The S1-sensitive form of d(C-T)n.d(A-G)n: chemical evidence for a three-stranded structure in plasmids

摘要: Homopurine-homopyrimidine sequences that flank certain actively transcribed genes are hypersensitive to single strand-specific nucleases such as S1. This has raised the possibility an unusual structure exists in these regions might be involved recognition or regulation. Several of sequences, including d(C-T)n.d(A-G)n, known undergo a transition plasmids underwound state is nucleases; this occurs under conditions moderately acid pH and negative supercoiling. Chemical probes were used examine reactivity restriction fragment from human U1 gene containing sequence d(C-T)18.d(A-G)18 function supercoiling pH, thus analyze region. Hyperreactivity was seen center at one end (C-T)n tract, continuously same (A-G)n presence less than equal 6.0. These results provide strong support for triple-helical model recently proposed inconsistent with other structures.