Taxonomic Structure of the Family
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Family |
Flaviviridae |
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Genus |
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Genus |
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Genus |
Virions are 40-60 nm in diameter, spherical in shape and contain a lipid envelope. The capsid is composed of a single capsid protein (C) and the envelope contains two or three virus-encoded membrane proteins. The behavior of hepaciviruses during filtration and its sensitivity to chemical and physical treatments suggest that its overall structural properties are similar to those of flaviviruses and pestiviruses. Specific descriptions of the three individual genera are given in the corresponding sections.
Physicochemical and Physical Properties
The virion Mr, buoyant density, sedimentation coefficient and other physicochemical properties differ among the members of the three genera and are described separately in the corresponding sections.
The genome RNA of all three genera is a positive sense ssRNA of approximately 11, 12.3, and 9.6 kb for flavi-, pesti-, and hepaciviruses, respectively. It lacks a 3-terminal poly(A) tract, and in the case of flaviviruses contains a 5-terminal type I cap. A cap structure has not been demonstrated for pesti- or hepaciviruses.
All members of the family have a single, small basic capsid protein and two (Flavivirus and Hepacivirus) or three (Pestivirus) membrane-associated proteins. The nonstructural proteins contain sequence motifs characteristic of a serine protease, RNA helicase, and RNA-dependent RNA polymerase that are encoded at similar locations along the genome in all three genera. Further details of specific functional properties are given in the corresponding section of the individual genera.
Lipids present in virions are derived from host cell membranes and make up 15 to 20% of the total virion weight in the case of flaviviruses. The lipid content of pesti- and hepaciviruses has not been determined.
Virions contain carbohydrates in the form of glycolipids and glycoproteins.
Genome Organization and Replication
The genome RNA of all three genera has a similar organization and is the only mRNA found in infected cells. It contains a single long ORF flanked by 5- and 3-terminal noncoding regions (NCRs) that form specific secondary structures required for genome replication. Translation-initiation is cap-dependent in the case of flaviviruses, whereas internal ribosomal entry sites have been demonstrated for pesti- and hepaciviruses. Viral proteins are synthesized as part of a polyprotein of more than 3000 amino acids which is co- and post-translationally cleaved by viral and cellular proteinases. The structural proteins are contained in the N-terminal portion of this polyprotein and the nonstructural proteins in the remainder. The latter include a serine proteinase, an RNA helicase, and an RNA-dependent RNA polymerase. RNA synthesis occurs in the cytoplasm in association with membranes via synthesis of a full-length negative-strand intermediate. Virion assembly and envelopment take place at intracellular membranes, and particles are transported in cytoplasmic vesicles through the secretory pathway before they are released by exocytosis.
The three genera are antigenically unrelated, but serological cross-reactivities exist among members within the genera Flavivirus and Pestivirus. Hepaciviruses have so far not been amenable to antigenic analysis.
The biological properties of the three genera exhibit different characteristics and are described in the corresponding sections.
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