We all know that chromosomes in the nucleus are highly compressed, and that DNA is wrapped around histones when folded. Zooming in on the histone region, we see a rosary where histones are strung together by a single DNA sequence.
Histone modifications is a type of post-translational modification (PTM) of proteins performed in a covalent manner. It includes: methylation (M), phosphorylation (P), acetylation (A) and many more. Since there are many types of histone modifications, we need to have a rule when calling histone modifications: histone structure plus amino acid name plus amino acid position plus modification type. In practical applications, we generally write: H3K4me3: Represent the trimethylation of the 4th lysine of the H3 histone. H3K14ac: Acetylation of lysine 14 representing the H3 histone. These modifications all affect the transcriptional activity of genes. Histone H3 is the most modified histone.
1. Type 1 of histone modifications: Histone methylation. Methylation is associated with inhibition or activation depending on its location and state. The sites of histone methylation are lysine and arginine. Lysine can be mono-, di-, and tri-methylated respectively, while arginine can only be mono- or di-methylated. Studies have shown that histone arginine methylation is a relatively dynamic marker, and arginine methylation is associated with gene activation. In contrast, lysine methylation appears to be a more stable marker in the regulation of gene expression. For example, methylation of H3K4 is associated with gene activation. H3K9 and H3K27 monomethylation are associated with gene activation, and trimethylation is associated with gene silencing. H3K9 and H3K27 methylation will mediate the formation of heterochromatin.
2. The second type of histone modifications: Histone acetylation. Histone methylation and acetylation occurs mainly in their N-terminal tails and can affect gene transcription. Histone acetylation is mainly related to gene activation. Histone acetylation mainly occurs at the relatively conserved lysine position at the N-terminus of H3 and H4, which is coordinated by histone acetyltransferase and histone deacetylase. Histone acetylation and deacetylation of specific gene regions occurs in a non-random, position-specific manner. Acetylation can regulate gene transcription through effects on histone charge and interacting proteins. These histone modifications can also work together to accomplish regulation. For example, H3K9ac also coexists with H3K14ac and H3K4me3 as markers of active gene promoters.