A model for nucleosome positioning sequences. Vertical dotted lines represent consistent nucleosome boundaries. Green and red filled plots with arbitrary axes are qualitative representations of sequence conservation and SymCurv, respectively, shown here to allow better description of the model (actual data presented in Figure 4). High-affinity binding is necessary for regulatory elements occupying the surrounding space and allowing nucleosomes to occupy regions of low sequence constraint. The consistent positioning of these nucleosomes does not require high affinity but imposes a secondary level of constraints of a structural type on the underlying sequences. The symmetry of DNA curvature may be seen as one constraint of this type.