2. Papovaviridae
• 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
3. 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
4. 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
5. Polyomavirus Papillomavirus
Genome
•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
Morphol
ogy
•Noneveloped
•~45 nm diameter
•icosohedral, skew, T= 7
•3 capsid proteins
•Nonenveloped
•~52-55nm diameter
•icosohedral, skew, T= 7
•2 capsid proteins
Other
Differen
ces
•Subfamily specific antigens
•Papillomavirus can't be grown in culture, while polyomaviruses are often grown in culture
Pathoge
nesis
Most are asymptomatic, although can be
oncogenic in hamsters
Cause various types of warts, epidermodysplasia verruciformis
Represe
ntative
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.
6. 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
8. A. EM of particles
and B. Circular
representation of
Human
papillomavirus
genome. Note that
transcription proceeds
from only one of the
two strands.
9. 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
10. 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
11. 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
12. 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
13. 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
14. 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.
15. A. EM of replicating SV40 DNA,
showing unwinding during
bidirectional replication.
B. Schematic of bidirectional
replication from single origin of
replication (Ori)
16. A. Continuous DNA synthesis from Ori, RNA primed (primase)5’>3’
B. Discontinuous DNA synthesis toward Ori, also RNA primed, also 5’>3” (Fig
9.2, Principles of Virology)
Semidiscontinuous DNA synthesis from SV40 bidirectional origin
17. 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
