title: Long (dA)n.(dT)n tracts can form intramolecular triplexes under superhelical stress.
abstract: Plasmids containing long tracts of (dA)n.(dT)n have been prepared and their conformations examined in linear and supercoiled DNA using a series of chemical and enzymic probes which are known to be sensitive to unusual DNA structures. Under superhelical stress and in the presence of magnesium the sequence T69.A69 adopts a conformation at pH 8.0 consistent with the formation of an intramolecular DNA triplex. Site specific cleavage of the supercoiled plasmid by single-strand specific nucleases occurs within the A.T insert; the 5'-end of the purine strand is sensitive to reaction with diethylpyrocarbonate while the central 5-6 bases of the pyrimidine strand are reactive to osmium tetroxide. By contrast shorter inserts of A33.T33 and A23.T23 do not appear to form unusual structures.
authors.affiliation: Department of Physiology and Pharmacology, University of Southampton, UK.
authors.forename: K R
authors.initials: KR
authors.lastname: Fox
mesh_terms: D001483:Base Sequence; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D004278:DNA, Superhelical / Q000737:chemistry*; D004047:Diethyl Pyrocarbonate; D006863:Hydrogen-Ion Concentration; D008274:Magnesium; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D009993:Osmium Tetroxide; D010957:Plasmids; D011067:Poly dA-dT / Q000737:chemistry*; D015719:Single-Strand Specific DNA and RNA Endonucleases / Q000378:metabolism
chemical_list: D004278:DNA, Superhelical; D011067:Poly dA-dT; D015719:Single-Strand Specific DNA and RNA Endonucleases; D008274:Magnesium; D004047:Diethyl Pyrocarbonate; D009993:Osmium Tetroxide