What are the types of histone modifications?

There are several types of histone modifications, but the most common ones are acetylation, methylation, ubiquitination, and phosphorylation. These are the most important histone modifications as they play a vital role in the regulation of chromatin structure and transcriptional activity.  Acetylation is most widely studied, as it plays an important role in transcriptional regulation. Acetylation neutralizes the positive charge on histones which in turn minimizes the interaction of histone and DNA, creating a weaker histone. This allows transcription factor binding to occur and also increases gene expression. Histone acetylation is also involved in cell proliferation, apoptosis, and cell cycle regulation. Acetyl groups are added to lysine residues of H3 and H4 histones by histone acetyltransferases and removed by deacetylases. Histone acetylation is often used in promoter regions. 

Methylation is added to arginine or lysine residues of H3 and H4 histones. Arginine methylation stimulates transcriptional activation, and lysine methylation is involved in transcriptional activation as well as repression (depends on the methylation site). Methylation doesn’t alter histone charge or have an effect on histone-DNA interactions. 

Histone phosphorylation has a crucial role in chromosome condensation during cell division, DNA damage repair and transcriptional regulation as well. Phosphorylation works to create interactions between other histone modifications and is mediated by kinases and phosphorylases. Phosphorylation occurs on all core histones. For example, phosphorylation occurs on H3 at serine 1- and 28 and on histone 2A on T 120. These both assist in chromatin compaction and regulation of chromatin structure during the process of mitosis.  

Histone ubiquitylation is a histone modification induced by DNA damage and has an important role in the DNA damage response. H2A and H2B are the most commonly ubiquitinated proteins in the nucleus. Monoubiquitylation H2A is correlated with gene silencing, and H2B is correlated with transcription activation. Poly-ubiquitination occurs less frequently, but still plays a role in DNA repair. Poly- ubiquitination of H2A allows for a recognition site to be made for DNA repair proteins.