Microbiology 6/e

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1. PLANT VIRUS 2. INFECTIOUS AGENTS (Virus-like
agents) VIROIDS, VIRUSOIDS AND PRIONS
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LESSON OUTCOME

• Definition viroids, virusoids and prions
• Explain these three
• Similarities and differentiation between these
  virus-like agents
• Compare between virus and viroids/virusoids.
• Characteristics to compare prions, viroids and
  viruses
• Relationship between virus and cancer

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Plant viruses

• Plant viruses face special problems initiating an
  infection.
• - The outer surfaces of plants are composed of
  protective layers of waxes and pectin, but more
  significantly, each cell is surrounded by a thick
  wall of cellulose overlying the cytoplasmic
  membrane.
• - no plant virus is known to use a specific
  cellular receptor of the type that animal and
  bacterial viruses use to attach to cells.
• - Plant viruses rely on a mechanical breach of
  the integrity of a cell wall to directly
  introduce a virus particle into a cell. - - This
  is achieved either by the vector associated with
  transmission of the virus or simply by mechanical
  damage to cells.

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Transmission Plant viruses

• Through sap - entry
• Seeds due to external contamination of the seed
  with virus particles, or due to infection of the
  living tissues of the embryo.
• Vectors Many different groups of living
  organisms can act as vectors and spread viruses
  from one plant to another
• Bacteria (e.g. Agrobacterium tumefaciens - the Ti
  plasmid of this organism has been used
  experimentally to transmit virus genomes between
  plants)
• Fungi
• Nematodes
• Arthropods Insects - aphids, leafhoppers,
  planthoppers, beetles, thrips, etc.
• Arachnids - mites
• Mechanical Mechanical transmission of viruses is
  the most widely used method for experimental
  infection of plants and is usually achieved by
  rubbing virus-containing preparations into the
  leaves, which in most plant species are
  particularly susceptible to infection. However,
  this is also an important natural method of
  transmission. Virus particles may contaminate
  soil for long periods and may be transmitted to
  the leaves of new host plants as wind-blown dust
  or as rain-splashed mud.

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RNA Viruses - Class IV ( sense)
Plant viruses example
ssRNA
Family Potyviridae (Genus Potyvirus) - potato
virus Y - papaya ringspot
virus (PRSV) Flexiviridae - potato
virus X Comoviridae- cowpea mosaic
virus Genus Tobamovirus- tobacco mosaic virus
Family Sequiviridae - Plant virus Genus
Waikavirus rice tungro spherical virus
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INFECTOUS AGENTS

• Virion a complete virus particle, including
  envelope (if any)
• Viroid an infectious RNA particle, smaller
  than a virus, lacking a capsid, that causes
  various plant diseases
• Virusoid (satellite nucleic acids) same as
  viroid small, ssRNA molecule, usually 500 to
  2000 nucleotides in length, lacking a capsid,
  lack genes required for the replication ?
  virusoid require a helper (satellite) virus to
  replicate, causes various plant diseases.

OTHER THAN VIRUS IN PLANT
- depend on virus to replicate, its nucleotide
only encode structural proteins
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SIMILARITIES
Viroid Nucleic acid ssRNA Virusoid Lack
capsid, cause plant diseases
DIFFERENCES
Viroids Virusoids
1. Does not require helper virus 1. Require helper virus for replication
2. Does not encode protein 2. Encode proteins
3. Replication takes place in host cell nucleus 3. Replication takes place in hosts cytoplasm
Animal virusoid hepatitis delta virusoid the
helper virus is Hepatitis B virus.
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Example
The potato spindle tuber viroid and tomato plant
macho viroid, members of the family Pospiviroidae,
Avsunviroidae, two known members are the avocado
sunblotch viroid and the peach latent mosaic
viroid.

