1. Poxviruses Poxviridae
Virion
Genome
Genes and proteins
Viruses and hosts
Diseases
Distinctive characteristics

2. Poxviruses Poxviridae
Virion
Complex, ovoid or brick shaped, nm
Surface ridges or “tubules.”
No typical symmetry elements.
Internal core and lateral bodies.
Virions exist in two forms:
Intracellular mature virus (IMV): one lipid membrane
Extracellular enveloped virus (EEV): two lipid membranes

3. Poxviruses Poxviridae
Genome
Linear ds DNA, 150–250 Kb (vaccinia virus: 200 Kb).
Covalently closed hairpin ends: no free 3 and 5 ends.
10 Kb inverted terminal repeats.

4. Poxviruses Poxviridae
Genes and proteins
150–250 genes (vaccinia virus: 200).
Each gene has own promoter.
No spliced mRNAs (no introns).
Genes distributed on both DNA strands.
Genes expressed in three temporal classes:
Early: dissolve core, direct DNA replication and intermediate gene expression
Intermediate: direct late gene expression
Late: virion structural proteins, RNA polymerase, initiation factors, RNA processing enzymes

5. Poxviruses Poxviridae
Viruses and hosts
Two subfamilies:
Chordopoxvirinae: infect vertebrates:
Humans: variola, vaccinia (vaccine strain), molluscum contagiosum
Viruses that infect a variety of birds and mammals: monkeys, cattle, etc.
Entomopoxvirinae: infect insects (beetles, butterflies, flies, etc.).

6. Poxviruses Poxviridae
Diseases
Smallpox: high fatality rate, now eradicated.
Molluscum contagiosum: the only existing natural infection of humans; relatively rare
but more common in immunocompromised patients.
Monkeypox, cowpox, tanapox are animal poxviruses that occasionally infect humans.

7. Poxviruses Poxviridae
Distinctive Characteristics
Unusually large and complex virus
Virus-coded enzymes for transcription and RNA processing packaged in virion core
Early RNAs made in intact core and extruded into cytoplasm
DNA genome replicates in the cytoplasm using exclusively virus-coded enzymes
Internal envelope not formed by budding but assembled de novo

8. Virion Smallpox was a debilitating and fatal worldwide disease
Variolation led to vaccination, which has eradicated smallpox worldwide
Poxviruses remain a subject of intense research interest
Provide ideal model system for studying mRNA metabolism
Host inflammatory and immune response against viral infection

9. Virion

10. Genome
Linear vaccinia virus genomes have covalently sealed hairpin ends and lack introns
Fig Vaccinia virus genome structure.

11. Genome
Fig Functional organization of the vaccinia virus genome.

12. Virion
Two forms of vaccinia virions have different roles in spreading infection
IMV (intracellular mature virus), EEV (extracellular enveloped virus): differ in membrane composition
IMV contains outer membrane composed of lipid and viral protein, EEV is an IMV particle wraped in additional lipid bilayer
Both forms are infectious
EEV taken by endocytosis, membrane ruptured due to low pH in endosome, release IMV, IMV fuse with vesicle, release viral core into cytosol
IMV enters by direct fusion with plasma membrane

13. Virion
Two forms of vaccinia virions have different roles in spreading infection
Different roles in nature
Fragile EEV is required for virus spread from cell-to-cell and tissue-to-tissue in infected animal
Stable IMV is responsible for transmission of virus between individuals

14. Virion

15. Virion

16. Virion
Poxviruses replicate in the cytoplasm

17. Genes and proteins
Poxvirus genes are expressed in a regulated transcriptional cascade controlled by viral transcription factors
Virus-coded enzymes packaged in the core carry out early RNA synthesis and processing

18. Genes and proteins
Enzymes that direct DNA replication are encoded by early mRNAs

19. Genes and proteins
Poxviruses produce large concatemeric DNA molecules that are resolved into monomers
Fig A model for vaccinia virus DNA replication.

20. Genes and proteins
Postreplicative mRNAs have 5’ end poly(A) extensions and 3’ end heterogeneity
30-50 A residues upstream of AUG codon
May aid in recognition of AUG by ribosome
Heterogeneous 3’ ends
RNA polymerase ignores termination sequence
Mature virions are formed within virus “factories”
viroplasm

21. Genes and proteins
Extracellular virions are extruded through the plasma membrane by actin tails
Poxviruses make several proteins that target host immune defenses

22. Genes and proteins

23. VVT7

24. Key Terms Antibody Complement Concatemer Cytokine Cytotoxic lymphocyte
Endemic
Extracellular enveloped virus (EEV)
Glycosaminoglycan
Guanylyltransferase
Interferons
Interleukin-1
Intracellular mature virus (IMV)
Micelle
Microvilli (singular: microvillus)
2, 5oligoadenylate synthetase
Phagocytosis
PKR (double-stranded RNA-dependent protein kinase)
Pock
Resolvase
Ribose methyltransferase
Serpin
Tumor necrosis factor (TNF)
Vaccination
Variolation
Viroplasm