A nanomechanical device based on the B-Z transition of DNA

The assembly of synthetic, controllable molecular mechanical systems(1-7) i s one of the goals of nanotechnology. Protein-based molecular machines, oft en driven by an energy source such as ATP, are abundant in biology(8,9). It has been shown previously that branched motifs of DNA can provide componen ts for the assembly of nanoscale objects(10), links(11) and arrays(12). Her e we show that such structures can also provide the basis for dynamic assem blies: switchable molecular machines. We have constructed a supramolecular device consisting of two rigid DNA 'double-crossover' (DX) molecules connec ted by 4.5 double-helical turns. One domain of each DX molecule is attached to the connecting helix. To effect switchable motion in this assembly, we use the transition between the B and Z(13,14) forms of DNA. In conditions t hat favour B-DNA, the two unconnected domains of the DX molecules lie on th e same side of the central helix, In Z-DNA-promoting conditions, however, t hese domains switch to opposite sides of the helix. This relative repositio ning is detected by means of fluorescence resonance energy transfer spectro scopy, which measures the relative proximity of two dye molecules attached fa the free ends of the DX molecules, The switching event induces atomic di splacements of 20-60 Angstrom.