• Viroid
• avocado sunblotch viroid, peach latent mosaic,
  potato spindle tuber, coconut cadang-cadang,
  tomato plant macho viroid, citrus bent leaf
  viroid, pear blister canker viroid
• Virusoid
• - barley yellow dwarf satellite RNA , tobacco
  ringspot virus satellite RNA

Disease
Viroid - cause lethal plant diseases potato
spindle tuber disease, chrysanthemum stunt
disease, cucumber pale fruit disease, coconut
cadang-cadang disease, chrysanthemum stunt
disease, tomato apical stunt disease. Virusoid -
cause tobacco necrosis
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PRIONS

• A unique infectious agent, protein infectious
  particle
• Prion a small infectious particle consisting
  of protein and lack nucleic acid.
• Prion features
• Resistant to inactivation by heating to 90o C,
  which inactivate virus
• The infection is not sensitive to radiation
  radiation damages virus genomes
• Prions are not destroyed by enzymes that digest
  nucleic acids
• Sensitive to protein denaturing agents urea,
  phenol
• Prions have direct pairing of amino acids

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PRIONS

• Diseases
• Creutzfeldt-Jakob mental degeneration, loss of
  motor function and death (human)
• Scrapie and bovine spongiform encephalopathy
  (BSE) loss of neuronal function that leads to
  death in sheep and dairy cattle.
• Kuru a neurological disorder in human

Mad cow disease
Holes brain spongiform
Cerebral cortex of a normal human brain (right),
patient with CJD (left).
Once present in the brain, prions cause normal
proteins to refold into abnormal shapes. As these
abnormal proteins multiply, they destroy neurons
and eventually cause brain tissue to become
riddled with holes. Prions can only be destroyed
through incineration.
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Viruses, Viroids and Prions

• Characteristic between virus, viroid and prion
• Nucleic acid type, NA strandedness, host range
  and structural features

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Virus and Cancer
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VIRUS AND CANCER - INTRODUCTION

• Cancers are the result of a disruption of the
  normal restraints on cellular proliferation. It
  is apparent that the number of ways in which such
  disruption can occur is strictly limited and
  there may be as few as forty cellular genes in
  which mutation or some other disruption of their
  expression leads to unrestrained cell growth.
• There are two classes of these genes in which
  altered expression can lead to loss of growth
  control 
• (a) Those genes that are stimulatory for growth
  and which cause cancer when hyperactive.
  Mutations in these genes will be dominant. These
  genes are called oncogenes.
• (b) Those genes that inhibit cell growth and
  which cause cancer when they are turned off.
  Mutations in these genes will be recessive. These
  are the anti-oncogenes or tumor-suppressor genes.
• Viruses are involved in cancers because they can
  either carry a copy of one of these genes or can
  alter expression of the cell's copy of one of
  these genes. tumor viruses

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REMEMBER Animal virus

• Not all virus infection will cause immediate
  lysis to host cell
• some maintain a carrier relationship, these so
  called persistent infections can last from a few
  weeks to the remainder of the hosts life.
• Example measles virus, herpes simplex viruses
  (cold sores and genital herpes) and herpes zoster
  virus (chicken pox and shingles).

Virulent virus
Latency/latent state
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Viruses and Cancer

• Some animal viruses enter their host cell and
  permanently alter its genetic material, leading
  to cancer.
• These viruses are termed oncogenic,
• Their effect on the cell is called
  transformation.
• - A startling feature of these viruses is
  that their nucleic acid is consolidated into the
  host DNA.
• - Transformed cells have an increased rate of
  growth alterations in chromosomes changes in
  the cells surface molecules and the capacity to
  divide for an indefinite period, unlike normal
  animal cells.
• Mammalian viruses capable of initiating tumors
  are called oncoviruses.
• DNA viruses such as papillomavirus (genital
  warts are associated with cervical cancer),
  herpesviruses (Epstein-Barr virus causes
  Burkitts lymphoma), and hepatitis B virus.

A process when virus catalyze the conversion of a
normal cell to a tumor cell.
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Latent state
Related with tumor formation eventually
producing cancerous cell
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TUMOR VIRUSES

• CLASSES OF TUMOR VIRUSES
• There are two classes of tumor viruses
• DNA tumor viruses
• RNA tumor viruses, the latter also being referred
  to as RETROVIRUSES.
• These two classes have very different ways of
  reproducing themselves but they often have one
  aspect of their life cycle in common the ability
  to integrate their own genome into that of the
  host cell. Such integration is not, however, a
  pre-requisite for tumor formation.

