1. Department of Biochemistry and Molecular Biology, Drexel University College of Medicine, Philadelphia, Pennsylvania 19102-1192, USA
2. Institute of Chemical Biology and Fundamental Medicine, Siberian Branch of Russian Academy of Science, Novosibirsk, 630090, Russia

Correspondence to: Alexander V. Mazin¹ Correspondence and requests for materials should be addressed to A.V.M. (Email: amazin@drexelmed.edu).

Abstract

Homologous recombination has a crucial function in the repair of DNA double-strand breaks and in faithful chromosome segregation^{1, 2, 3}. The mechanism of homologous recombination involves the search for homology and invasion of the ends of a broken DNA molecule into homologous duplex DNA to form a cross-stranded structure, a Holliday junction (HJ)^{4, 5, 6, 7}. A HJ is able to undergo branch migration along DNA, generating increasing or decreasing lengths of heteroduplex. In both prokaryotes and eukaryotes, the physical evidence for HJs, the key intermediate in homologous recombination, was provided by electron microscopy⁸. In bacteria there are specialized enzymes that promote branch migration of HJs⁷. However, in eukaryotes the identity of homologous recombination branch-migration protein(s) has remained elusive. Here we show that Rad54, a Swi2/Snf2 protein⁹, binds HJ-like structures with high specificity and promotes their bidirectional branch migration in an ATPase-dependent manner. The activity seemed to be conserved in human and yeast Rad54 orthologues. In vitro, Rad54 has been shown to stimulate DNA pairing of Rad51, a key homologous recombination protein^{10, 11, 12}. However, genetic data indicate that Rad54 protein might also act at later stages of homologous recombination, after Rad51 (ref. 13). Novel DNA branch-migration activity is fully consistent with this late homologous recombination function of Rad54 protein.

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