Preferential Nucleosome Occupancy at High Values of DNA Helical Rise

Nucleosomes are the basic structural units of eukaryotic chromatin and play a key role in the regulation of gene expression. Nucleosome formation depends on several factors, including properties of the se-quence itself, but also physical constraints and epigenetic factors such as chromatin-remodelling enzymes. In this view, a sequence-dependent approach is able to capture a general tendency of a region to bind a histone octamer. A reference data set of positioned nucleosomes of Saccharomyces cerevisiae was used to study the role of DNA helical rise in histone–DNA interaction. Genomic sequences were trans-formed into arrays of helical rise values by a tetranucleotide code and then turned into profiles of mean helical rise values. These profiles resemble maps of nucleosome occupancy, suggesting that intrinsic histone–DNA interactions are linked to helical rise. The obtained results show that preferential nucleo-some occupancy occurs where the mean helical rise reaches its largest values. Mean helical rise profiles obtained by using maps of positioned nucleosomes of the Drosophila melanogaster and Plasmodium fal-ciparum genomes, as well as Homo sapiens chromosome 20 confirm that nucleosomes are mainly located where the mean helical rise reaches its largest values. Key words: nucleosome; tetranucleotide; helical rise 1