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Polyomaviruses
Polyomaviridae
Virion
Genome
Genes and proteins
Viruses and hosts
Diseases
Distinctive characteristics
Polyomaviruses
Polyomaviridae
 Virion
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Naked icosahedral capsid (T=7). Diameter 45 nm.
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Formed from 72 capsomers, pentamers of VP1.
Polyomaviruses
Polyomaviridae
 Genome
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Circular double-stranded DNA, 5.3 Kb.
DNA is packaged as “minichromosome” with
nucleosomes formed from cellular histones.
Polyomaviruses
Polyomaviridae
 Genes and proteins
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Two transcription units: early and late.
Divergent transcription from a central control
region containing origin of DNA replication.
Differential splicing produces 3–4 mRNAs from
each transcription unit.
Early proteins (T antigens) regulate cell cycle and
direct DNA replication.
Late proteins (VP1, 2, 3) make virus capsid.
Polyomaviruses
Polyomaviridae
 Viruses and hosts
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Mouse polyomavirus; simian virus 40;
polyomaviruses infecting birds, rodents, cattle;
and two human viruses: BK and JC virus.
Polyomaviruses
Polyomaviridae
 Diseases
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Usually persistent, nonsymptomatic infections.
Progressive multifocal leukoencephalopathy, a
rare demyelinating disease caused by human
polyomavirus JC.
Polyomaviruses
Polyomaviridae
 Distinctive characteristics
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Polyomaviruses are dependent on cellular RNA
synthesis and RNA processing enzymes, and DNA
replication systems.
T antigens interact with multiple signaling
pathways and activate the cell cycle to facilitate
viral DNA replication.
T antigens are oncogenes that can transform
nonpermissive cells in vitro.
Polyomaviruses are tumorigenic in animals when
administered at high concentrations, but rarely
cause tumors in nature.
Virion
 Mouse polyomavirus was discovered as a
tumor-producing infectious agent
 Simian virus 40 was found as a contaminant
of Salk poliovirus vaccine
 Polyomaviruses are models for studying
DNA virus replication and tumorigenesis
 Polyomavirus capsids are constructed from
pentamers of the major capsid protein
Virion
Fig. 10.1 Computer-generated image of mouse polyomavirus capsid.
Genome
 The circular DNA genome is packaged with
cellular histones
 Circular DNA becomes supercoiled upon
removal of histones
 Supercoiled DNA can be separated from
relaxed or linear DNA molecules
Genome
(a) A schematic diagram of a virion
in cross-section, showing
pentamers of VP1 (orange
spheres), with single molecules
of VP2 or VP3 (gray ovals)
Circular viral DNA is bound to
nucleosomes formed from
cellular histones.
(b) A larger view of a nucleosome,
around which DNA is wrapped as
a toroidal coil.
Fig. 10.2 Components of polyomavirus virion.
Genome
Fig. 10.3 Gymnastics of circular DNA molecules.
Genes and proteins
 Polyomavirus genes are organized in
two divergent transcription units
Genes and proteins
(a) The intergenic region, a
noncoding region of 400 nt,
bounded by the divergent
promoters for early and late
transcription (PE and PL)
(b) Region where overlapping 3
ends of mRNAs are located
Fig. 10.5 Control regions on mouse polyomavirus genome.
Genes and proteins
 Virions enter cells in caveolae and are
transported to the nucleus
 The viral minichromosome is transcribed by
cellular RNA polymerase II
 Four early mRNAs are made by differential
splicing of a common transcript
Genes and proteins
Fig. 10.6 Alternative splicing of early mouse polyomavirus transcripts generates four different T antigens.
Genes and proteins
 T antigens share common N-terminal
sequences but have different C-terminal
sequences
 T antigens bring resting cells into the DNA
synthesis (S) phase of the cell cycle
 Small T antigen inhibits protein phosphatase
2A and induces cell cycling
 Middle T antigen stimulates protein tyrosine
kinases that signal cell proliferation and
division
Genes and proteins
Fig. 10.7 Small T antigen binding to protein phosphorylase 2A stimulates MAP kinase pathway.
Genes and proteins
Fig. 10.8 Middle T antigen binding to c-Src activates pathways that stimulate cell metabolism and the cell cycle.
Genes and proteins
 Large T antigen activates or suppresses
transcription of cellular genes by binding to
a number of important cellular regulatory
proteins
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Retinoblastoma protein (pRb) controls cell-cycle
and S-phase gene expression.
The DNA J domain acts as a co-chaperone to
dissociate pRb from E2F.
p53 blocks the cell cycle and induces apoptosis in
response to virus infection.
p300 and general transcription factors control
levels of transcription of viral and cellular genes.
Genes and proteins
 Large T antigen hexamers bind to the origin
of DNA replication and locally unwind the
two DNA strands
Fig. 10.9 Functional and protein-binding domains of simian virus 40 large T antigen.
Genes and proteins
 Large T antigen hexamers assemble cellular
DNA synthesis machinery to initiate viral
DNA replication
Genes and proteins
(a) Initiation of DNA synthesis at
viral replication origin
(b) Extension of DNA chains
beyond the initiation region by
DNA polymerase
(c) Joining of growing DNA
strands
Fig. 10.11 Mechanism of bidirectional replication of polyomavirus DNA.
Genes and proteins
 High levels of late transcripts are made after
DNA replication begins
1. Repression of early transcription by large T
antigen.
2. Activation or induction of new transcription
factors.
3. Derepression of late transcription by dilution of a
repressor.
4. Increased efficiency of processing and export of
lateRNAs.
5. Readthrough RNAs hybridize with early
transcripts and lead to their degradation.
Genes and proteins
 Three late mRNAs are made by alternative
splicing
 How do polyomaviruses transform cells in
vitro and cause tumors in vivo?
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abortive infection
Transform nonpermissive cells (10-5)
Key Terms
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Abortive
Apoptosis
Caveolae
Co-chaperone
Ethidium bromide
Gangliosides
Intergenic region
Myristate
Nonpermissive
Nucleosomes
Oncogene
Polyadenylation signal
Progressive multifocal
leukoencephalopathy (PML)
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Cyclins
Divergent transcription
DNA gyrase
DNA helicase
Protein tyrosine kinase
Sialic acid
Sodium dodecyl sulfate
Spliceosome
Supercoiled
TATA box
Topoisomerase I
Toroidal coil
Transcription factors
Transcriptional enhancer