title: The S1-sensitive form of d(C-T)n.d(A-G)n: chemical evidence for a three-stranded structure in plasmids.
abstract: Homopurine-homopyrimidine sequences that flank certain actively transcribed genes are hypersensitive to single strand-specific nucleases such as S1. This has raised the possibility that an unusual structure exists in these regions that might be involved in recognition or regulation. Several of these sequences, including d(C-T)n.d(A-G)n, are known to undergo a transition in plasmids to an underwound state that is hypersensitive to single strand-specific nucleases; this transition occurs under conditions of moderately acid pH and negative supercoiling. Chemical probes were used to examine the reactivity of a restriction fragment from a human U1 gene containing the sequence d(C-T)18.d(A-G)18 as a function of supercoiling and pH, and thus analyze the structure in this region. Hyperreactivity was seen in the center and at one end of the (C-T)n tract, and continuously from the center to the same end of the (A-G)n tract, in the presence of supercoiling and pH less than or equal to 6.0. These results provide strong support for a triple-helical model recently proposed for these sequences and are inconsistent with other proposed structures.
authors.affiliation: Department of Biology, Massachusetts Institute of Technology, Cambridge 02139.
authors.forename: B H
authors.initials: BH
authors.lastname: Johnston
mesh_terms: D001483:Base Sequence; D055598:Chemical Phenomena; D002621:Chemistry; D004247:DNA; D004278:DNA, Superhelical; D004720:Endonucleases / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D010957:Plasmids; D015719:Single-Strand Specific DNA and RNA Endonucleases
chemical_list: D004278:DNA, Superhelical; D004247:DNA; D004720:Endonucleases; D015719:Single-Strand Specific DNA and RNA Endonucleases