A few Important Mechanisms of Viral Gene Expression

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A few Important Mechanisms of Viral Gene
Expression

• DNA viruses
• Use nucleus to copy DNA (Exception Pox)
• Code for viral protein(s) to preferentially
  initiate DNA synthesis on the viral DNA
• Large DNA viruses bring their own DNA Pol, etc.
• RNA viruses
• Code for a RNA Pol(s)
• Use cytoplasm to replicate (Exception Influenza
  needs a nucleus)

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A few Important Mechanisms of Viral Gene
Expression

• Transcription to produce mRNA
• mRNA in a eukaryotic cell is translated as ONE
  protein
• Viruses therefore
• Make mRNA coding for a polyprotein(s) which is
  cleaved by a protease(s), or..
• Make monocistronic mRNAs
• Some can use splicing machinery to make mRNAs if
  replication is in the nucleus

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A few Important Mechanisms of Viral Gene
Expression

• Transcription continued
• mRNA in a eukaryotic cell is made in the nucleus
  and capped at the 5 prime end
  (m7G(5')ppp(5')N---- 3), and polyadenylated at
  the 3 prime end. (m7G(5')ppp(5')N---- AAAA3)
• Viruses that replicate in the cytoplasm
  therefore
• Make their own CAP or a substitute, or have an
  IRES internal ribosomal entry site
• Make RNA that may or may not be polyadenylated

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EMC virus IRES 5end Polio type 1
IRES 5 end
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Translation to produce protein(s)
Translation is sometimes controlled by
read-through of stop codons or by reading frame
changes The retrovirus example
Splicing of mRNA by nuclear enzymes in
retroviruses
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Translation to produce protein(s)
Reading frame changes a slippery sequence
allows the ribosome to slip and start reading
in another frame The retrovirus example
RF 1 RF 2
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A Reading Frame Shift Ribosomal Frameshifting
on a retrovirus mRNA
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A Reading Frame Shift What Causes it? A RNA
hairpin structure causes the slippage
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Translation to produce polyprotein(s)-contd Polyp
roteins are processed via viral proteases to
produce functional proteins
Polio Protease 3C Adenovirus
Protease HIV Protease
Proteins may also be processed by
glycosylation, myristylation