The rate of branch migration in doublestranded DNA has been measured by the use of a unique substrate formed by the action of the EcoRI restriction endonuclease on the dimeric figure-S configuration of the replicative form DNA of phage G4. The figureS and the X-form derived from it contain a junction of the kind postulated to occur in the Holliday structure and to be an essential feature of a number of models of recombination. In the X-form this junction can branch migrate to an irreversible terminal configuration consisting of two linear monomers. The disappearance of X-forms was measured by electron microscopy. A treatment of branch migration as a random walk process was developed to permit the determination of the rate of the intrinsic process, a step movement of the junction by a distance of one base pair. A value of about 6 kilobase pairs per sec at 370 was obtained. The figure-8 configuration of dimeric replicative form (RF) DNA of phages 4X174 and S13 was shown by Thompson et al. (1) to comprise up to 7% of the dimers. It was proposed to be an intermediate in recombination in these phages chiefly because it was assumed to be able to engage in branch migration (1, 2). A junction of the type present in the figure-8 is an essential feature of the Holliday structure (3), and its migration is basic to a number of models of recombination described in recent reviews (4-6). The figure-8 is, in fact, a Holliday structure for the simple, circular genome of the RF of the small DNA phages.Evidence that branch migration is a physically possible process was given by Lee et al. (7). They studied a repetitious DNA structure with single-stranded tails by electron microscopy and concluded from the distribution of tail lengths that branch migration had occurred. Kim et al. (8) provided similar evidence for double-stranded branch migration of the kind that occurs in the figure-8. Structures interpreted as involving both single-and double-stranded migration were observed by electron microscopy to result from recombination in phage T4 by Broker and Lehman (9, 10). They developed models showing the relevance of branch migration to recombination and suggested the figure-8, among other configurations, as an intermediate. Largely because of the reversibility of the direction of branch migration, the methods that have been used thus far cannot give information about the dynamics of the process or even provide more than indirect evidence that it can occur.The RF of phage G4 has one site per monomer for hydrolysis by the EcoRI restriction endonuclease. We have used this enzyme to convert figure-8 forms from G4-RF to an X-shaped configuration. These X-forms are a new type of substrate for studying branch migration because the migration terminates in an irreversible step, the formation of two linear monomers.We report here a preliminary study of branch migration in the X-form, including measurements of its rate. Electron Microscopy. Samples were prepared for electron microscopy as previously described (1). Some of the sprea...

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