Post translational modifications on histones include acetylation, methylation, phosphorylation and some new acylation modifications found in recent years. These histone modifications directly affect the binding of chromatin to transcription factors or other epigenetic regulators, and change genome stability and gene transcription. Histone methylation usually occurs in lysine and arginine residues of core histones. Histone methylation can promote or inhibit gene transcription, depending on whether histone methylation occurs on lysine or arginine and the number of methylation groups (lysine can be monomethyl, dimethyl and trimethylated, arginine can be monomethyl, symmetric and asymmetric dimethyl). Histone lysine methylation usually occurs on lysine 4, 9, 27, 36, 79 of histone H3 and lysine 20 of histone H4; Arginine methylation usually occurs on arginine 2, 8, 17, 26 of histone H3 and arginine 3 of histone H4. Protein methylase (HMT) and histone demethylase (HDM) are the main regulators of methylation modification. H3K122me2 is a newly discovered modification site.
Histone post-translational modifications (PTMs) are key mechanisms of epigenetics that modulate chromatin structures, termed as “histone code”. The PTMs on histone including acetylation, methylation, phosphorylation and novel acylations directly affect the accessibility of chromatin to transcription factors and other epigenetic regulators, altering genome stability, gene transcription, etc. Histone methylation occurs primarily at lysine and arginine residues on the amino terminal of core histones. Methylation of histones can either increase or decrease transcription of genes, depending on which amino acids (Lys or Arg) in the histones are methylated and how many methyl groups are attached (mono-, di-, Trimethylation on Lys, mono-di-symmetric/asymmetric methylation on Arg). Mostly, lysine methylation occurs primarily on histone H3 Lys4, 9, 27, 36, 79 and H4 Lys20, while Arginine methylation occurs primarily on histone H3 Arg2, 8, 17, 26 and H4 Arg3. histone methyltransferases (HMTs) and histone demethylases (HDMs) are major regulating factors.
Histones are basic nuclear proteins that are responsible for the nucleosome structure of the chromosomal fiber in eukaryotes. This structure consists of approximately 146 bp of DNA wrapped around a nucleosome, an octamer composed of pairs of each of the four core histones (H2A, H2B, H3, and H4). The chromatin fiber is further compacted through the interaction of a linker histone, H1, with the DNA between the nucleosomes to form higher order chromatin structures. This gene is intronless and encodes a replication-dependent histone that is a member of the histone H3 family. Transcripts from this gene lack polyA tails; instead, they contain a palindromic termination element. This gene is found in the large histone gene cluster on chromosome 6p22-p21.3.
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