Histone code

The histone code is a hypothesis that the transcription of genetic information encoded in DNA is in part regulated by chemical modifications to histone proteins, primarily on their unstructured ends. Together with similar modifications such as DNA methylation it is part of the epigenetic code. Histones associate with DNA to form nucleosomes, which themselves bundle to form chromatin fibers, which in turn make up the more familiar chromosome. Histones are globular proteins with a flexible N-terminus (taken to be the tail) that protrudes from the nucleosome. Many of the histone tail modifications correlate very well to chromatin structure and both histone modification state and chromatin structure correlate well to gene expression levels. The critical concept of the histone code hypothesis is that the histone modifications serve to recruit other proteins by specific recognition of the modified histone via protein domains specialized for such purposes, rather than through simply stabilizing or destabilizing the interaction between histone and the underlying DNA. These recruited proteins then act to alter chromatin structure actively or to promote transcription. For details of gene expression regulation by histone modifications see table below.

The hypothesis is that chromatin-DNA interactions are guided by combinations of histone modifications. While it is accepted that modifications (such as methylation, acetylation, ADP-ribosylation, ubiquitination, citrullination, and phosphorylation) to histone tails alter chromatin structure, a complete understanding of the precise mechanisms by which these alterations to histone tails influence DNA-histone interactions remains elusive. However, some specific examples have been worked out in detail. For example, phosphorylation of serine residues 10 and 28 on histone H3 is a marker for chromosomal condensation. Similarly, the combination of phosphorylation of serine residue 10 and acetylation of a lysine residue 14 on histone H3 is a tell-tale sign of active transcription.

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