title: Condensation prevails over B-A transition in the structure of DNA at low humidity.
abstract: B-A transition and DNA condensation are processes regulated by base sequence and water activity. The constraints imposed by interhelical interactions in condensation compromise the observation of the mechanism by which B and A base-stacking modes influence the global state of the molecule. We used a single-molecule approach to prevent aggregation and mechanical force to control the intramolecular chain association involved in condensation. Force-extension experiments with optical tweezers revealed that DNA stretches as B-DNA under ethanol and spermine concentrations that favor the A-form. Moreover, we found no contour-length change compatible with a cooperative transition between the A and B forms within the intrinsic-force regime. Experiments performed at constant force in the entropic-force regime with magnetic tweezers similarly did not show a bistable contraction of the molecules that could be attributed to the B-A transition when the physiological buffer was replaced by a water-ethanol mixture. A total, stepwise collapse was found instead, which is characteristic of DNA condensation. Therefore, a low-humidity-induced change from the B- to the A-form base-stacking alone does not lead to a contour-length shortening. These results support a mechanism for the B-A transition in which low-humidity conditions locally change the base-stacking arrangement and globally induce DNA condensation, an effect that may eventually stabilize a molecular contour-length reduction.
authors.affiliation: Instituto Madrileño de Estudios Avanzados en Nanociencia Cantoblanco, Madrid, Spain.
authors.forename: Silvia
authors.initials: S
authors.lastname: Hormeño
authors.forename: Fernando
authors.initials: F
authors.lastname: Moreno-Herrero
authors.forename: Borja
authors.initials: B
authors.lastname: Ibarra
authors.forename: José L
authors.initials: JL
authors.lastname: Carrascosa
authors.forename: José M
authors.initials: JM
authors.lastname: Valpuesta
authors.forename: J Ricardo
authors.initials: JR
authors.lastname: Arias-Gonzalez
mesh_terms: D020029:Base Pairing; D001696:Biomechanical Phenomena; D004247:DNA / Q000737:chemistry*; D044704:DNA, A-Form / Q000737:chemistry*; D004269:DNA, Bacterial / Q000737:chemistry; D004548:Elasticity; D000431:Ethanol / Q000737:chemistry; D006813:Humidity; D008280:Magnetics; D052898:Optical Tweezers; D044367:Phase Transition; D013302:Streptomyces; D014867:Water / Q000737:chemistry
chemical_list: D044704:DNA, A-Form; D004269:DNA, Bacterial; D014867:Water; D000431:Ethanol; D004247:DNA