title: A stable interaction between separated pyrimidine.purine tracts in circular DNA.
abstract: Pyrimidine.purine tracts are widespread in eukaryotic genomes and have the potential to form a number of unusual structures including triplexes. Two such tracts, which could form triplexes with each other but not with themselves, were cloned into a plasmid at separate sites. Upon lowering the pH, linear, open circular and relaxed plasmid molecules formed a number of novel structures that were observed on agarose gels and directly by electron microscopy. In open circles a stable join was formed between the two Pyr.Pur tracts giving rise to molecules resembling dumbells, trefoils and tetrafoils, which collectively are termed T-loops. The structure was stable at pH 8 and contains a single-stranded region that was sensitive to P1 nuclease. Thus, there is no apparent topological impediment to the formation of triplex-mediated loops in circular molecules. These structures may be important for gene regulation and chromosome condensation.
authors.affiliation: Department of Biochemistry, University of Saskatchewan, Saskatoon, Canada.
authors.forename: J S
authors.initials: JS
authors.lastname: Lee
authors.forename: C
authors.initials: C
authors.lastname: Ashley
authors.forename: K J
authors.initials: KJ
authors.lastname: Hampel
authors.forename: R
authors.initials: R
authors.lastname: Bradley
authors.forename: D G
authors.initials: DG
authors.lastname: Scraba
mesh_terms: D001483:Base Sequence; D004270:DNA, Circular / Q000737:chemistry* / Q000648:ultrastructure; D004355:Drug Stability; D004587:Electrophoresis, Agar Gel; D006863:Hydrogen-Ion Concentration; D008854:Microscopy, Electron; D008969:Molecular Sequence Data; D010957:Plasmids / Q000737:chemistry / Q000648:ultrastructure; D011687:Purines; D011743:Pyrimidines; D015719:Single-Strand Specific DNA and RNA Endonucleases; D013096:Spermine
chemical_list: D004270:DNA, Circular; D011687:Purines; D011743:Pyrimidines; D013096:Spermine; D015719:Single-Strand Specific DNA and RNA Endonucleases