Long range structural communication between sequences in supercoiled DNA. Sequence dependence of contextual influence on cruciform extrusion mechanism

Sequence context may profoundly alter the character of structural transitions in supercoiled DNA (Sullivan, K. M., and Lilley, D. M. J. (1986) Cell 47, 817-827). The A + T-rich sequences of ColE1, which flank the inverted repeat, are responsible for cruciform extrusion following a mechanistic pathway which proceeds via a relatively large denatured region. This C-type mechanism results in kinetic properties which are very different from those of the S-type pathway, the normal mechanism of cruciform extrusion in the absence of the ColE1 flanking sequences. We have analyzed the sequence requirements for the induction of the C-type pathway. The 100-base pair left side sequence of ColE1 (colL) was subjected to systematic deletion using Bal31 exonucleolysis, showing that removal of 30 base pairs from its right end abolished extrusion by the C-type process. A cloned oligonucleotide of the same 30-base pair sequence was sufficient to confer C-type cruciform extrusion on an adjacent inverted repeat. An A + T-rich sequence from Drosophila was found to act like the ColE1 sequences. We have studied the effects of introducing sequences between the A + T-rich colL, and the inverted repeat on which it acts. A range of such fragments was found, from those which augment the effect of colL to those which block it completely. In general, it appears that the ability of a sequence to block the effect of colL depends on both the length and G + C content of the fragment. The sequences which are responsible for the extrusion by the C-type pathway are termed C-type inducing sequences, while sequences which are interposed between the inducing sequence and the inverted repeat, and which may either augment or attenuate the effect, but which cannot function as inducing sequences in isolation, are termed transmitting sequences. The results of these studies are most readily consistent with long range destabilization of DNA structure via telestability effects.