RESUMEN
Fluorescence resonance energy transfer (FRET) measurements on four-way DNA junctions have been carried out in order to analyze the global structure and its dependence on the concentration of several types of ions. The stereochemical arrangement of the four DNA helices that make up the four-way junction was established by a global comparison of the efficiency of FRET between donor and acceptor molecules attached pairwise in all possible permutations to the 5' termini of the duplex arms of the four-way structure. The results indicate that the four-way junction isomerizes from an unstacked extended square arrangement of the four duplex arms at low ion concentration to an antiparallel stacked X-structure as the salt is added. The ion-related conformational change progresses in a continuous non-cooperative manner as the ionic strength of the solution increases.