Histone H3

Histone H3 is 15-16 kDa protein consisting of ~135 amino acid residues (except for a variant CENP-A). H3-H4 heterotetramer forms a core component of the 'nucleosome'. The Histone H3 family contains 216 different members reported from diverse species. Human histone H3 has 6 different variants. These are expressed in a replication dependent as well as independent manner. CENPA is a variant that is bigger in size and specifically incorporates into centromeric nucleosomes. Asf1a and Asf1b are histone chaperones required for the deposition of H3 variants. H3 is coded by total 18 genes. All genes coding for H3.1 are present in cluster 1 and those for H3.2 are present in cluster 2.

H3 harbours maximum number of sites that are reported to undergo post translational modifications in context of various cellular process. Certain modifications such as H3K4me are used as a universal transcriptional activation mark whereas H3K9me and H3K27me are universally accepted as marks of transcriptional repression. Many of these variants are supposed to be incorporated at the sites of DNA damage in order to provide a damage memory.

Cytogenetic map of histone H3 coding genes
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Variants of histone H3
Variant (UniprotKB recommended name) UniprotKB Accession No. of coding gene(s)
Histone H3.1P6843110 (HIST1H3A, HIST1H3D, HIST1H3C, HIST1H3E, HIST1H3I, HIST1H3G, HIST1H3J, HIST1H3H, HIST1H3B, HIST1H3F)
Histone H3.3P842432 (H3F3A, H3F3B)
Histone H3.2Q71DI33 (HIST2H3C, HIST2H3A, HIST2H3D)
Histone H3.1tQ166951 (HIST3H3)
Histone H3-like centromeric protein AP494501 (CENPA)
Histone H3.3CQ6NXT21 (H3F3C)