Flaviviridae. Flaviviridae

Description of Flaviviridae adapted from ICTVdb This virus family contains ethiologic agents of serious human diseases, such as viral hepatitis, meningitis, encephalitides, hemorragic fevers and others. About a half of known flaviviruses are transmitted to vertebrates from mosquitos, and less than a third from ticks; others are zoonotic viruses. Morphology Virions consist of an envelope and a nucleocapsid. Virus capsid is enveloped. Virions are spherical to pleomorphic measuring 40-60 nm in diameter. Surface projections are small spikes surrounded by a prominent fringe. Capsid/nucleocapsid is round and exhibits polyhedral symmetry. The core is isometric and has a diameter of 25-30 nm. Capsids can be penetrated by stain and some appear dark in the center. Physicochemical and Physical Properties The molecular mass (Mr) of virions is 60 x 106. Virions have a buoyant density in CsCl of 1.07-1.24 g cm-3; sucrose of 1.1-1.23 g cm-3. The sedimentation coefficient is 140-200 S20w. Under in vitro conditions virions are stable in alkaline environment of pH 8. Virions are sensitive to treatment with heat, organic solvents, and detergents. Nucleic Acid The genome is not segmented and contains a single molecule of linear positive-sense, single-stranded RNA. The complete genome is 9500-12500 nucleotides long. The 5'-end of the genome has a methylated nucleotide cap, or genome-linked protein (VPg). The 3'-terminus has no poly (A) tract, or a poly (A) tract (in some strains of tick-borne encephalitis complex of flaviviruses). Proteins The viral genome encodes structural proteins and non-structural proteins. Virions consist of 3-4 structural protein(s). Lipids Lipids are present and located in the envelope. Virions are composed of 15-20% lipids by weight. The composition of viral lipids and host cell membranes are similar. The lipids are of host origin that derived from host cell membranes. Genome Organization and ReplicationBy itself, genomic nucleic acid is infectious. Replication cycle Features: the genome has 5' end encodes structural proteins which has a stem and loop structure. The non-structural proteins including protease, helicase and polymerase, are encode at the 3' end.