 Immediately after strand invasion , the Holliday junction moves along the linked DNA during the branch migration process . It is in this movement of the Holliday junction that base pairs between the two homologous DNA duplexes are exchanged . To catalyze branch migration , the RuvA protein first recognizes and binds to the Holliday junction and recruits the RuvB protein to form the <unk> complex . Two sets of the RuvB protein , which each form a ring @-@ shaped ATPase , are loaded onto opposite sides of the Holliday junction , where they act as twin pumps that provide the force for branch migration . Between those two rings of RuvB , two sets of the RuvA protein assemble in the center of the Holliday junction such that the DNA at the junction is sandwiched between each set of RuvA . The strands of both DNA duplexes — the " donor " and the " recipient " duplexes — are unwound on the surface of RuvA as they are guided by the protein from one duplex to the other . 
