CpG promoters have a constitutive nucleosome-depleted region and, when active, an unstable H4K20me3-modified nucleosome at the start site. (a, b) Highly expressed CpG promoter and non-CpG promoter genes have an upstream nucleosome-depleted region and well-positioned nucleosomes within the gene body (a, top). However, CpG promoters also contain a nucleosome-depleted region when repressed (a, bottom left) whereas, in inactive non-CpG promoters (a, bottom right), nucleosome occupancy more closely resembles that predicted by the intrinsic binding preferences of nucleosomes for DNA (b). (c, d) In expressed CpG promoter genes the transcription initiation site within the nucleosome-depleted region (a) is precisely marked by H4K20me3.
Vavouri and Lehner Genome Biology 2012 13:R110 doi:10.1186/gb-2012-13-11-r110