Natural Nanomachine Caught on Camera
January 16, 2000
Embargoed until 11 a.m. PST Jan. 17.
the first time, scientists from the University of California, Davis,
and the Lawrence Livermore National Laboratory have filmed a
molecular machine at work unwinding a DNA double helix.
"This is not only a technical tour de force, but it also
reveals behavior of the enzyme that before could only be surmised,"
said UC Davis molecular biologist Stephen Kowalczykowski, who led
the research team. The study will be published in the Jan. 18 issue
of the journal Nature.
The scientists studied an enzyme
called RecBCD. RecBCD is a helicase, an enzyme that attaches to DNA
and moves along the molecule, unwinding the double helix as it goes.
This allows other enzymes to access the DNA strands, so that the DNA
sequence can be copied or repaired.
To film RecBCD at work,
the researchers attached DNA molecules labeled with a fluorescent
dye to polystyrene beads one-millionth of a meter in size. Under the
microscope, the bead looks like a white sphere with a bright string
of DNA attached.
They let RecBCD attach to the free end of
the DNA strand, and used laser beams as "optical tweezers" to move
the beads into position under a microscope. To start the enzyme,
they added ATP, a chemical fuel that powers many enzymes.
RecBCD unwound the DNA strands, the fluorescent dye was removed, so
the bright string of DNA seemed to get shorter.
unwinds DNA in a process that is continuous from start to finish,
with no detectable pausing," said Kowalczykowski. New data from the
study could take months or even years to fully understand, he said.
Note for editors: A movie of RecBCD at work is available. It
will be posted as supplemental data on the Nature Web site
<http://www.nature.com>, or contact Andy Fell for details.
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