Lecture 5: DNA virus overview and polyomavirus Flashcards
why don’t small DNA viruses encode entire replication systems
no DNA virus encodes everything they need
- encodes proteins that orchestrate the host
- papillomaviridae, polyomaviridae, parvoviridae
where does the polymerase come from
small DNA viruses
- no DNA virus encodes everything they need
- encodes proteins that orchestrate the host
- papillomaviridae, polyomaviridae, parvoviridae
large DNA viruses
- encode their own replication system
- herpes, adenoviridae, poxviridae
basic rules of DNA replication in eukaryotic cells
- DNA synthesized by template-directed incorporation of nucleotides into 3’-OH of DNA chain
- always synthesized 5’ to 3’ (semiconservative)
- replication origins
- catalyzed by DNA dependent DNA polymerase and accessory proteins
- always primer dependent
host cell nuclear functions
- DNA replication (DNA polymerases, helicases, RNA primase, ligase, DNA binding proteins
- RNA transcription (initiation factor, RNA polymerase II)
- RNA processing (capping/splicing)
all DNA viruses encode an initiation protein that
- bind to ORI region
- recruit host DNA replication proteins
all DNA viruses require ______ for DNA synthesis
RNA primer
replication fork vs strand displacement (primer)
replication fork
- in both directions
- papillomavirus, polyomavirus, herpesvirus, retroviral proviruses
strand displacement
- one direction
- adenoviruses (protein), parvoviruses (DNA hairpin), poxvirus (DNA hairpin)
describe the “end replication” problem
all linear DNA shortens when replicated
- RNA primers added and removed during replication
- primers located at 5’ end of linear DNA not replaced
what is the “solution” for DNA viruses
- circular genomes
- circularized linear genomes: inverted repeats
- protei primers
- hairpin loops
- rolling circle
- reverse transcription
28 mn no envelope capsid: icosahedral (T=1) baltimore class 2 linear, ssDNA (hairpin termini) segments: 1 genes: 6 mRNAs genome size: 5kb members: B19, FPV, CPS unique traits: integrate into host DNA
parvoviruses
replicate in cells that normally cycle and frequently enter S phase
autonomous parvoviruses
replicate in cells infected with a helper virus which indices entry of the cell into S phase
-may enter latent phase by integrating in host genome if helper virus is absent
dependent parvoviruses
gene products in viruses
multiple proteins from one open reading frame by staggering promoter
- alternative splicing od mRNA
- post-translational proteolytic cleavage
- internal translation initiation
describe parvovirus replication
extremely host dependent
- dependent on host cell DNA rep system
- require actively dividing cells (s phase)
- rbc precursors
- cancer cells (lethal)
regulatory proteins (NS1 and Rep78/68
- bind to viral DNA- powerful transcription activator for recruiting host polymerase
- freeze cell cycle in s phase
- helicase activity unwinds circular
- endonuclease activity
45 nm no envelope capsid: icosahedral (T=7) baltimore class 1 circular, dsDNA; "minichromosome" segments: 1 genes: 6-7 proteins genome size: 5.3 kb members: simian virus 40, BK and JC virus unique traits: T antigens= oncogenes, tumorigenic in animals
polyomaviruses
polyomavirus structure
72 pentomers (no hexamers) inner: nucleosome with viral DNA wrapped around, VP2/VP3, VP1: outer
describe the mouse polyomavirus: divergent transcription, early/late proteins, differential splicing
divergent transcription- complementary strands transcribed in opposite directions
early proteins: regulatory genes transcribed first
- sm, middle, and lg T antigens
- stimulate the cell cycle to enhance viral DNA replication
late proteins: transcription of structural genes is delayed
-capsid proteins
differential splicing of a common mRNA transcript maximizes coding capacity
all mRNAs are coded at the polyadenylation signal region
- functions by indirectly enhancing the transcription of cyclin D1
- binds and inactivates protein phosphatase 2A
- end result is binding of AP-1 to cyclin D1 transcription enhancer
small T antigen
how do small T antigen function by indirectly enhancing the transcription of cyclin D1
activates the cell cycle and brings host cell into S phase
why do small T antigen bind and inactivate protein phosphatase 2A
allows phosphorylation of MSP kinase pathway
- activates signaling pathways
- cell metabolism stimulated and initiates the cell cycle
middle T antigen
how are middle T antigen pathway signals activated
- middle T antigen anchored in cell membrane
- associates with several protein tyrosine kinases
- initiates cascade of signaling events
tyrosine kinase protein that a potent oncogene, activator of cell cycle that has the ability to over stimulate cell growth but normally regulates the cell
C-Src
tyrosine kinase proteins attached to middle T antigen
-C-Src, activated MAP kinase, phosphatidyl inositol 3 phosphate, inositol triphosphate (middle T antigen serves as dock for them)
