SM L5 Flashcards
Poxviridae
Baltimore Group I - dsDNA linear
Enveloped, complex shape
Only DNA viruses to replicate in cytoplasm - must provide their own mRNa & DNA enzymes
Small pox, cowpox - inflection or disease
Name the two subfamilies in Poxviridae
Entomopoxivirinae (invertebrates) 3 Genera A,B & C
Chordopoxvidinae (vertebrates) 8 Genera: One genus called Orthopoxvirus
What is the name of the live virus in small pox vaccine?
Vaccina - is a type of poxvirus that is used as a vaccine to protect against smallpox.
What is the origin of vaccinia?
Exact origin is unknown.
Does not appear to be related to any other knwon pox virus
Characteristics of Vaccine
Very large (300+nm) Complex Capsid
Linear dsDNA genome (190kb: 250 genes)
Replicates in cytoplasm of host
Outer membrane
“core” envelope and nucleoprotein/caspid
Viral enzymes
True or False: Viruses have cytoplamic and metabolic activity
False, viruses have no cytoplasm or metabolic activity.
What are the infectious forms of vaccina that are produced during replication?
intracellular mature virion (IMV)
intracellular enveloped virion (IEV)
cell-associated enveloped virion (CEV)
extracellular enveloped virion (EEV)
IMV is most infectious form
Describe vaccinia genome organisation
ds DNA genome, strands covalently linked by hairpin loops
190kb coding for >180 proteins (Fully sequenced)
flanked by multiple terminal repeat sequences (TR)
Coding sequences are densely packed, may be in TR different reading frames (overlap) and be on either strand
Where does vaccinia replication occur
occurs in cytoplasm
replication may occur in enucleated cells but maturation is blocked
What receptors are on vaccinia?
Receptors: unknown but probably >1 on different cell types
How does gene expression work in vaccinia replication?
Gene Expression: viral enzymes & divided into three phases
– Early gene: ~50% genome, expressed before genome replication
- Intermediate: genome replication & late gene proteins produced
- Late genes: expressed after genome replication & late promoters are dependent on DNA replication for activity
Early Phase
Early proteins cause cells to divide, block immunity disassemble core induce intermediate phase
Early proteins: viral DNA polymerase and viral thymidine kinase (TK)
Intermediate Phase
Int. proteins induce late phase genes DNA replication
Late Phase
-Late proteins= Virus structural proteins Assembly of immature virion in cis Golgi Maturation in endosome
-Late proteins = structural proteins, enzymes that turn off early viral protein synthesis and promote virion production
-Mature Virus disseminates to external environment or neighbouring cells
Viral entry and uncoating
The outer viral membrane of the vaccinia virus fuses with the host cell’s plasma membrane, releasing the virus into the cell’ cytoplasm.
Following release, the virus undergoes a second uncoating step to release nucleoproteins and the DNA.
Within minutes after infection, viral enzymes that enter the cell with the vaccinia virus cores initiate transcription, the copying of messenger RNA from the DNA viral chromosome.
Host DNA, RNA and protein synthesis are switched off almost immediately by viral factors carried into the cell with the virion.
Early Transcription and Early Proteins
Early viral transcription is transcribed by a multisubunit viral RNA polymerase. About one half the genome are transcribed before the DNA is replicated.
Major protein products are viral DNA polymerase and viral thymidine kinase (TK). Early viral mRNAs are translated on cellular ribosomes soon after they are transcribed.
Vaccinia Virus DNA Replication
Occurs in the cytoplasm
– the virus is sufficiently complex to have acquired all the
functions necessary for genome replication. (some
contribution from the host)
* replication may occur in enucleated cells but maturation is
blocked
* Receptors: unknown but probably >1 on different cell types
* Penetration: complex & >1 mechanism
* Gene Expression: viral enzymes & divided into three phases:
– Early gene: ~50% genome, expressed before genome
replication
– Intermediate: genome replication & late gene proteins
produced
– Late genes: expressed after genome replication & late
promoters are dependent on DNA replication for activity
Late Transciption and Late Proteins
Late proteins=
Virus structural proteins
Assembly of immature virion in cis GolgiMaturation in endosome
Virus Assembly, Maturation and Release
Immature viral particles appear circular or spherical in three dimensions in special virion assembly areas within the
cytoplasm of the cell. Mature vaccinia virus virions are moved out of assembly areas and transported to the cell membrane.
Mature virions are drawn to the tips of specialized, actin-containing microvilli and are released.
The double-layer membrane surrounding virions is thought to be derived from the Golgi apparatus. Seven viral glycoproteins
stud the viral envelope
What is an Interferon ?
Interferon (IFN) is a signalling protein released by a host cell in response to the
presence of virus.
IFN is a cytokine that triggers the immune system to help eradicate the virus. IFNs
are named for their ability to “interfere” with viral replication by protecting cells against
viral infections.
VV Evasion of host
VV shows considerable resistance to
antiviral effects of interferons
* an early gene of VV, K3L, inhibits the
action of PKR
* E3L protein binds to dsRNA and
sequesters it & so it inhibits dsRNA
mediated activation of PKR
* other vaccinia proteins interfere with the
actions of the complement system & IL-1
