title: B-X transition in synthetic and natural (dA-dT)n.(dA-dT)n sequences.
abstract: AB-X transition of polyh(dA-dT).poly(dA-dT) was observed to occur in methanol-water mixtures with methanol concentrations higher than 50% in the presence of a specific combination of monovalent and divalent cations. In the presence of Na+, divalent cations induce denaturation of poly(dA-dT).poly(dA-dT) accompanied by condensation and/or aggregation, and effect similar to that observed previously with random sequence DNA (Votavová, Kucerová, Felsberg and Sponar, J. Biomol. Struct. Dyn. 4,477-489, 1986). In the presence of Cs+ cations a B-X transition was induced by addition of Ca2+ or Mn2+ but not Mg2+ or Ni2+ ions. Circular dichroism and ultraviolet spectroscopy demonstrate that the X conformation is a double stranded form of poly(dA-dT).poly(dA-dT) belonging presumably to the B family which, however has an altered base stacking. The X conformation of poly(dA-dT).poly(dA-dT) found in methanol-water mixtures is a condensed and/or aggregated form. In contrast, the X conformation characterized by similar CD spectra observed in high salt concentrations is not aggregated up to a concentration of 6 M CsF. In methanol-water mixtures (A+T)-rich bacterial DNA behaves essentially as a random sequence DNA revealing no detectable amount of the X form. On the other hand crab (Cancer pagurus) satellite and crab non-satellite DNAs containing varying amounts of (dA-dT)n.(dA-dT)n sequences were shown to undergo a B-X transition, at least partly, in both methanol-water mixtures and 6 M CsF solutions.
authors.affiliation: Institute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Sciences, Prague.
authors.forename: H
authors.initials: H
authors.lastname: Votavová
authors.forename: J
authors.initials: J
authors.lastname: Sponar
mesh_terms: D000818:Animals; D003386:Brachyura; D002942:Circular Dichroism; D004247:DNA / Q000648:ultrastructure; D004276:DNA, Satellite / Q000648:ultrastructure; D009690:Nucleic Acid Conformation; D009691:Nucleic Acid Denaturation; D011067:Poly dA-dT / Q000138:chemical synthesis; D011089:Polydeoxyribonucleotides / Q000138:chemical synthesis; D013056:Spectrophotometry, Ultraviolet
chemical_list: D004276:DNA, Satellite; D011089:Polydeoxyribonucleotides; D011067:Poly dA-dT; D004247:DNA