Dissolution of double Holliday junctions by the concerted action of BLM and topoisomerase IIIalpha.

In eukaryotic cells, topoisomerase III forms an evolutionarily conserved complex with a RecQ family helicase and two OB-fold containing proteins, replication protein A (RPA) and RMI1. One role for this complex is to catalyze the completion of homologous recombination reactions in which the recombining DNA molecules are covalently interlinked by a double Holliday junction structure. This process, which requires the single-stranded DNA decatenation activity of topoisomerase III, is termed Holliday junction "dissolution" to distinguish it from Holliday junction "resolution" catalyzed by endonucleases (resolvases) that simply cleave the four-way junction. Holliday junction dissolution gives rise exclusively to non-cross-over recombinant products, which would have the effect of suppressing sister chromatid exchanges and loss of heterozygosity between homologous chromosomes. In this chapter, we provide a detailed experimental protocol for the preparation of an oligonucleotide-based, double Holliday junction substrate and for the biochemical analysis of dissolution in vitro.