Abstract
Two factors contribute to structural diversity in DNA molecules. These are base sequence and DNA supercoiling. Certain DNA sequence arrangements have an inherrent propensity to adopt a geometry which is recognisably different from B-DNA, which in the present context means an inverted repeat having a two-fold symmetry axis. However, many perturbed DNA structures are unstable relative to B-DNA in the absence of an energy source, which may be provided by negative supercoiling. Cruciform structures (paired stem-loop structures) were first recognised experimentally about eight years ago [Geliert et al, 1979; Lilley, 1980; Panayotatos & Wells, 1981], twenty five years after their first theoretical description [Platt, 1955; Gierer, 1966]. The crucial role of supercoiling was overlooked in earlier studies. Cruciform formation has subsequently been demonstrated for many inverted repeat sequences in a variety of supercoiled plasmids and phage.
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