Fig. 1.

Scheme for observing branch migration by a single RuvAB complex under an applied force. A junction with two very long arms formed from seven oligonucleotides (see Materials and Methods) and two ≈20-kbp segments of the λ-phage DNA tethers a paramagnetic bead to a glass surface. Regions of heterology (denoted by shaded regions in the four arms; 12 bp in the vertical arms and 40 bp in the horizontal arms) prevented spontaneous branch migration. The gray square at the crossover point and the pair of black ovals sandwiching it represent the RuvA and RuvB components of the complex, respectively. Dotted arrows represent the direction of migration for the shown orientation of the RuvAB complex after addition of ATP. The pair of magnets induces a field (gray dashed lines) that effects a force opposing branch migration (hatched arrow). When the RuvB rings assemble on the vertical arms, the direction of branch migration is reversed. The motion of the bead was followed through a microscope and recorded at video rate.