20.12.2011 -
The mechanism involved in the transcription of genetic
information from DNA to RNA through RNA polymerase enzyme activity
has been known for some time. It has been discovered that this
mechanism operates at a speed up to 10-20 times faster than
previously proposed.
The discovery was made by a group of researchers at the
International Centre for Genetic Engineering and Biotechnology
(ICGEB) in Trieste, headed by Alessandro Marcello, in partnership
with the physicist Paolo Maiuri.
The prestigious magazine EMBO reports made this the
cover story of its December issue. The Italian team examined the
HIV virus, the pathogen that causes AIDS, which is integrated
within the genome of the infected cell and uses the cellular RNA
polymerase to transcribe its own genome.
By using a novel fluorescence microscopy method for the first
time, which makes it possible to observe the transcription process
in live cells, the researchers were able to measure the speed of
the polymerase on the HIV genome in real time. The result? The
measurements obtained were 10-20 times faster than those measured
using other techniques.
The method works like a sort of molecular 'speed camera' by
tracing the build-up of RNA transcribed by the virus genome in the
cell nucleus. By 'switching off' the fluorescence on the
transcription site and monitoring recovery of the signal in time
the researchers were able to calculate how long it took for a new
transcription cycle to be completed. The mathematical analysis of
these measurements made it possible to calculate the speed of the
polymerase.
"Transcription is a fundamental process closely controlled by
the cell," explains Alessandro Marcello, Group Leader of the
Molecular Virology Laboratory at ICGEB. "Our measurements indicate
that the speed of the polymerase may also be an important factor in
regulating gene expression. Let's take dystrophin for example, the
longest known gene, the lack of which causes muscular dystrophy:
until today it was estimated that a good sixteen hours at least
were needed for a single transcription cycle. A very long time,
which increases the risk of not completing the process. However,
according to the new polymerase speed measurements, the time needed
for the transcription of dystrophin may be much lower, greatly
reducing the risk of not completing the transcription of an
essential gene."
The next step for the researchers will be directed at gaining an
understanding of the factors on which the RNA transcription speed
depends, investigating, for example, whether the polymerase is
modulated by a molecular accelerator or by the morphological
context of the nucleus.
In fact, like a car, the speed also depends on the straightness
of the route and the obstacles in its path. In the case of the
polymerase these obstacles are DNA and protein packaging in the
chromatin. Once all the factors that influence the process have
been ascertained, it could be possible to see whether any
alterations in the transcription speed can be associated with
pathologies such as genetic diseases or cancer.
"Changing the scale of a simple measurement of a fundamental
molecular process has many implications, including pathogenic ones.
Basic research is responsible for understanding the mechanisms that
could then be targeted by new therapeutic approaches," concludes
Marcello.