVdb
Virion Properties
Distinct viral structures are visible in thin sections of
infected tissue. Particles contain nucleic acid which is
encapsidated.
Morphology
Virions consist of an
envelope and a nucleocapsid. During their life cycle, virions
have an extracellular phase; occur in one phenotype only and are
encapsidated during extracellular phase. Virus may be sequestered
within inclusion bodies that are not occluded and typically
contain one nucleocapsid. Virus capsid is enveloped by a single
layer envelope. Virions are spherical to pleomorphic. Have no
protrusions. Virions measure (80-)100(-120) nm in diameter. The
envelope surrounds three nucleocapsids; has surface projections.
Surface projections are distinctive spikes covering evenly the
surface which are embedded in a lipid bilayer that is 5 nm thick.
Surface projections are 5-10 nm long. Host ribosomes are not seen
inside the envelope. A regular capsid structure is present.
Capsid/nucleocapsid is elongated with helical symmetry. The
ribonucleocapsid is filamentous with a length of 200-3000 nm
(depending on arrangement) and a width of 2-2.5 nm. The
nucleocapsid is not segmented. Nucleocapsids are circular.
Virion populations are comprised of particles of uniform size
and only one species is recovered in preparations.
Physicochemical and Physical Properties
The molecular mass (Mr) of
virions is 300-400 x 10
6. Virions have a buoyant
density in CsCl of 1.2-1.21 g cm
-3; sucrose of
1.16-1.18 g cm
-3. The sedimentation coefficient is
350-500
S20w. Virions are sensitive to
treatment with detergents, formaldehyde, heat.
Nucleic Acid
The Mr of the genome constitutes 1-2% of the virion
by weight. The genome is monomeric; segmented and consists of
three segments of circular; sometimes supercoiled, negative-sense
and ambisense, single-stranded RNA that forms a non-covalently
closed circle (through hydrogen-bonded ends). Minor species of
non-genomic nucleic acid are not found in virions. The complete
genome is 10500-22700 nucleotides long. The RNA-L is sequenced,
but only an estimate is available, complete sequence is
6300-12000 nucleotides long. RNA-M is sequenced, complete
sequence is about 3500-6000 nucleotides long. RNA-S is sequenced
and complete sequence is about 1000-2200 nucleotides long. The
genome has terminally redundant sequences. The terminally
redundant sequences have reiterated terminal sequences. The
genome sequence is repeated at both ends. Terminal nucleotides
are base-paired, forming non-covalently closed, circular RNAs.
Nucleotide sequences at the 3'-terminus are complementary to the
5' end and form a panhandle. The 5'-terminal sequence has
conserved regions and repeats complementary to the 3'-terminus;
terminal repeats at the 5'-end are 8-11 nucleotides long. The
5'-terminus has no poly (C) tract. The 3'-terminus has conserved
nucleotide sequences; of 8-11 nucleotides in length; in each gene
segment. The multipartite genome is found in one type of particle
only. Each virion contains a single copy of the genome.
Proteins
Proteins constitute about over 50% of
the particle weight.
The viral genome encodes structural proteins and
non-structural proteins.
Lipids
Lipids are present and located in the
envelope. Virions are composed of 20-30% lipids by weight. The
composition of viral lipids is known. The composition of viral
lipids and host cell membranes are similar. The lipids are of
host origin and are derived from plasma membranes. Viral
membranes include phospholipids, sterols, fatty acids, and
glycolipids. Lipids are not essential for infectivity.
Genome Organization and Replication
Coding Strategy of Segment 1:
Sequence has no gene blocks.
Translation: Replication
does not involve a reverse transcription step.
Virions are not dependent on a helper virus for
replication.
Release: The outer
envelope acquired by budding. The outer envelope lipids are
derived from cellular Golgi membranes, or cell surface membranes
(occasionally).