ohungnes at bioslave.uio.no
Fri Dec 2 10:12:01 EST 1994
n.panjwani at ic.ac.uk wrote:
: I have two questions about HIV that arose in a discussion I had today, and
: we reached no satisfactory conclusion. Perhaps one of you is
:  When HIV RNA is reverse transcribed, why is the polyA tail not
: transcribed to polyT?
Minus-strand synthesis does not start at the extreme 3' end. It starts
from a tRNA primer annealed to a primer binding site (PBS) a couple of
hundred nt from the 5'end of the genomic RNA.
The first and last ca.100 bases (that is, last 100 bases before the polyA
tail) of the virus RNA are identical (R regions). This means that the
newly synthesised cDNA initiated at the PBS is complementary to the
downstream R region also, and anneals to this during the "first jump" of
retroviral reverse transcription. From this downstream position,
minus-strand synthesis proceeds leftwards, probably until it has
reached and copied the 5' end of the PBS.
The polymerase and RNaseH activities reside on the same enzyme, and the
RNA moiety of the RNA-DNA duplex is probably degraded shortly after it
has been copied.
Initiation of plus strand synthesis occurs from the two polypurine tracts
(PPTs), whose characteristics seem to make them immune to RNaseH
degradation, so that a RNA PPT-oligo is left on the DNA minus strand and
act as plus-strand primers (this is what I used to work on).
The second and final "jump" occurs upon hybridization between the
resulting terminal PBS sequences, allowing further elongation of the
plus and minus strands to form a full-length double-stranded proviral DNA.
(I really should have made a drawing).
:  How many copies of provirus can exist in a cell, i.e. how many HIV cDNAs
: can integrate? Does multiplicity have any known effect on pathogenesis?
A cell infected in cell culture can harbor as many as 80 proviruses. This
happens multiple (secondary) infection events, as a anti-CD4 MAb which
blocks infection will also block this provirus accumulation when added to
the cells immediately after the initial infection.
What happens in vivo, and has it any significance in pathogenesis? I
: Thank you for your time,
: Naveed Panjwani
: Dept of Clinical Sciences
: London School of Hygiene and Tropical Medicine
: University of London, UK.
: E-mail: n.panjwani at ic.ac.uk
Olav Hungnes olav.hungnes at embnet.uio.no
National Institute Phone (+47)22042200
of Public Health FAX (+47)22353605
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