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Updated: Jun 2, 2026

Real-time Observation of the DNA Strand Exchange Reaction Mediated by Rad51
Published on: February 13, 2019
Single-molecule analysis reveals the molecular bearing mechanism of DNA strand exchange by a serine recombinase
Hua Bai1, Mingxuan Sun, Pallavi Ghosh
1Department of Physics, University of Illinois at Chicago, Chicago, IL 60607, USA.
Abstract:
Structural and topological data suggest that serine site-specific DNA recombinases exchange duplex DNAs by rigid-body relative rotation of the two halves of the synapse, mediated by a flat protein-protein interaction surface. We present evidence for this rotational motion for a simple serine recombinase, the Bxb1 phage integrase, from a single-DNA-based supercoil-release assay that allows us to follow crossover site cleavage, rotation, religation, and product release in real time. We have also used a two-DNA braiding-relaxation experiment to observe the effect of synapse rotation in reactions on two long molecules. Relaxation and unbraiding are rapid (averaging 54 and 70 turns/s, respectively) and complete, with no discernible pauses. Nevertheless, the molecular friction associated with rotation is larger than that of type-I topoisomerases in a similar assay. Surprisingly we find that the synapse can stay rotationally "open" for many minutes.
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