2. Abstract:-
• Reverse transcription and integration are the defining features of
the Retroviridae; the common name “retrovirus” derives from the
fact that these viruses use a virally encoded enzyme, reverse
transcriptase (RT), to convert their RNA genomes into DNA.
Reverse transcription is an essential step in retroviral replication.
3. The Process of Reverse Transcription:-
• When a mature HIV-1 virion infects a susceptible target cell,
interactions of the envelope glycoprotein with the coreceptors on
the surface of the cell brings about a fusion of the membranes of
the host cell and the virion . This fusion introduces the contents of
the virion into the cytoplasm of the cell, setting the stage for
reverse transcription.
4. • Reverse transcription initiates from an RNA primer provided by a
specific cellular tRNA (tRNA Lys3). The primer hybridizes to the
primer Binding Site (PBS) of the genomic RNA.
• Reverse transcriptase catalyzes RNA dependent DNA synthesis.
Synthesis of the minus-strand DNA generates an RNA-DNA hybrid.
Rnase H activity of reverse transcriptase degrades the RNA
strand, leaving the nascent minus-strand DNA single stranded.
• The newly synthesized minus-strand DNA hybridizes with the
terminal R-region of the 3’ – end of the viral RNA ( Called the first
jump or minus-strand transfer).
5. • After the nascent DNA hybridizes to R, minus--strand DNA synthesis
can continue along the viral RNA. As DNA synthesis proceeds,
RNase H degrades the RNA strand.
6. • There is a Purine-rich sequence ( called the polypurine tract or PPT)
near the 3’ end of the viral RNA which is relatively resistant to RNase
H cleavage, and it serves as the primer for plus -- strand DNA
synthesis.
• Plus-strand DNA synthesis proceeds until reverse transcriptase starts
to copy the tRNA primer. Once the 3’ end of the tRNA has been
reverse transcribed , an RNA/DNA hybrid that is a substrate for
RNase H is created.
7. • The tRNA associated with the nascent viral DNA is then removed by
the RNase H activity. This may facilitate annealing to the PBS
complement on the minus-strand DNA, providing the
complementarity for the Second jump or Plus-strand transfer.
• Once this Second transfer happens, both the minus – and plus – DNA
strands are extended until the entire DNA is double stranded and
creating a DNA that has the same sequences at both ends (these
repeats are called the Long Terminal Repeats or LTRs).
8.
9. Why Left hand LTR and Right hand LTR is necessary ?
• RNA polymerase that initate transcription with in the Left hand LTR
can dislodge transcription factor bound to the Right hand LTR as
they passed through that region on the proviral DNA inactivating
it’s ability to initiate for transcription. This is called Promotar
occlusion.
This concept is supported by the observation that mutation or
deletion of the Left hand LTR can lead to the initiation of the
transcription in the Right LTR resulting in transcription of the cellular
genes downstream of the proviral DNA.
10. • The cellular cleavage and poly adenylation machinery recognizes
the highly conserved AAUAAA poly adenylation signal in RNA.
