DNA bending induced by cruciform formation

Abstract

Cruciform structures^1,2 in DNA are of considerable interest, both as extreme examples of sequence-dependent structural heterogeneity and as models for four-way junctions such as the Holliday junction³ of homologous genetic recombination. Cruciforms are of lower thermodynamic stability than regular duplex DNA, and have been observed only in negatively supercoiled molecules^4–6, where the unfavourable free energy of formation is offset by the topological relaxation of the torsionally stressed molecule. From an experimental viewpoint this can be a disadvantage, as cruciform structures can be studied only in relatively large supercoiled DNA circles, and are destabilized when a break is introduced at any point. We therefore set out to construct a pseudo-cruciform junction—by generating hereroduplex formation between two inverted repeat sequences. Stereochemically, this should closely resemble a true cruciform but remain stable in a linear DNA fragment. We have now created such a junction and find that it has the expected sensitivities to endonucleases. These DNA fragments exhibit extremely anomalous gel electrophoretic mobility, the extent of which depends on the relative position of the pseudo-cruciform along the length of the molecule. Our results are very similar to those obtained by Wu and Crothers⁷ using kinetoplast DNA, and we conclude that the pseudo-cruciform junction introduces a bend in the linear DNA molecule.