Conserved patterns of bending in satellite and nucleosome positioning DNA

Intrinsic DNA curvature has previously been implicated in the condensation of satellite DNA in chromatin. In this article, electrophoretic methods and computer programs for predicting DNA structure from nucleotide sequence were used to determine if curvature is a conserved feature of satellite DNA. The results revealed that satellite sequences display wide variation in magnitude of intrinsic bending. Less than half of the satellites examined were strongly bent when compared with control DNA. However, a conserved pattern of bending was seen in all 57 satellite sequences that were studied. The pattern consisted of repeating units of two 50-60-base pair bending elements which were separated by a 20-30-base pair region of low curvature. This pattern resembles qualitatively the bending of DNA in the nucleosome where the helix is folded approximately two turns around the histone octomer with the turns interrupted by a less bent segment in the center of the particle. The distance between successive pairs of bending elements was also similar to the average spacing of nucleosomes in chromatin. Thus, the conserved structures could play some role in the positioning of nucleosomes along satellite chromatin. In order to strengthen the correlation between DNA structure and nucleosome positioning, sequences were examined which have been shown to position nucleosomes at single major sites. This analysis revealed a conserved pattern of DNA structure resembling the two peak units seen in satellites. In addition, the nucleotide sequence patterns responsible for the conserved patterns of bending were similar in both satellite and nucleosome positioning DNA. Likewise, nucleotide sequence patterns that are thought to direct the rotational orientation of DNA in the nucleosome were similar in both sequence sets. These results are considered in terms of a general model for the role of DNA bending and nucleotide sequence in the control of nucleosome positioning and chromatin condensation in eukaryotes.