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Researchers advance understanding of enzyme that regulates DNA
http://www.physorg.com/news201527712.html
August 20, 2010
Illinois physics professor Taekjip Ha and graduate student Jeehae Park
used fluorescence resonance energy transfer to study how an important
DNA enzyme regulates damages sections of DNA. The red and green
fluorescent dyes allow them to track a single enzyme's activity.
Thanks to a single-molecule imaging technique developed by a
University of Illinois professor, researchers have revealed the
mechanisms of an important DNA-regulating enzyme.
Helicase enzymes are best known for "unzipping" DNA for replication,
but have many other functions for DNA repair and maintenance. The
Illinois team focused on a particular bacterial helicase called PcrA
involved in preventing unwanted recombination.
A DNA double helix consists of two strands twisted around each other.
When one strand is damaged or breaks, the surrounding area is
degraded, leaving a single-stranded region. Specialized proteins then
start the process of recombination - rebuilding the second strand
using the intact DNA as a template.
"Recombination is essential for DNA repair, but if it runs amok, it
causes problems," said U. of I. physics professor Taekjip Ha. "This
helicase controls recombination by removing recombination proteins
from the DNA."
Using a technique called single molecule fluorescence resonance energy
transfer (FRET), Ha and his team were able to identify one of the
mechanisms that PcrA uses to regulate recombination. The system uses
two dyes that change in relative intensity depending on their
proximities to one another. The researchers attached the two dyes to
the opposite ends of the single-stranded DNA tail.
Helicases are motor proteins, a class of enzymes that use chemical
energy to move along a DNA molecule like a train on a track. But using
FRET, the researchers observed the two dyes gradually moving closer to
each other, then flying apart, repeatedly. Instead of moving along the
single-stranded tail, PcrA binds at the point of the break, where the
double- and single-stranded regions meet. Then, it uses its motor
function to "reel in" the tail, like a fisherman pulling in a rope.
"By combining the structure-specific binding of the enzyme to the DNA
and the motor function, the enzyme can reel in the DNA and in the
process kick off recombination proteins," said Ha, who also is a
Howard Hughes Medical Institute investigator.
When PcrA reaches the end of its DNA rope, it releases it and starts
the reeling in process over again, removing any additional problematic
proteins that have bound to the damaged DNA as it reels.
By using FRET, a technique Ha developed, the team also was able to
answer another question about PcrA: How consistent is its motor
function? Researchers agree that on average, PcrA moves one DNA unit,
called a base pair, for each unit of cellular energy it uses, called
ATP. But because researchers traditionally study the enzyme in
relatively large samples, broad distributions of data have led to
conflicting views on whether the helicase moves in uniform steps or
those of varying lengths - even up to six base pairs per ATP.
Since FRET is a single-molecule technique, the researchers were able
to document a single enzyme's function, step by step, and found that
PcrA does, in fact, move in uniform steps of one base pair per ATP.
Next, the team plans to create a reaction environment more similar to
that in vivo, using three and four colors of FRET dyes to measure
activities of multiple proteins simultaneously. They are also working
toward understanding why helicase moves only in one direction.
"This is an ideal marriage of a new technology and an interesting
biological problem," Ha said.
More information: The team published its findings in the Aug. 20
edition of the journal Cell. The paper, "PcrA helicase dismantles RecA
filments by reeling in DNA in uniform steps," is available online.
Provided by University of Illinois at Urbana-Champaign
"Ethics" is simply a last-gasp attempt by deist conservatives and
orthodox dogmatics to keep humanity in ignorance and obscurantism,
through the well tried fermentation of fear, the fear of science and
new technologies.
There is nothing glorious about what our ancestors call history,
it is simply a succession of mistakes, intolerances and violations.
On the contrary, let us embrace Science and the new technologies
unfettered, for it is these which will liberate mankind from the
myth of god, and free us from our age old fears, from disease,
death and the sweat of labour.
Rael
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