Retroviruses and Human T Cell Lymphotropic Virus Flashcards
What was the first retrovirus discovered?
Rous Sarcoma virus
What enzyme is required reverse transcription?
RNA-dependent DNA polymerase (Reverse transcriptase)
What was the first human retrovirus discovered?
Human T cell leukaemia
How were retroviruses first classified?
- Morphology/assembly
- Biological properties
What are the basic properties and morphology of retroviruses?
- Lentivirus is slow growing
- Spherical enveloped viruses
- Approximately 100 nm diameter
- Prominent peplomers - spikes
- Contain dense core 70-80 nm - shape variable
- Positive stranded, diploid RNA genome - 2 identical copies
How are the RNA genome copies held together?
By non-covalent interaction base-pairing at the 5’ end
What does the surface glycoprotein, SU, aid?
Helps recognition of target cells
What does transmembrane protein, TM, aid?
Helps fusion with target cells
What are the 3 common retrovirus genes?
- GAG
- POL
- ENV
What does GAG encode?
- Group-specific antigens
- Internal proteins - includes matrix, nucleocapsid and in some viruses the protease
What does POL encode?
- Reverse transcriptase
- Integrase
- For specific viruses it encodes the protease
What does ENV encode?
Glycoprotein spikes
What ends of the genome have regulatory sequences?
Both 5’ and 3’
What is found in the regulatory sequences?
- Primer binding site
- Untranslated region in 5’ and 3’
- Polypurine tract in the 3’
- Repeat region found on both ends
What is the function of the tRNA bound to the PBS?
Acts as a primer for reverse transcription
Describe the retrovirus life cycle
- First have to have recognition between surface envelope proteins and specific receptors on the host cell
- Following this entry proceeds via fusion - fusion-mediated entry
- Fusion between the viral envelope and the cellular envelope
- Once in the cytoplasm the capsid disassociates and releases the RNA genome
- Reverse transcription produces a DNA copy of the genome
- Proviral DNA is transported into the nucleus where it’s integrated into the host chromosome by integrase
- Viral DNA can be transcribed and used to make viral proteins and multiple copies of viral RNA
- New infectious particles are assembled and bud out of the infected cell
- Particle that buds out isn’t a mature virion - processed by retroviral protease to adopt the final structure
What are the properties of reverse transcriptase?
- RNA-dependent DNA polymerase
- Able to synthesise DNA remnant that is complementary to the RNA
- Has RNase H activity - able to degrade RNA molecule hybridised to the DNA fragment
Describe the reverse transcription process
- Once the viral capsid it releases the single stranded RNA genome
- The negative strand is primed by tRNA bound at tb
- Reverse transcriptase produces a DNA copy of the 5’ sequence
- RNase H exposes the DNA copy of R and U5
- As the RNA genome has identical sequences at the 5’ and 3’ ends, the newly extended DNA fragment is able to jump from the 5’ end to the 3’ end and anneal onto the repeat segment
- This promotes elongation of the full stretch of the DNA strand
- Template RNA degradation by RNase H
- Polypurine oligo (polyP) segment on the RNA is specifically spared degradation to prime positive strand synthesis - acts as a primer
- Positive strand uses the new DNA as a template and continues until a modified base in tRNA blocks extension
- tRNA is degraded by RNase H
- tb and tb’ at 3’ ends of the positive and negative strands base pair - second template jump
- Extension of both strands produces a complete double stranded DNA copy of the RNA genome
Describe the integration of viral DNA into the host’s genome
- Integrase recognises the 5’ and 3’ end of the proviral double stranded DNA
- Integrase creates random nicks in the host chromosome
- Integrase repairs these nicks with the viral DNA - this is reversible
- Repair occurs by the host cell machinery - this is irreversible
- End result is the viral genome integrated in random places within the host chromosome
What levels are the retrovirus genes regulated at?
- Transcriptional level
- Translational level
Which retrovirus genes are needed to be expressed in a large quantity and what genes are needed in a smaller quantity?
- Gag-pro and Env are structural genes needed in large amount
- Pol is needed in a smaller amount
How are retrovirus genes regulated at the transcriptional level?
ENV genes are made from a separate spliced mRNA in large amount to generate ENV proteins separately from the others
How are retrovirus genes regulated at the translational level?
- Full-length mRNA has to be used as mRNA for Gag (large quantity) and Pol (smaller quantity)
- Retroviruses have 2 methods of translational control depending on the reading frame
- Gag and Pol in the same reading frame or in different reading frames
Describe translational control when Gag and Pol are in the same reading frame
- The coding regions are separated by a stop codon
- Readthrough is by suppression of termination
- 90% of ribosomes translate Gag-pro and terminate at the stop - don’t translate Pol
- 10% insert GIn residue (CAG codon) and continue translating - called a readthrough event
- Therefore, Pol is translated less than Gag-pro