DNA bending and its relation to nucleosome positioning

X-ray and solution studies have shown that the conformation of a DNA double helix depends strongly on its base sequence. Here we show that certain sequence-dependent modulations in structure appear to determine the rotational positioning of DNA about the nucleosome. Three different experiments are described. First, a piece of DNA of defined sequence (169 base-pairs long) is closed into a circle, and its structure examined by digestion with DNAase I: the helix adopts a highly preferred configuration, with short runs of (A, T) facing in and runs of (G, C) facing out. Secondly, the same sequence is reconstituted with a histone octamer: the angular orientation around the histone core remains conserved, apart from a small uniform increase in helix twist. Finally, it is shown that the average sequence content of DNA molecules isolated from chicken nucleosome cores is non-random, as in a reconstituted nucleosome: short runs of (A, T) are preferentially positioned with minor grooves facing in, while runs of (G, C) tend to have their minor grooves facing out. The periodicity of this modulation in sequence content (10.17 base-pairs) corresponds to the helix twist in a local frame of reference (a result that bears on the change in linking number upon nucleosome formation). The determinants of translational positioning have not been identified, but one possibility is that long runs of homopolymer (dA) X (dT) or (dG) X (dC) will be excluded from the central region of the supercoil on account of their resistance to curvature.