Transcription against an Applied Force|
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Movie format (with superimposed graphics, 1.48 Mb)]
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This movie shows an experiment from:
"Transcription against an Applied Force"
Hong Yin, Michelle D. Wang, Karel Svoboda, Robert Landick, Steven M.
Block, and Jeff Gelles
(1995) Science 270, pp. 1653-1657.
The movie is a video recording, made through the microscope, of the experiment
that produced the data in Fig. 2A of the paper. The movie shows a DNA-tethered
bead exhibiting tethered Brownian motion, being captured by the laser trap,
and being moved by the bound RNA polymerase molecule. Since this experiment
was performed at relatively low laser power (25 mW), the polymerase does
Sequence of events visible in the movie:
Events 1 and 2 are shown in real time. The portion of the movie showing
event 3 is speeded up twenty-fold in order to make the RNA polymerase-driven
bead movement more visible
Brownian motion of a DNA-tethered bead. The differential interference
contrast optics used in the microscope image the single spherical bead
as apposed black and white ovoids on a gray background. The DNA tether
(invisible) confines the random Brownian motion of the bead to a region
near the center of the frame.
Capture and movement of the bead by the laser trap. When the trap
is activated by opening the laser shutter, an image of the laser beam appears.
The beam image, which consists of a central bright spot and surrounding
interference fringes, is produced by a weak reflection of the beam at the
cover slip surface. The bead image is displaced somewhat downward and to
the right of the true trap position.
When the bead is captured by the trap, the visible Brownian motion stops.
The trap center is then moved toward the upper right corner of the frame,
dragging the bead with it and placing the DNA tether under light tension.
Translocation of the bead by the DNA-bound RNA polymerase molecule.
As transcriptional elongation occurs, the force exerted on the DNA by the
RNA polymerase molecule moves the bead away from the trap center and towards
the point at which the polymerase molecule is attached to the coverslip.
This movement is visible in the movie as a slow displacement of the bead
towards the lower left corner of the frame. Also visible are the periodic
movements of the laser beam in the same direction; these movements serve
to keep the bead positioned within the central, calibrated portion of the
The width of the frame is ~3200 nm.
A second version of the movie is identical to the first except
that it has superimposed graphics. In these graphics, the number in the
upper left corner of the frame indicates the time acceleration. During
event 3, a new white line that points to the trap center appears each time
the trap is repositioned.
Many thanks to Daniel
Peisach for help constructing the movie.