STABILITY OF TRIPLE HELICES CONTAINING RNA AND DNA STRANDS - EXPERIMENTAL AND MOLECULAR MODELING STUDIES

UV-absorption spectrophotometry and molecular modeling have been used to study the influence of the chemical nature of sugars (ribose or deo xyribose) on triple helix stability. For the Pyrimidine.Purine Pyrimi dine motif, all eight combinations were tested with each of the three strands composed of either DNA or RNA. The chemical nature of sugars h as a dramatic influence on triple helix stability. For each double hel ix composition, a more stable triple helix was formed when the third s trand was RNA rather than DNA. No stable triple helix was detected whe n the polypurine sequence was made of RNA with a third strand made of DNA. Energy minimization studies using the JUMNA program suggested tha t interactions between the 2'-hydroxyl group of the third strand and t he phosphates of the polypurine strand play an important role in deter mining the relative stabilities of triple-helical structures in which the polypyrimidine third strand is oriented parallel to the polypurine sequence. These interactions are not allowed when the third strand ad opts an antiparallel orientation with respect to the target polypurine sequence, as observed when the third strand contains G and A or G and T/U. We show by footprinting and gel retardation experiments that an oligoribonucleotide containing G and A or G and U fails to bind double helical DNA, while the corresponding DNA oligomers form stable triple -helical complexes.