Poxvirus virus

Properties of Poxviruses

• Virions in most genera are brick-shaped virions, 250×200×200 nm, with an irregular arrangement of surface tubules. Virions of members of the genus Parapoxvirus are ovoid, 260×160 nm, with regular spiral arrangement of surface tubules
• Virions have a complex structure with a core, lateral bodies, outer membrane, and sometimes an envelope
• Genomes have the capacity to encode approximately 200 proteins, as many as 100 of which are contained within virions. Poxviruses, unlike other DNA viruses, encode all of the enzymes required for transcription and replication
• Cytoplasmic replication, enveloped virions released by exocytosis; non-enveloped virions released by cell lysis

Poxviruses are enveloped, large, double-stranded DNA viruses. Members of the poxvirus family infect a broad range of animal species, from insects to fish to birds to mammals. They pose a significant health threat for animals and humans. The best known is variola virus (VARV), the cause of smallpox, an extinct disease that claimed millions of victims and influenced human history.

Classification of Poxvirus

The family Poxviridae is divided into two subfamilies: Chordopoxvirinae (poxviruses of vertebrates) and Entomopoxvirinae (poxviruses of insects). The subfamily Chordopoxvirinae is divided further into nine genera, four of which (Orthipoxvirus, Parapoxvirus, Molluscipoxvirus, and Yatapoxvirus) contain viruses that cause human infections.

See All Related Proteins of Rabies-Virus

The poxvirus genome consists of a single molecule of linear double-stranded DNA varying in size from 130 kbp (parapoxviruses), to 280 kbp (fowlpox virus), and up to 375 kbp (entomopoxviruses). The genomes of vaccinia virus (191,636 bp) and many other poxviruses have been sequenced. Poxvirus genomes have covalent linkage joining the two DNA strands at both ends; the ends of each DNA strand have long inverted tandem repeated nucleotide sequences forming single-stranded loops (Figure).

Poxvirus genomes have the capacity to code for up to 200 proteins, as many as 100 of which are contained within virions. The genome can be divided functionally into a conserved central region coding for those proteins essential for virus replication (e.g., nucleic acid synthesis, virion structural component synthesis). Examples include a DNA-dependent DNA polymerase, DNA ligase, DNA-dependent RNA polymerase, enzymes involved in capping and polyadenylation of messenger RNAs and thymidine kinase. The two flanking regions toward the termini code for a large number of proteins that determine host range, virulence, and immunomodulation. In the case of vaccinia virus, these regions account for nearly half the genome.

Figure 1.Schematic diagram, genus Parapoxvirus. Part of the two diagrams shows the surface structure of an unenveloped virion, the other part shows a cross-section through the center of an enveloped virion.