Artificial nucleosome positioning sequences.

We have used the emerging rules for the sequence dependence of DNA bendability to design and test a series of DNA molecules that incorporate strongly into nucleosomes. Competitive reconstitution experiments showed the superiority in histone octamer binding of DNA molecules in which segments consisting exclusively of A and T or G and C, separated by 2 base pairs (bp), are repeated with a 10-bp period. These repeated (A/T)3NN(G/C)3NN motifs are superior in nucleosome formation to natural positioning sequences and to other repeated motifs such as AANNNTTNNN and GGNNNCCNNN. Studies of different lengths of repetitive anisotropically flexible DNA showed that a segment of approximately 40 bp embedded in a 160-bp fragment is sufficient to generate nucleosome binding equivalent to that of natural nucleosome positioning sequences from 5S RNA genes. Bending requirements along the surface of the nucleosome seem to be quite constant, with no large jumps in binding free energy attributable to protein-induced kinks. The most favorable sequences incorporate into nucleosomes more strongly by 100-fold than bulk nucleosomal DNA, but differential bending free energies are small when normalized to the number of bends: a free energy difference of only about 100 cal/mol per bend (1 cal = 4.184 J) distinguishes the best bending sequences and bulk DNA. We infer that the distortion energy of DNA bending in the nucleosome is only weakly dependent on DNA sequence.