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Science 30 September 1988:
Vol. 241. no. 4874, pp. 1800 - 1804
DOI: 10.1126/science.2845572
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Articles

Science, Vol 241, Issue 4874, 1800-1804
Copyright © 1988 by American Association for the Advancement of Science

articles

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

BH Johnston

Department of Biology, Massachusetts Institute of Technology, Cambridge 02139.

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.


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Topology and formation of triple-stranded H-DNA.
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Single strands, triple strands, and kinks in H-DNA.
H Htun and J. Dahlberg (1988)
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Science. ISSN 0036-8075 (print), 1095-9203 (online)