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

Poxviruses Poxviridae Virion Complex, ovoid or brick shaped, 300 250 150 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

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.

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

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.).

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.

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

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

Virion

Genome Linear vaccinia virus genomes have covalently sealed hairpin ends and lack introns Fig. 15.1 Vaccinia virus genome structure.

Genome Fig. 15.2 Functional organization of the vaccinia virus genome.

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

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

Virion

Virion

Virion Poxviruses replicate in the cytoplasm

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

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

Genes and proteins Poxviruses produce large concatemeric DNA molecules that are resolved into monomers Fig. 15.5 A model for vaccinia virus DNA replication.

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

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

Genes and proteins

VVT7

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