The global folding of four-way helical junctions in RNA, including that in U1 snRNA

Helical junctions are important elements in the architecture of folded RNA molecules. The global geometry of fully base-paired four-way junctions between RNA helices has been analyzed by comparative gel electrophoresis. Junctions appear to fold by pairwise coaxial helical stacking in one of two possible stereochemically equivalent isomers based upon alternative selections of stacking partners. In the presence of 1 mM Mg2+, the two continuous helical axes are approximately at right angles to each other for all junctions studied, but the RNA junctions exhibit significant sequence-dependent differences in their structures as a function of ionic conditions. The four-way junction found in the U1 snRNA folded by coaxial helical stacking. It retained the 90 degrees crossed stacked structure under all ionic conditions tested, despite the presence of a G.A mismatch at the point of strand exchange.