Understanding the paradoxical mechanical response of in-phase A-tracts at different force regimes

ABSTRACTA-tracts are A:T rich DNA sequences that exhibit unique structural and mechanical properties associated with several functions in vivo. The crystallographic structure of A-tracts has been well characterized. However, their response to forces remains unknown and the variability of their flexibility reported for different length scales has precluded a comprehensive description of the mechanical properties of these molecules. Here, we rationalize the mechanical properties of A-tracts across multiple length scales using a combination of single-molecule experiments and theoretical polymer models applied to DNA sequences present in the C. elegans genome. Atomic Force Microscopy imaging shows that phased A-tracts induce long-range (∼200 nm) bending. Moreover, the enhanced bending originates from an intrinsically bent structure rather than as a consequence of larger flexibility. In support of this, our data were well described with a theoretical model based on the worm-like chain model that includes intrinsic bending. Magnetic tweezers experiments confirm that the observed bent is intrinsic to the sequence and does not rely on particular ionic conditions. Using optical tweezers, we assess the local rigidity of A-tracts at high forces and unravel an unusually stiff character of these sequences, as quantified by their large stretch modulus. Our work rationalizes the complex multiscale flexibility of A-tracts, shedding light on the cryptic character of these sequences.