Virtually all models for recombination between homologous DNA sequences invoke a branched intermediate known as a Holliday structure. The terminal steps of recombination are postulated to involve a specific cleavage through the four-way junction of a Holliday structure, in a process known as resolution. We have constructed a synthetic Holliday structure in which the position of the junction of the DNA duplexes can branch migrate through approximately 185 bp. Using this structure, we have found that a component of a cytoplasmic extract of Hela cells is capable of cleaving the central junction of the substrate in a manner consistent with resolution. The activity requires a divalent cation but does not require an exogenous energy source. This is the first reported resolution activity from a mammalian source.
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