CSB351Y1 Lecture Notes - Lecture 41: Endogenous Retrovirus, Reverse Transcriptase, Ribonuclease H
26 May 2018
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Lecture 41
Retrovirus physical property
• Enveloped, 80-100nm,
• Env/spike gene (TM (transmembrane) and SU (surface)), GsAg gene (MA
(matrix), CA (capsid), NC (nucleocapsid)), Pol gene (PR (protease), RT
(Rtranscriptase), IN (integrase))
• ALV simplest (3 main ORFs), HTLV very complicated (extra ORFs)
• Each end of retrovirus has LTR
- R (repeat region) at the ends of genome
- U5 (unique region) has kissing loop (two RNA molecules will interact with each other and form diploid)
- Primer binding site (PBS) tRNA primer is found here and begins reverse transcription
- Leade seuee 5’ UTR of RNA is aea efoe ORF
- In the other LTR – polypurine tract (highly conserved and A/G residues) involved in +sense synthesis
- U3 (unique region)
Human endogenous retrovirus (HERVs)
• Previous exposure to retroviruses (fossil viruses), constitutes 1% of human genome, transmitted vertically
• Has reverse transcriptase region found in all retroviruses
Retrovirus replication
• Virus enters by fusing to plasma membrane (SU glycoprotein attach, TM mediates fusion)
• Uncoat, viral RNA released, reverse transcriptase making dsDNA from ssRNA(+) → inserted into host genome
using integrase (randomly integrated)
• Cellular machinery transcription (spliced/unspliced mRNA from provirus) and incompletely spliced exit nucleus
• Translated by ribosome (viral proteins produced), spliced genomic RNA encapsidated and released by budding
• Proteolytic processing of precursors polyproteins by viral protease and maturation of virions
• Need cellular tRNA for replication
Mechanism of retrovirus replication
1. Retrovirus-specific cellular tRNA binds PBS, reverse transcriptase starts to synthesize using PBS as primer →
-strand produced and chews up the RNA template at the same time
2. Jumps to the other end and binds R seuees ad evese tasiptase sythesizes the est of the DNA fo 3’
3. Most viral RNA removed by RNaseH (polypurine tract resistant to RNaseH activity)
4. Polypurine tract act as primer for second DNA (+) synthesis, RNaseH removes tRNA and polypurine tract
5. Second jump with PBS to PBS and synthesis of 2nd strand is completed (2 LTR repeats and dsDNA)
Retrovirus resemblance to Hepadnavirus (Hepatitis B)
• Both have reverse transcriptase, contain direct repeat regions at end, have RNase and polymerase domain,
order of genes are same (gag,pol,env → C,P,S), both associated with cancer and both transmit through blood
• HBV sometimes can integrate but retrovirus ALWAYS integrates itself into host genome
• Two class of oncogenic retrovirus – acute transforming (rapidly transform cells, induce cancer within short time
after inoculation) and nontransforming (induce cancer after long latency period)
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