FIGURE 9.

Models for Rec-X-HJ isomerization, and for Rec-X resolution via Top3 versus a HJ resolvase. Top: model for how a Rec-X can isomerize into a HJ. The outside strands are capable of base pairing (A), which would be limited by stress induced by the positive twist in the region of base pairing and paranemic coiling. This stress could be absorbed by compensatory negative twist in the adjacent stretch(es) of the outside strands (not shown), and so base pairs should be able to form at least transiently. Here such base pairing is shown adjacent to the right junction, thus forming a HJ (B). Lower left, model for how a Rec-X can be resolved by Top3. Top3 binds the Rec-X and cleaves one of the plectonemically coiled strands (red; C), enables passage of the intact strand (black), then religates the strand (D). The resulting negative twist is relieved by duplex rotation (E), leading to a Rec-X with one less linkage (F). G and H, the products expected after steps C–F are repeated one and two more times, respectively. Lower right, model for how a Rec-X can be resolved by a HJ resolvase. The transient HJ described above is cleaved by the HJ resolvase (I), allowing the original outside strands to coil plectonemically around each other (J). This enables the linkage on the left to form a HJ that is subsequently cleaved by the HJ resolvase (K). Note that crossing-over does not occur with resolution by Top3, but can occur with resolution via HJ resolvases.