Sequence and supercoiling-dependent effects on the structural dynamics of DNA minicircles

The degree of over-/underwinding of the DNA double helix, quantified by the superhelical density, is a key feature modulating critical biological processes such as gene expression and regulation. In fact, DNA molecules are able to channel the excess levels of mechanical stress into local defective and denatured states that are promptly detected by, e.g., transcription factors and nuclease enzymes. The occurrence and stability of these motifs are dictated by a complex interplay between topological and sequence-dependent effects, ultimately affecting the global conformational dynamics of the DNA molecule itself. Here, we characterize the impact of the sequence and of the superhelical density on the structural evolution of a 672-bp DNA minicircle via classical molecular dynamics simulations employing the coarse-grained oxDNA force field. We observe that moderately-to-highly undercoiled regimes are associated with the occurrence of stable, somewhat broad denaturation bubbles, typically co-localizing with flexible nucleotide sequences on the DNA minicircle. These defects are hardly reabsorbed by the system, thereby pinning the subsequent dynamics of the molecule. In fact, a similar behavior was recapitulated by enforcing "synthetic,"adjoining DNA mismatches, regardless of the underlying nucleotide sequence, suggesting an effective manner of DNA manipulation.