Torsional stress stabilizes extended base unpairing in suppressor sites flanking immunoglobulin heavy chain enhancer

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Abstract
DNA sequences surrounding the immunoglobulin heavy chain (IgH) enhancer contain negative regulatory elements which are important for the tissue specificity of the enhancer. We have shown that sequences located both 5'' and 3'' of the enhancer, corresponding to the negative regulatory elements, become stably and uniformly unpaired over an extended length when subjected to torsional stress. These DNA sequences are also included within matrix association regions. The ability of the sequences to assume a stably unpaired conformation was shown by reactivity with chloroacetaldehyde which is specific for unpaired DNA bases, as well as two-dimensional gel electrophoresis of topoisomers. The sequences located 3'' of the enhancer induce base unpairing in the direction of the enhancer. This unpaired region progressively expands to include as much as 200 base pairs as the ionic concentration decreases or superhelical density increases. When an ATATAT motif within a negative regulatory element located 3'' of the enhancer was mutated, the extensive base-unpairing property was abolished. This base-unpairing property of DNA may be important for negative regulation of gene expression and attachment to the nuclear matrix.