The method of DNA cyclization kinetics reveals special properties of the TATAAACGCC sequence motif found in DNA sequences that have high affinity for core histones. Replacement of 30 bp of generic DNA by three 10-bp repeats of the motif in small cyclization constructs increases cyclization rates by two orders of magnitude. We document a 13°bend in the motif and characterize the direction of curvature. The bending force constant is smaller by nearly 2-fold and there is a 35% decrease in the twist modulus, relative to generic DNA. These features are the likely source of the high affinity for bending around core histones to form nucleosomes. Our results establish a protocol for determination of the ensembleaveraged global solution structure and mechanical properties of any Ϸ10-bp DNA sequence element of interest, providing information complementary to that from NMR and crystallographic structural studies.D NA in cells is highly compacted from the unfettered size it would have if free in solution. The first stage of packaging is achieved by wrapping DNA around core histones in a welldefined conformation (1), forming a beads-on-a-string structure that is further compacted by additional proteins into folded chromatin and ultimately into metaphase chromosomes. Interphase chromatin is generally less condensed, but it contains regulatory sites such as promoters at which DNA must be distorted to enable interactions between proteins that bind distant sites on linear DNA. As a relatively stiff polymer, DNA resists the bending required for packaging, which requires free energy assistance from strong interactions with core histones and other proteins.Variations in DNA sequence can, in principle, modulate the energy cost of bending and packaging. For example, intrinsic curvature of the kind provided by A-tracts (2) should, if in the proper direction, lessen the energy required for bending. Increased local bending flexibility could reduce the energetic cost of packaging, as could an increase in twist flexibility (3), assuming that the helical phasing between distal regulatory sequence elements must be altered to form a protein complex.The stiffness of DNA (the inverse of its flexibility) is traditionally described by the persistence length P, defined as the average projection of the end-to-end vector of a very long chain on its initial direction. Classical techniques, such as light scattering (4) and rotational diffusion (5), supplemented by DNA cyclization kinetics (6), have led to estimates of around 140-180 bp for P. However, these methods are generally unable to deconvolute rigorously the contributions of curvature and flexibility to apparent persistence length (7), unless special constructs are used (8).Parameter sets exist for predicting DNA curvature based on roll and tilt values for dinucleotide (9-13) and tetranucleotide (14) steps, but the dinucleotide parameters vary considerably from set to set (15). Nor is there sufficient data to define unambiguously the sequence dependence of the experimental persistence length, ...

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