Geometry of a branched DNA structure in solution.

An approximate geometry of a stable, four-way DNA junction has been determined in free solution by applying the technique of transient electric birefringence. The current approach consists of (i) construction of a set of six molecules in which two of the four arms of a synthetic junction are elongated by approximately 9-fold (in each of the six possible two-arm combinations), (ii) determination of the ratios of the longest birefringence decay time of each elongated junction to the decay time of a linear control molecule, and (iii) comparison of the experimental ratios with corresponding ratios computed as a function of the junction interarm angle. The result is a set of six angles that define the geometry of the junction. In the presence of magnesium ions, the junction adopts a geometry in which particular pairs of arms approach colinearity. Furthermore, the geometry of the junction is significantly altered in the absence of magnesium, adopting a more uniform structure, although such an effect is not apparent on gels. The application of transient electric birefringence, as described in the current study, should be useful for the characterization of a broad range of tertiary structures in both DNA and RNA.