Researchers
at the University of California, San Diego have found that DNA packs more
easily into the tight confines of a virus when given a chance to relax.
DNA is a
long, unwieldy molecule that tends to repel itself because it is negatively
charged, yet it can spool tightly. Within the heads of viruses, DNA can be
packed to near crystalline densities, crammed in by a molecular motor.
"These
are among the most powerful molecular motors we know of," says Douglas Smith,
a professor of physics whose group studies them.
Within an
infected cell, viruses assemble in a matter of minutes. Smith's group studies
the process by isolating components of this system to watch single molecules in
action.
They attach
the empty head of a single virus, along with the molecular motor, to a
microscopic bead that can be moved about using a laser. To another bead, they
tether a molecule of viral DNA.
"It's
like fishing," Smith says. "We dangle a DNA molecule in front of the
viral motor. If we're lucky, the motor grabs the DNA and starts pulling it
in."
Packaging
proceeds in fits and starts, with slips and pauses along the way. These pauses
increase, along with forces the motor counters, as the viral head becomes full.
This image shows cross-sections of
the empty prohead of the bacteriophage phi29 (left) and the fully assembled
virus (right). A molecular motor transports the DNA (red) into the prohead
through a portal.
Scientists
who model this process have had to make assumptions about the state of the DNA
within. An open question is whether the DNA is in its lowest energy state, that
is at equilibrium, or in a disordered configuration.
"In
confinement, it could be forming all kinds of knots and tangles," said
Zachary Berndsen, a graduate student in biochemistry who works with Smith. To
figure this out, Berndsen stalled the motor by depriving it of chemical energy,
and found that packaging rates picked up when the motor restarted. The longer
the stall, the greater the acceleration.
DNA takes
more than 10 minutes to fully relax inside the confines of a viral head where
there's little wiggle room, the team found. That's 60,000 times as long as it
takes unconfined DNA to relax.
Beyond a
clearer understanding of how viruses operate, the approach offers a natural
system that is a model for understanding and studying the physics of long
polymers like DNA in confined spaces.
By
Keerti
Mishra
