Escherichia coli RuvA and RuvB proteins specifically interact with Holliday junctions and promote branch migration.

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Abstract
The Escherichia coli ruvA and ruvB genes are involved in DNA repair and in the late step of homologous genetic recombination. We have demonstrated previously that the RuvA-RuvB protein complex in the presence of ATP promotes reabsorption of cruciform structures extruded from a supercoiled plasmid with an inverted repeat sequence. Because the cruciform structure is topologically analogous to the Holiday structure, we have proposed that the role of the RuvA and RuvB proteins in recombination is to promote a strand exchange reaction at the Holliday junction. Here, we studied the specific interaction of the RuvA-RuvB complex with the Holliday structure using synthetic analogs prepared by annealing four oligonucleotides. The affinities of the RuvA protein for synthetic Holliday junctions are much higher (> 20-fold) than for duplex DNA, and the affinities of the RuvA protein for the junctions are further enhanced (> 4-fold) by the interaction with the RuvB protein. The RuvA-RuvB protein complex in the presence of ATP promotes dissociation of the synthetic Holliday junction with homology in the central core into two halves by catalyzing branch migration to the DNA ends, but it does not affect the structure of the synthetic Holliday junction without the homology. The separation of the synthetic Holliday junction is a result of the activity of the RuvA-RuvB complex that promotes strand exchange and DNA unwinding. Furthermore, RuvA and RuvB promote the strand exchange reaction at the Holliday junctions made by RecA. These results provide further evidence that the RuvA-RuvB complex recognizes the Holliday junction and promotes branch migration in homologous recombination.