Chapter 13 Viruses, Viroids and Prions

Adolf Mayer, 1886 –tobacco mosaic disease (TMD) transmissible Dimitri Iwanowski, 1892 –Filtered sap still caused TMD –contagious fluid or filterable agent Walter Reed,1901 –Yellow Fever

Felix d’Herelle, 1917 –Bacteriophage –Suggested phage therapy 1930’s, term virus introduced and electron microscopy invented Wendell Stanley, 1935 –Isolated tobacco mosaic virus

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Viral Features –Obligate intracellular pathogen –Host range highly specific or generalists –DNA or RNA –Protein coat –Surface proteins highly susceptible to mutations –Cause synthesis of specialized structures to transfer viral particles to other cells

Virion –complete, fully developed, infectious viral particle –Living or non-living entities?????

Oldest system based on symptomology International Committee on Taxonomy of Viruses (ICTV), 1966 –Describe viruses as elementary bio-systems –Classified into orders, families, genera and species –Over 1,500 officially recognized species Viral Classification

3 primary means of classification: –Nucleic acid –Replication strategy –Morphology of protein coat (capsid)

Helical Viruses Polyhedral Viruses

Viral Envelope –Acquired from host cell –Phospholipids and proteins –Some glycoproteins are virally coded spikes –Often play role in host recognition

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Complex Viruses

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Viral Taxonomy Order –virales Family –viridae Genus – virus Species –Common names –Subspecies designated by a number

Viral Taxonomy Retroviridae– family –Lentivirus – genus Human Immunodeficiency Virus– species Herpesviridae –Simplexvirus Human herpesvirus 1, HHV 2, HHV 3

Isolation and Cultivation of Viruses Viruses must be grown in living cells –Cytopathic effects

Animal viruses may be grown in living animals or in embryonated eggs

Animal & plants viruses may be grown in cell cultures –Primary cell lines –Continuous cell lines (transformed cells )

Serological tests –Detect antibodies against viruses in a patient Nucleic acids –RFLPs –PCR Virus Identification

Multiplication of Bacteriophages Lytic cyclePhage causes lysis and death of host cell Lysogenic cycleTemperate phages incorporate DNA into host DNA (prophage)

AttachmentPhage attaches by tail fibers to host cell PenetrationPhage lysozyme opens cell wall, tail sheath contracts to force tail core and DNA into cell BiosynthesisProduction of phage DNA and proteins MaturationAssembly of newly synthesized phage particles ReleasePhage lysozyme breaks cell wall Lytic Cycle

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One-step Growth Curve

Lysogenic Cycle

3 Important results of lysogeny –Immunity to re-infection –Phage conversion –Specialized transduction

Specialized Transduction Prophage exists in galactose-using host (containing the gal gene). Phage genome excises, carrying with it the adjacent gal gene from the host. Phage matures and cell lyses, releasing phage carrying gal gene Prophage gal gene Bacterial DNA Galactose-positive donor cell gal gene Phage infects a cell that cannot utilize galactose (lacking gal gene). 4 Galactose-negative recipient cell Along with the prophage, the bacterial gal gene becomes integrated into the new host’s DNA. 5 Lysogenic cell can now metabolize galactose. 6 Galactose-positive recombinant cell

AttachmentViruses attaches to cell membrane PenetrationBy endocytosis or fusion UncoatingBy viral or host enzymes BiosynthesisProduction of nucleic acid and proteins MaturationNucleic acid and capsid proteins assemble ReleaseBy budding (enveloped viruses) or rupture Multiplication of Animal viruses

Attachment of animal viruses –Chemical attraction –No tails or tail fibers –Glycoprotein spikes or other attachment molecules

Replication of Animal Viruses –Biosynthesis Each virus requires different strategy depending on its nucleic acid DNA viruses often enter the nucleus RNA viruses typically replicate in cytoplasm Must consider: –What serves as template for replication and how mRNA is transcribed

Multiplication of DNA Virus Virion attaches to host cell Virion penetrates cell and its DNA is uncoated Viral DNA penetrates host nucleus Early transcription and translation; enzymes are synthesized DNA Late transcription; DNA is replicated 4 Late translation; capsid proteins are synthesized 5 Virions mature 6 Capsid Papovavirus Host cell DNA Cytoplasm Virions are released 7 Capsid proteins mRNA

[INSERT FIGURE 13.13] Multiplication of RNA Virus

Multiplication of a Retrovirus Retrovirus penetrates host cell. Virion penetrates cell and its DNA is uncoated The new viral DNA is tranported into the host cell’s nucleus and integrated as a provirus. The provirus may divide indefinitely with the host cell DNA DNA Transcription of the provirus may also occur, producing RNA for new retrovirus genomes and RNA that codes for the retrovirus capsid and envelope proteins. 4 Mature retrovirus leaves host cell, acquiring an envelope as it buds out. 5 Capsid Reverse transcriptase Virus Two identical + stands of RNA DNA of one of the host cell’s chromosomes Provirus Host cell Reverse transcriptase Viral RNA RNA Viral proteins Identical strands of RNA

Assembly and release of animal viruses Most DNA viruses assemble in and are released from nucleus into cytosol Most RNA viruses develop solely in cytoplasm Enveloped viruses cause persistent infections Naked viruses are released by exocytosis or may cause lysis and death of host cell

Oncogenes transform normal cells into tumor cells –Activated by mutagenic chemicals, radiation, viruses –Causes increased growth, loss of contact inhibition –Cells tend to be misshapen and exhibit chromosomal abnormalities Viruses and Cancer

Oncoviruses Viral DNA integrated into host DNA Induces tumors

Oncogenic DNA Viruses –Adenoviridae –Herpesviridae –Poxviridae –Papovaviridae –Hepadnaviridae Oncogenic Viruses Oncogenic RNA viruses –Retroviridae DNA HTLV 1 HTLV 2

Latent Viral Infections –Virus remains dormant in asymptomatic host cell for long periods Cold sores, shingles Persistent Viral Infections –Disease progresses slowly over a long period, generally fatal Subacute sclerosing panencephalitis (measles virus)

Plant Viruses –Plant viruses enter through wounds or via biting insects –May be transmitted in pollen Viroids –infectious naked RNA Potato Spindle Tuber Viroid

Proteinaceous infectious particle Inherited and transmissible diseases Spongiform encephalopathies –Sheep scrapie, Creutzfeldt-Jakob disease, Gerstmann- Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow disease Prions

PrP C, normal cellular prion protein PrP Sc, scrapie protein

[INSERT FIGURE 13.23] scrapie proteins accumulate in brain cells forming large vacuoles