A cartoon of an AOT generating plectonemes in DNA.
In vivo, DNA topology is altered by motor proteins and our lab pioneered an angular trapping technique for simultaneous torque and force generation and detection (La Porta and Wang, PRL, 2004, Deufel et al., Nature Methods, 2007). When a birefringent particle is trapped in a polarized laser beam, rotation of the laser polarization induces rotation of the particle, while torque exerted on the particle is detected as a change in the polarization of the trapping beam. This technique allows the control and detection of the torque of a biological molecule attached to the particle and has opened new dimensions for applications of optical trapping techniques.
An SEM image of nanofabricated quartz cylinders before they are separated from their substrate.
We have also developed specialized particles for optical rotation experiments, making use of the world-class nanofabrication facilities located on Cornell’s Ithaca campus (Deufel et al., Nature Methods, 2007). Highly homogenous quartz nano-cylinders have been produced in mass quantities, with specific biological functionalization for use in DNA torsional experiments.
This instrument has enabled the Wang lab to measure the torque it takes to buckle DNA (Forth et al., PRL, 2008), melt DNA(Sheinin et al., PCCP, 2009), and migrate a Holliday junction(Forth et al., BJ, 2011). Our lab has investigated the role of DNA torque and topology during transcription and the AOT directly measured the torque required to stall RNA polymerase (Ma et al., Science, 2013).