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How viruses cause cancer

• - DNA tumor viruses can exist as provirus
• RNA tumor viruses use own RTase to transcribe
  () sense RNA into DNA that integrates as
  provirus
• Cancer is due to unregulated viral proteins in
  cell causing the host cells to divide
  uncontrollably. Some viral proteins blocks the
  tumor-suppressor genes (prevent uncontrolled cell
  divisions) therefore a tumor develops.

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Viruses and Cancer

• Oncogenes gene that when mutated and expressed
  at high level will form tumor due to uncontrolled
  host cell divison
• In DNA tumor virus, oncogenes also contain the
  information for synthesizing viral proteins for
  viral replication
• Proto-oncogenes normal gene (from normal host
  cells), that under the control of a virus can
  cause uncontrolled cell division can act as
  oncogene. The oncogenes are not needed for virus
  replication.

Oncogene a gene that can bring about malignant
transformation
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VIRUS AND CANCER

• Examples of viruses that are associated with
  human cancers
• Epstein-Barr virus (EBV)
• Human papillomavirus (HPV)
• Hepatitis B virus (HBV)

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EBV

• What is the Epstein-Barr virus? Human herpesvirus
  4 (HHV-4),
• Herpesviridae so dsDNA linear virus,
  icosahedral capsid and has envelope
• EBV is a member of the herpesvirus family and
  causes more than 90 of cases of mononucleosis.
  EBV is a double-stranded DNA virus named for the
  English virologists professor Sir Anthony Epstein
  and Yvonne M. Barr.
• The designation "mononucleosis" refers to an
  increase in a special type of white blood cells
• (lymphocytes) in the bloodstream
• relative to the other blood
• components as a result of the EBV
• infection.

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EBV

• Mono is usually spread by person-to-personcontact
  with saliva as the primary method of
  transmission.
• person with mono can also pass on the disease by
  coughing or sneezing, causing small droplets of
  infected saliva and/or mucus to be suspended in
  the air and inhaled by others. Sharing food or
  beverages from the same container or utensil can
  also transfer the virus from one person to
  another, since contact with infected saliva may
  result.

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EBV

• The initial symptoms of mono are a generallack
  of energy (malaise), loss of appetite, and
  chills. The more common intense symptoms include
  a severe sore throat, fever, and swollen glands
  (lymph nodes) in the neck area. It is typically
  the severe sore throat that prompts people to
  contact their doctor. have recovered will
  continue to secrete the EBV in their saliva for
  years due to periodic "reactivations" of the
  viral infection.

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EBV

• This disease rarely fatal in healthy man, Mono
  tends to be more aggressive in patients with
  abnormal immune systems, such as people with AIDS
  or those who are taking medications that suppress
  immune function. The EBV has been associated with
  some types of cancers, most commonly lymphomas.
  As well, some studies have linked EBV to the
  development of at least one subtype of Hodgkin's
  disease.
• Burkitts Lymphoma - caused by Epstein-Barr
  Virus  2
• In 1958 he reported that the jaw tumours affected
  children between the ages of two and fourteen
  years, with a peak incidence occurring at five
  years. What became known as Burkitts lymphoma
  (BL) was found to occur in a band across tropical
  Africa, tailing partly down the east coast, and
  showing a similar distribution to holoendemic
  malaria. Also cause Nasopharyngeal Carcinoma

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Human papillomavirus

• Non-enveloped ds DNA virus
• Family Papillomaviridae
• HPV has vaccine
• Spread through epithelial skin contact
• Some cause only benign warts.
• HPV-cause of cervical cancer, genital warts. HPVs
  may play a role in some cancers of the anus,
  vulva, vagina, and penile cancer (cancer of the
  penis) eg. HPV-8 and HPV-16.