Tension induces a base-paired overstretched DNA conformation

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 109; no. 38; pp. 15179 - 15184
• Main Authors: Bosaeus, Niklas, El-Sagheer, Afaf H, Brown, Tom, Smith, Steven B, Åkerman, Björn, Bustamante, Carlos, Nordén, Bengt
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 18.09.2012; National Acad Sciences
• Subjects: acid; Bacterial proteins; Base Pairing; Biological Sciences; Chemical bases; complexes; denaturation; Deoxyribonucleic acid; dichroism; DNA; DNA - chemistry; DNA conformation; DNA stretching; double helix; elongated DNA; force; GC Rich Sequence - genetics; Guanine - chemistry; homologous recombination; Humans; Hydrogen Bonding; Hydrogen-Ion Concentration; linear; mechanical properties of DNA; mechanism of recombination; Melting; Molecular physics; Molecular structure; molecule; Molecules; Nucleic Acid Conformation; Nucleic acids; Oligonucleotides; Oligonucleotides - chemistry; optical traps; Optical Tweezers; Physical Sciences; Rad51 Recombinase - chemistry; Rec A Recombinases - chemistry; Research design; Salts; single molecule; stranded-dna; Stress, Mechanical; structural transitions; Table salt; Temperature; terminal alkynes; Thermodynamics
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1213172109

Mixed-sequence DNA molecules undergo mechanical overstretching by approximately 70% at 60–70 pN. Since its initial discovery 15 y ago, a debate has arisen as to whether the molecule adopts a new form [Cluzel P, et al. (1996) Science 271:792–794; Smith SB, Cui Y, Bustamante C (1996) Science 271:795–799], or simply denatures under tension [van Mameren J, et al. (2009) Proc Natl Acad Sci USA 106:18231–18236]. Here, we resolve this controversy by using optical tweezers to extend small 60–64 bp single DNA duplex molecules whose base content can be designed at will. We show that when AT content is high (70%), a force-induced denaturation of the DNA helix ensues at 62 pN that is accompanied by an extension of the molecule of approximately 70%. By contrast, GC-rich sequences (60% GC) are found to undergo a reversible overstretching transition into a distinct form that is characterized by a 51% extension and that remains base-paired. For the first time, results proving the existence of a stretched basepaired form of DNA can be presented. The extension observed in the reversible transition coincides with that produced on DNA by binding of bacterial RecA and human Rad51, pointing to its possible relevance in homologous recombination.