Histone adalah

Histones are small, basic proteins required to condense DNA into chromatin. They have been first described and named in 1884 by Albrecht Kossel. There are five main histones: H1, H2A, H2B, H3 and H4. An octamer of core histones H2A, H2B, H3 and H4 is located inside a nucleosome, the central building block of chromatin, with about 150 base pairs of DNAwrapped around. The basic nature of histones, mediated by the high content of lysine and arginine residues, allows a direct interaction with the acidic phosphate back bone of DNA. The fifth histone H1 is located outside at the junction between nucleosomes and is referred to as the linker histone. Besides the main histones, so-called histone variants are known, which replace core histones in certain locations

like centromers.

Histone Acetyl Transferases (HAT) An enzyme activity ascribed to many coactivators, which transfers acetyl groups to lysine residues of histone tails of the nucleosomes and thereby facilitate their disruption and the opening of the chromatin.

▶Histone acetylation is a reversible and covalent modification of histone proteins introduced at the ε-amino groups of lysine residues. Histones and DNA

form a complex.

▶chromatin – which condenses DNA and controls gene activity. Current models interpret histone acetylation as a means to regulate chromatin activity.

Basic Mechanisms

All organisms must deal with the problem of packing DNA molecules of enormous size into the limited space of their cellular nuclei. Eukaryotes solve this problem by organizing their DNA into chromatin, a complex of DNA and dedicated packing proteins, so-called histones. This strategy allows an enormous reduction of the DNA’s spacial requirement, up to 10,000-fold. Chromatin consists of repetitive building blocks called.

▶nucleosome s comprising four pairs of histones (H2A, H2B, H3, and H4) with about 150 base pairs of DNA wrapped around. The nucleosome is only the first stage of packing DNA and further condensation into higherorder chromatin structures takes place in order to reach most efficient packaging. The structure of condensed higher-order chromatin is still unknown, but the nucleosomal building blocks with their histone cores are maintained as the basic structural unit. Histone proteins share distinct features including a high content of basic amino acids, lysines and arginines, which represent up to 25% of all residues. They consist of C-terminal globular domains located inside the

nucleosome, which interact with the DNA and other histones. In addition, histones possess N-terminal.

HDAC

Definition

Enzyme activity ascribed to corepressors, which is the removal of acetyl groups from lysine residues of histone tails. Thereby the assembly of nucleosomes is maintained,

which leads to a dense, transcriptional inactive chromatin structure.

Histone Methylation

Histone methylation is a common posttranslational modification fond in histones. Histone methylations have been identified on lysine and arginine residues. In case of lysines S-adenosyl-methionine (SAM) dependent methyl transferases catalyze the transfer of one, two or three methyl groups. Lysine methylation is reversible and lysine specific demethylases have been identified recently. Histone methylation seems to mediate long-term repression of transcription by recruitment of potent transcriptional repressors, such as heterochromatin protein-1 (HP-1).

Histone Phosphorylation

Histone phosphorylation is a common posttranslational modification fond in histones, primarily on the N-terminal tails. Phosphorylation sites include serine and threonine residues, tyrosine phosphorylation has not been observed so far. Some phosphorylation events occur locally whereas others occur globally throughout all chromosomes during specific events like mitosis. Histone phosphorylation is catalyzed by kinases. Removal of the phosphoryl groups is catalyzed by phosphatases.

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