Topological insulators inhibit diffusion of transcription-induced positive supercoils in the chromosome of Escherichia coli

The double helical nature of DNA implies that progression of transcription machinery that cannot rotate easily around the DNA axis creates waves of positive supercoils ahead of it and negative supercoils behind it. Using topological reporters that detect local variations in DNA supercoiling, we have characterized the diffusion of transcription-induced (TI) positive supercoils in plasmids or in the chromosome of wild type Escherichia coli cells. Transcription-induced positive supercoils were able to diffuse and affect local supercoiling several kilobases away from the site of origin. By testing the effect of various DNA sequences, these reporters enabled us to identify elements that impede supercoil diffusion, i.e. behave as topological insulators. All the elements tested correspond to DNA gyrase catalytic targets. These results correlate the ability of a DNA sequence to be cleaved by DNA gyrase with topological insulator activity. Implications of the asymmetry in supercoil diffusion for the control of DNA topology are discussed.