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Chromatin is the DNA-protein complex that makes up chromosomes. The major protein component of chromatin is the nucleosome octamer. One of the four proteins that make up the nucleosome octamer is histone H4. The particular interest in histone H4 stems from the fact that it is acetylated in several important processes, including gene activation, chromatin assembly and during spermatogenesis the histone replacement by protamine . Two of these processes are described below: gene activation and chromatin assembly. Evidence for the fundamental biological importance of histone H4 acetylation is not limited to Drosophila, but has been gleaned from studies in yeast, ciliates, Drosophila, frogs and mammals. Histone acetylation is an evolutionarily conserved process that performs conserved biological functions.
Gene activation: Histone acetylation plays an active role in facilitating the entry of transcription factors into nucleosomal DNA. The idea that acetylated histones are associated with transcriptionally active chromatin is more than three decades old. However, only in the past decade have experimental systems been sufficiently refined to provide convincing evidence.
Chromatin assembly: controlling gene accessibility to transcription factors is not the only role for histone H4 acetylation in cell biology. Acetylation is involved in the assembly of histones into nucleosomes. The cytoplasmic enzyme histone transacetylase B (HAT B) is involved in the evolutionarily conserved acetylation of newly synthesized histone H4 protein on lysine 12.
Chromatin is essential for genome packaging and regulation. The basic unit of chromatin is the nucleosome, which consists of 147 DNA base pairs wrapped around a histone octamer, which contains two copies of each of histone H3 protein, histone H4 protein, histone H2A protein, and histone H2B protein. A fifth "linker histone", H1, dynamically binds DNA located between histone octamers in a subset of nucleosomes. Histones not only provide a binding platform for DNA; they are also actively involved in DNA-related processes such as transcription. One mechanism by which histones perform these functions is through post-translational modifications.
Over the past 20 years, more than 20 PTMs have been identified on histones, including acetylation, methylation, phosphorylation, ubiquitination, and crotonylation. Among these PTMs, 12 were added to lysine residues. N-terminal flexible "tail" domains are the most heavily modified parts of histones, presumably because they are more accessible to histone-modifying enzymes than other domains. However, PTMs have also been detected in the globular core domains of histones. Histone PTMs are thought to regulate chromatin structure and gene expression either directly or by recruiting specific chromatin-associated proteins.
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