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In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei and in most Archaeal phyla.
Each nucleosome is composed of a little less than two turns of DNA wrapped around a set of eight proteins called histones, which are known as a histone octamer. Each histone octamer is composed of two copies each of the histone proteins H2A, H2B, H3, and H4. DNA must be compacted into nucleosomes to fit within the cell nucleus. [2]
Histones are responsible for maintaining the shape and structure of a nucleosome. One chromatin molecule is composed of at least one of each core histones per 100 base pairs of DNA. [ 2 ] There are five families of histones known to date; these histones are termed H1/H5, H2A, H2B, H3, and H4. [ 3 ]
Basic units of chromatin structure. Histone H3 is one of the five main histones involved in the structure of chromatin in eukaryotic cells. [1] [2] Featuring a main globular domain and a long N-terminal tail, H3 is involved with the structure of the nucleosomes of the 'beads on a string' structure.
Histones can be methylated on lysine (K) and arginine (R) residues only, but methylation is most commonly observed on lysine residues of histone tails H3 and H4. [7] The tail end furthest from the nucleosome core is the N-terminal (residues are numbered starting at this end).
Core histones are four proteins called H2A, H2B, H3 and H4 and they are all found in equal parts in the cell. All four of the core histone amino acid sequences contain between 20 and 24% of lysine and arginine and the size or the protein ranges between 11400 and 15400 daltons, making them relatively small, yet highly positively charged proteins. [6]
The cis position induces compact histones and decreases the ability of proteins to bind to the DNA, thus preventing methylation of K36 and decreasing gene transcription. Conversely, the trans position of P38 promotes a more open histone conformation, allowing for K36 methylation and leading to an increase gene transcription.
Following elution, the protein readily binds DNA, indicating the protein's high affinity for DNA. Histone-like proteins were unknown to be present in bacteria until similarities between eukaryotic histones and the HU-protein were noted, particularly because of the abundancy, basicity, and small size of both of the proteins. [8]