title: Tension induces a base-paired overstretched DNA conformation.
abstract: 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.
authors.affiliation: Department of Chemical and Biological Engineering, Chalmers University of Technology, Gothenburg S41296, Sweden.
authors.forename: Niklas
authors.initials: N
authors.lastname: Bosaeus
authors.forename: Afaf H
authors.initials: AH
authors.lastname: El-Sagheer
authors.forename: Tom
authors.initials: T
authors.lastname: Brown
authors.forename: Steven B
authors.initials: SB
authors.lastname: Smith
authors.forename: Björn
authors.initials: B
authors.lastname: Akerman
authors.forename: Carlos
authors.initials: C
authors.lastname: Bustamante
authors.forename: Bengt
authors.initials: B
authors.lastname: Nordén
mesh_terms: D020029:Base Pairing; D004247:DNA / Q000737:chemistry*; D020862:GC Rich Sequence / Q000235:genetics; D006147:Guanine / Q000737:chemistry; D006801:Humans; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D009690:Nucleic Acid Conformation; D009841:Oligonucleotides / Q000737:chemistry; D052898:Optical Tweezers; D051135:Rad51 Recombinase / Q000737:chemistry; D011938:Rec A Recombinases / Q000737:chemistry; D013314:Stress, Mechanical; D013696:Temperature; D013816:Thermodynamics
chemical_list: D009841:Oligonucleotides; D006147:Guanine; D004247:DNA; C495153:RAD51 protein, human; D051135:Rad51 Recombinase; D011938:Rec A Recombinases