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EM of purified papillomavirus particles

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EM of purified papillomavirus particles. Papovaviruses. Papovaviridae. Name from: Pa pilloma Po lyoma Va cuolating agent Suffix “oma” means swelling or tumor Two subfamilies: Papillomavirinae, Polyomavirinae

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  • Name from: Papilloma Polyoma Vacuolating agent
  • Suffix “oma” means swelling or tumor
  • Two subfamilies: Papillomavirinae, Polyomavirinae
  • Not very important as lethal human pathogens, but cause warts and in some cases cancer
  • The most common viral STD
  • Highly restricted host ranges
major papovavirus diseases
Major papovavirus diseases
  • Papillomavirus:
    • Most important of papovaviruses
    • More than 100 distinct strains identified to date
    • Cause common and genital warts, sometimes associated with cancer
  • Polyomaviruses:
    • JC Virus
      • Infect only humans
      • Common, causes disease only in immunosuppressed
    • BK Virus
      • Also found in immunosuppressed
      • Both JC and BK associated with cancer
    • Role of SV40 in human cancer still debated
papovaviridae properties
Papovaviridae properties
  • Simple T=7 particles, 45-55 nm, no envelope
  • Small dsDNA genomes, 5-7 kbp
  • 3 capsid proteins subunits
  • Host-derived histones account for 20% of viral protein



  • double-stranded
  • circular
  • DNA
  • ~5kbp
  • Uses overlapping genes and both strands of DNA to pack all 6 genes into a tiny space
  • genome ~5000 nucleotides
  • double-stranded
  • circular
  • DNA
  • ~8kbp
  • Uses overlapping genes and one strand of DNA to pack at least 12 genes into 8kbp
  • Genome ~ 8000 nucleotides


  • Noneveloped
  • ~45 nm diameter
  • icosohedral, skew, T= 7
  • 3 capsid proteins
  • Nonenveloped
  • ~52-55nm diameter
  • icosohedral, skew, T= 7
  • 2 capsid proteins

Other Differences

  • Subfamily specific antigens
  • Papillomavirus can't be grown in culture, while polyomaviruses are often grown in culture


Most are asymptomatic, although can be oncogenic in hamsters

Cause various types of warts, epidermodysplasia verruciformis

Representative Viruses

  • JC Virus
    • Associated with Progressive multifocal leukoencephalopathy, found mainly in the elderly and immunocompromised
  • BK Virus
    • Results in mild respiratory illness in kids
    • Found in some tumors
  • Simian Virus 40
  • A completely sequenced animal virus used frequently as a cloning vector.
  • At least 62 strains of Human Papillomaviruses
    • Widespread
    • Cause growths or warts
    • Many are associated with cancer

Summary from Robert Siegel, Stanford U.

papillomavirus properties
Papillomavirus properties
  • No tissue culture system available, so relatively poorly studied
  • Productively infect only fully differentiated squamose epithelial cells (dead end cells)
  • Result in warts, sometimes become malignant
  • All ORFs located on one strand; all transcripts from that strand
  • 70% of genome for transformation and plasmid maintenance
Papillomavirus genome organization

Viral capsid



Minor capsid






high copy

Linear representation of circular genome

A. EM of particles and B. Circular representation of Human papillomavirus genome. Note that transcription proceeds from only one of the two strands.
polyomavirus properties
Polyomavirus properties
  • SV40 the best known
  • Infectious closed circular DNA 5.2-5.3 kbp
  • Infection:
    • Establish lytic infection in permissive cells, may transform non-permissive cells
  • Entry
    • By receptor-mediated endocytosis
    • Virions enter nucleus for genome expression and replication
polyomavirus properties10
Polyomavirus properties
  • Transcription of T-antigens before replication
    • Large T
      • Induces cellular DNA synthesis
      • Required for transformation and maintenance off transformed state
      • Required for viral DNA synthesis (has ATPase and helicase activities
      • Regulates its own synthesis and that of other T antigens
    • Small T
      • Regulation of viral DNA synthesis
    • Middle T
      • Required for transformation
polyomavirus replication
Polyomavirus replication
  • Occurs 12-15 hours post-infection
  • Proceeds bidirectionally from origin
  • Large T is the only viral protein involved in DNA replication
  • Other replication-associated proteins are from host
polyomavirus late transcription
Polyomavirus late transcription
  • Late mRNA:
    • transcribed after DNA replication
    • transcribed from the strand complementary to the strand used for early RNA transcription
    • transcribed from progeny, not parental genomes
    • transcribed in much greater amounts than early
    • encodes three structural proteins, by differential splicing
polyomavirus assembly and release
Polyomavirus assembly and release
  • Assembly
    • Takes place in the nucleus
    • Viral genome complexed with histones encapsidated into preformed particles
    • Particle assembly process is mediated by host chaperone sp70 and Large T
  • Release
    • Initially by exocytosis of virus-containing vesicles
    • Later by cell death and lysis
A. SV40 has a distinctive T=7 structure made up of three proteins. Circular dsDNA inside is complexed with histone proteins. B. Genome organization is compact; transcription bidirectional from ORI. Transcription of early T-antigens is from input DNA; transcription of late structural proteins is from newly synthesized DNA.
EM of replicating SV40 DNA, showing unwinding during bidirectional replication.
  • Schematic of bidirectional replication from single origin of replication (Ori)
Semidiscontinuous DNA synthesis from SV40 bidirectional origin
  • Continuous DNA synthesis from Ori, RNA primed (primase)5’>3’
  • Discontinuous DNA synthesis toward Ori, also RNA primed, also 5’>3” (Fig 9.2, Principles of Virology)
SV40 infection cycle

1. Entry and release of core

2. Entry of DNA/nucleosome complex to nucleus

3-5. Synthesis and alternative splicing of early (T) transcripts; proteins synthesized

6-7. LT imported back to nucleus where it initiates DNA synthesis with host enzymes and potentiates late gene transcription

8-12. Late gene mRNAs synthesized, spliced, exported, translated, and products imported back to nucleus for particle assembly and release by unknown mechanism