Laser trapping is an important tool in the burgeoning field of biophysics, providing information about both mechanical and dynamical properties of biological systems. Traps, acting as linear springs, are used to stretch a single biological macromolecule between two trapped dielectric handles while simultaneously measuring the length and tension in that molecule. New opto-magnetic manipulation techniques can now be used to measure twist of and apply torque to optically trapped single molecules.
Opto-magnetic tweezers are applied to a biological molecular motor, bacteriophage phi29, which is known to exert forces up to 60 pN while pulling DNA into its capsid during the self-assembly or packaging process. Direct observations of DNA twist during packaging reveals the full, three-dimensional trajectory of the DNA as it enters the capsid. When the capsid is empty, the motor slightly underwinds the downstream DNA; equivalently, the DNA takes a left-handed, helical path into the capsid. During this process, the motor exerts torques sufficient to melt the DNA. As the capsid fills, the degree of twisting increases. The observations at low filling reveal details of the coordination of the packaging motor’s five subunits. The twist at late stages of packaging can be used to estimate the degree of supercoiling within the capsid, permitting the construction of a simple model for the organization of the packaged DNA.