Fig. 1.

Experimental strategy. Shown is a DNA with two BbvCI sites 30 bp apart, in either directly repeated (a) or inverted (b) orientations, as marked by the arrowheads. The points of cleavage in each site are marked by white arrows connected by a jagged line. The distances between the scissile bonds are indicated for both CC → CC and GC → GC separations. In both a and b, the BbvCI restriction endonuclease is shown bound to both sites: its R₁ and R₂ subunits are shown as blue and green triangles, respectively, with dark blue and dark green hemispheres for their catalytic centers. The subunits are placed on their target strands, R₁ on GC and R₂ on CC. In both a and b, the minimal motion needed to reposition the enzyme from the left-hand to the right-hand site is shown with red arrow(s): a linear translocation between the directly repeated sites in a and a rotation around and a turnover along the DNA with the inverted sites in b.