Optical instrumentation has been developed to provide simultaneous transmitted light and fluorescent video images of single chromatin fibers. The chromatin fiber was suspended between attachment to the silanized surface of a square cuvette and a micron-size magnetic bead. A white light source was filtered to provide both excitation light for fluorescent tags attached to the chromatin and non-competing transmission light to monitor the position of the bead. Magnetic tweezers allow manipulation (rotation) of forces applied to the magnetic bead-chromatin complex. This instrumentation allowed real time observation of chaperone-mediated chromatin assembly/disassembly and demonstrated the dynamic equilibrium between assembly and disassembly at the single chromatin fiber level.
