Structure of the four-way DNA junction and its interaction with proteins

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Abstract
The four-way DNA junction is an important intermediate in recombination processes; it is, the substrate for different enzyme activities. In solution, the junction adopts a right-handed, antiparallel-stacked X -structure formed by the pairwise coaxial-stacking of helical arms. The stereochemistry is determined by the juxtaposition of grooves and backbones, which is optimal when the smaller included angle is 60°. The antiparallel structure has two distinct sides with major and minor groove-characteristics, respectively. The folding process requires the binding of metal cations, in the absence of which, the junction remains extended without helix-helix stacking. The geometry of the junction can be perturbed by the presence of certain base-base mispairs or phosphodiester discontinuities located at the point of strand exchange. The four-way DNA junction is selectively cleaved by a number of resolving enzymes. In a number of cases, these appear to recognize the minor groove face of the junction and are functionally divisible into activities that recognize and bind the junction, and a catalytic activity. Some possible mechanisms for the recognition of branched DNA structure are discussed.