Near-field-magnetic-tweezer manipulation of single DNA molecules

We have developed an instrument for micromanipulation of single DNA molecules end labeled with [Formula presented]-diameter paramagnetic particles. A small, permanent magnet that can be moved as close as [Formula presented] to the particle being manipulated can generate forces in excess of [Formula presented], significantly larger than obtained in other recent “magnetic-tweezer” studies. Our instrument generates these forces in the focal plane of a microscope objective, allowing straightforward real-time observation of molecule extension with a position resolution of approximately [Formula presented]. We show how our magnetic manipulation system can be combined with manipulation and force measurement using glass micropipettes to allow rapid switching between measurements in fixed-force and fixed-extension ensembles. We demonstrate the use of our system to study formation of DNA loops by an enzyme which strongly binds two copies of a specific 6-base-pair sequence.