Eukaryotic chromatin is tightly packaged into an array of nucleosomes, each consisting of a histone octamer core wrapped around by 147 bp of DNA and separated by linker DNA [1–3]. The nucleosomal core consists of four histone proteins [1] that can be post-translationally altered by at least 80 known covalent modifications [4, 5] or replaced by histone variants [6–8]. Positioning of nucleosomes throughout a genome has a significant regulatory function by modifying the in vivo availability of binding sites to transcription factors (TFs) and the general transcription machinery and thus affecting DNA-dependent processes such as transcription, DNA repair, replication and recombination [9]. Experiments designed to decipher how nucleosome positioning regulates gene expression have led to the understanding that transcriptional activation coincides with nucleosome perturbation, whereas transcriptional regulation requires the repositioning of nucleosomes throughout the eukaryotic lineage [10–18].
