Post on 20-Dec-2015
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How did viruses originate?
“Viral Mutation Evolved”Media: Pastel on paperWALTER DUPRIEST acanthusarts.org
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• Virus first hypothesis– Originated before cells
• Viruses from all three domains have similar characteristics
• Predate LUCA
LUCA
Original virus prebiotic RNA molecule
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• Escape hypothesis– Fragments of escaped genome
• Bacteriophages originated in the prokaryotic genome
– Derived from an autonomously replicated plasmid
• Eukaryotic viruses originated from the eukaryotic genome
• Archaean viruses originated from the Archaean genome
– DNA developed from RNA viruses
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OriginalRNA organism
RNA viruses
DNA viruses
DNA is more stableLess prone to mutationsLarger more complex proteinsProtection against enzymes that degrade RNA
Viral DNA become incorporated into cellsDNA genes had a selective advantage
Did DNA organisms evolve 3 separate times or just once?
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• Regressive hypothesis– Reduction of a parasite
• Facultative parasite become dependent upon the host
• Evolved towards an obligate parasite / host relationship
• Similar to the reduction of endosymbiont to the mitochondria and photosynthesis
• Parasite looses genes and become more dependent upon the host
– Rickettsia like organism Chlamydia like organisms Pox like virus
Prangishvili et al. Viruses of the Archaea: a unifying view. Nature Reviews Microbiology 4:837-848.
The Biology of Viruses (2nd ed.) by Voyles. McGraw-Hill Co., Inc.
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Rickettsia
Pox virusds DNA100 genes
ChlamydiaNo cell wall0.3 and 1.0 m in diameter
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MimivirusA unique virus
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• Pathogen of amoebae – Acanthamoeba polyphaga – Microbiologists observed a
Gram + coccus within the amoeba
• Large virus– Diameter of capsid is 400
nm– 80 nm fibrils
• Nucleocytoplasmic Large DNA virus– NCLDV
MimivirusMimicking microbe
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Gram stain
Electron micrograph
ASM News 71(6):278-284
TEM images
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Mimivirus400 nm = 0.4 m
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• Linear ds DNA • Approx 1.2 million bp
– Largest viral genome– Larger genome than 20 prokaryotic organisms
• 1262 open reading frames– Putative genes
• Contains 21 genes that are found in all NCLDV• Contains metabolic genes not found in any
other virus– Contains genes for nucleotide synthesis,
protein synthesis, DNA repair, polysaccharide synthesis genes
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There are 7 minivirus genes that are present in Eukaraya, Bacteria, and Archaea.
A DNA sequence analysis places mimivirus closest to the Eukaraya in the 3 domain system
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Mimivirus is hypothesized to be the ancestor of a virus which contained more eukaryotic genes
15Raoult 2005 ASM News 71(6):278-284
Mimiviruses are an exception that provide evolutionary clues to the origin of life
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Proteinaceous Infectious AgentsTransmissible Spongiform Encephalopathies
• Prions • Normal proteins that become misfolded in
the mammalian brain• PrP
• Spongiform encephalopathies • Holes in the brain
• Human diseases• Creutzfeldt-Jakob Disease• New variant CJD• Kuru• Gerstmann-Straussler-Scheinker Syndrome • Fatal Familial Insomnia
• Several animal diseases• Scrapie in sheep & goats• Bovine spongiform encephalopathies (BSE)• Chronic wasting in elk and mule deer• Similar diseases in cat & mink
Dementia; early neurologic signs
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• Originally thought to be a viral disease• No RNA or DNA has been isolated• Treatment with UV did not reduce
virulence
• Prions have been produced in yeast cells
• Useful experimental model
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• Amino acid sequence can fold into two different structures– Cellular PrP PrPc
– Prion PrP PrPsc
• Prion Prp can cause a conformational change in the shape of the cellular PrP
• Ingesting the prion PrP can cause cellular PrP to convert to disease causing form
• Prion PrP are not denatured by…– Cooking– UV irradiation– Degradative enzymes
Stanley B. PrusinerThe Nobel Prize in Physiology or Medicine 1997
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Cellular PrP PrPc Prion PrP PrPsc
helicesPleated sheets
22nobelprize.org
the PrPsc accumulates primarily in the cerebral cortex
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How does PrPsc arise?• Horizontal transmission from
consuming contaminated food or animal feed– Sheep to a cow
• Veritcal transmission– Mutations in the wildtype prion gene
are transmitted from parent to child• Spontaneously
– Approximately 1 in a million humans develop CJD disease
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• Creutzfeldt-Jakob disease (CJD)– 1920’s– 1 in a million individuals
• Mutations in the human PrP gene produced the first prion PrP
• CJD is inherited– Families of Ashkenazi Jews – Median age of onset is 68 years– Median length of disease 5 months
• Kuru– Papua New Guinea– Cannibalism
nobelprize.org
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• European BSE– Meat and bone meal from sheep were
commonly added to animal feed as a protein supplement
• Scrapie-infected• 1980’s
– Healthy cattle consumed the PrPsc protein– Developed BSE
• 1990’s– Approximately 100 individuals in the U.K. and
France contracted the new variant CJD• Median age of onset is 28• Median length of disease is 14 months• Prominent psychiatric/behavioral symptoms; painful
dyesthesiasis; delayed neurologic signs – Unpleasant sensations that are produced in response to
normal stimuli– Painful tingling, burning and numbness
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BSE in the USA
• December 2003– An adult Holstein cow from
Washington State– The BSE infected cow was imported
from Canada in August 2001
• 2004– A single BSE American cow
• March 2006– A single BSE American cow
28www.cdc.gov
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Viroids
• Smallest known pathogens
• Naked ss RNA molecules with no protein
• ~ 246-399 nucleotides
• No protein encoding genes!
• Only known to infect plants (e.g., potato spindle tuber viroid, citrus exocortis viroid)
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Viroids (Continued)
• Infection can be symptomless or severe in symptoms, even lethal
• Severe syptoms tend to be growth related suggesting that viroid is a form of “regulatory RNA” that “rebelled”
• None known in animals or prokaryotes as yet
• Single-stranded but may appear to be ds based on secondary structure