Abstract:

Measurements of force and extension on single molecules of DNA have allowed
for direct measurement of the molecule's mechanical properties, provided 
rigorous tests of theories of polymer elasticity, and established an 
experimental and conceptual framework for mechanical assays of enzymes that 
act on DNA.  In this talk I present the development and application of a novel 
assay to study the torque and twist in a single molecule of DNA.  The 
technique involves the attachment of a small "rotor" bead to the side of a 
stretched DNA molecule; this rotor bead spins in response to torque in 
the DNA.  By observing the magnitude of angular fluctuations of this rotor 
bead we are able to measure the torsional rigidity of DNA, and find a value 
~ 50% higher than the value accepted from bulk experiments.  Next we directly 
measure the change in DNA helical twist when the molecule is stretched.  In 
contrast to both simple physical intuition and previous fitting of 
experimental data, we find that DNA overwinds when stretched.  Finally, we use 
the rotor bead technique to study the mechanochemical cycle of DNA gyrase, the 
molecular machine that is responsible for introducing essential negative 
supercoils into the bacterial genome.