Histone Modification

A nucleosome contains a protein core that is made up of four histone core proteins: H2A, H2B, H3, and H4.

In addition to these four standard core histones, eukaryotes also possess a few variants of each histone, with the exception of H4. 

The amino-terminal tails of the core standard and variant core histones protrude from the nucleosomes and are highly unstructured and mobile.

These tails, comprising of about 30 amino acids, are subjected to several forms of covalent modifications such as the acetylation of lysines, phosphorylation of serines, and mono-, di-, or tri-methylation of lysines. 

The reactions that lead to these modifications are catalyzed by different enzymes such as methyltransferases, acetylases, kinases. These groups of enzymes are collectively referred to as the ‘writers.’ 

The reactions that catalyze the removal of these chemical groups are catalyzed by enzymes such as demethylases, deacetylases, and phosphatases. These enzymes are collectively referred to as the ‘erasers.’

Amongst the numerous possible combinations of different histone variants and amino-terminal end modifications, only certain coordinated sets are known to occur. Some of these combined sets of m