DNA methylation, histone acetylation, and chromatin structure. DNA is packaged as chromatin, the basic subunit of which is the nucleosome. Each nucleosome consists of two turns of DNA wrapped around a core histone of histone proteins, the tails of which protrude. Transcriptionally active chromatin is characterized by unmethylated DNA and acetylated (green triangles) histone tails. (a) The DNA is exposed and accessible to transcription factor binding. (b) Methylation of cytosine bases in the DNA (red dots) attracts methylcytosine-binding proteins which in turn attract and tether chromatin inactivation complexes containing histone deacetylases and other proteins. (c) These complexes deacetylate the histones and promote condensation of the chromatin into a compressed structure inaccessible to the transcription initiation complexes. DNMT, DNA methyltransferase; HAT, histone acetylase; HDAC, histone deacetylase; MBD, methyl-CpG-binding domain; RNA-PII, RNA polymerase II; TF, transcription factor. Figure reprinted with permission from Michigan Creative.
Patel and Richardson Arthritis Research & Therapy 2013 15:201 doi:10.1186/ar4125